ITS RELATION TO THE EARTH AND MAN

A wide interest, if not a deep one, attaches to the general question as
to the existence of living beings, or at least the possibility of
organic existence, on planetary bodies other than our own. The question
has been examined in all ages, by the lights of the science peculiar to
each. With every important accession to our astronomical knowledge it
has been re-raised: every considerable discovery has given rise to some
new step or phase in the discussion, and in this way there has grown up
a somewhat extensive literature exclusively relating to mundane
plurality. It will readily be understood that the moon, from its
proximity to the earth, has from the first received a large, perhaps the
largest, share of attention from wanderers into this field of
speculation: and we might add greatly to the bulk of this volume by
merely reviewing some of the more curious and, in their way, instructive
conjectures specially relating to the moon as a world—to imaginary
journeys towards her, and to the beings conjectured to dwell upon and
within her. This, however, we feel there is no occasion to do, for it is
our purpose merely to point out the two or three almost conclusive
arguments against the possibility of any life, animal or vegetable,
having existence on our satellite.

We well know what are the requisite conditions of life on the earth; and
we can go no further for grounds of inference; for if we were to start
by assuming forms of life capable of existence under conditions widely
and essentially different from those pertaining to our planet, there
would be no need for discussing our subject further: we could revel in
conjectures, without a thought as to their extravagance. The only
legitimate phase of the question we can entertain is this:—can there be
on the moon any kind of living things analogous to any kind of living
things upon the earth? And this question, we think, admits only of a
negative answer. The lowest forms of vitality cannot exist without air,
moisture, and a moderate range of temperature. It may be true, as recent
experiments seem to show, that organic germs will retain their vitality
without either of the first, and with exposure to intense cold and to a
considerable degree of heat; and it is conceivable that the mere germs
of life may be present on the moon.[16] But this is not the case with
living organisms themselves. We have, in Chapter V., specially devoted
to the subject, cited the evidence from which we know that there can be
at the most, no more air on the moon than is left in the receiver of an
air-pump after the ordinary process of exhaustion. And with regard to
moisture, it could not exist in any but the vaporous state, and we know
that no appreciable amount of vapour can be discovered by any
observation (and some of them are crucial enough) that we are capable of
making. We may suppose it just within the verge of possibility that some
low forms of vegetation might exist upon the moon with a paucity of air
and moisture such as would be beyond even our most severe powers of
detection: but granting even this, we are met by the temperature
difficulty; for it is inconceivable that any plant-life could survive
exposure first to a degree of cold vastly surpassing that of our arctic
regions, and then in a short time (14 days) to a degree of heat capable
of melting the more fusible metals—the total range being equal, as we
have elsewhere shown, to perhaps 600 or 700 degrees of our thermometric
scale.

The higher forms of vegetation could not reasonably be expected to exist
under conditions which the lower forms could not survive. And as regards
the possibility of the existence of animal life in any form or condition
on the lunar surface, the reasons we have adduced in reference to the
non-existence of vegetable life bear still more strongly against the
possibility of the existence of the former. We know of no animal that
could live in what may be considered a vacuum and under such thermal
conditions as we have indicated.

[Illustration: PLATE XXI.
NORMAL LUNAR CRATER.]

As to man, aëronautic experience teaches us that human life is
endangered when the atmosphere is still sufficiently dense to support 12
inches of mercury in the barometer tube; what then would be his
condition in a medium only sufficiently dense to sustain one-tenth of an
inch of the barometric column? We have evidence from the most delicate
tests that no atmosphere or vapour approaching even this degree of
attenuation exists around the moon’s surface.

Taking all these adverse conditions into consideration we are in every
respect justified in concluding that there is no possibility of animal
or vegetable life existing on the moon, and that our satellite must
therefore be regarded as a barren world.

* * * * * * * *

After this disquisition upon lunar uninhabitability it may appear
somewhat inconsistent for us to attempt a description of the scenery of
the moon and some other effects that would be visible to a spectator,
and of which he would be otherwise sensible, during a day and a night
upon her surface. But we can offer the sufficient apology that an
imaginary sojourn of one complete lunar day and night upon the moon
affords an opportunity of marshalling before our readers some phenomena
that are proper to be noticed in a work of this character, and that have
necessarily been passed over in the series of chapters on consecutive
and special points that have gone before. It may be urged that, in
depicting the moon from such a standpoint as that now to be taken, we
are describing scenes that never have been such in the literal sense of
the word, since no eye has ever beheld them. Still we have this
justification—that we are invoking the conception of things that
actually exist; and that we are not, like some imaginary voyagers to the
moon, indulging in mere flights of fancy. Although it is impossible for
a habitant of this earth fully to realise existence upon the moon, it is
yet possible, indeed almost inevitable, for a thoughtful
telescopist—watching the moon night after night, observing the sun rise
upon a lunar scene, and noting the course of effects that follow till it
sets—it is almost inevitable, we say, for such an observer to identify
himself so far with the object of his scrutiny, as sometimes to become
in thought a lunar being. Seated in silence and in solitude at a
powerful telescope, abstracted from terrestrial influences, and gazing
upon the revealed details of some strikingly characteristic region of
the moon, it requires but a small effort of the imagination to suppose
one’s self actually upon the lunar globe, viewing some distant landscape
thereupon; and under these circumstances there is an irresistible
tendency in the mind to pass beyond the actually _visible_, and to fill
in with what it knows must exist those accessory features and phenomena
that are only hidden from us by distance and by our peculiar point of
view. Where the material eye is baffled, the clairvoyance of reason and
analogy comes to its aid.

Let us then endeavour to realize the strange consequences which the
position and conditions of the moon produce upon the aspect of a lunar
landscape in the course of a lunar day and night.

The moon’s day is a long one. From the time that the sun rises upon a
scene[17] till it sets, a period of 304 hours elapses, and of course
double this interval passes between one sunrise and the next. The
consequences of this slow march of the sun begin to show themselves from
the instant that he rises above the lunar horizon. Dawn, as we have it
on earth, can have no counterpart upon the moon. No atmosphere is there
to reflect the solar beams while the luminary is yet out of actual
sight, and only the glimmer of the zodiacal light heralds the approach
of day. From the black horizon the sun suddenly darts his bright
untempered beams upon the mountain tops, crowning them with dazzling
brilliance while their flanks and valleys are yet in utter darkness.
There is no blending of the night into day. And yet there is a growth of
illumination that in its early stages may be called a twilight, and
which is caused by the slow rise of the sun. Upon the earth, in central
latitudes, the average time occupied by the sun in rising, from the
first glint of his upper edge till the whole disc is in sight, is but
two minutes and a quarter. Upon the moon, however, this time is extended
to a few minutes short of an hour, and, therefore, during the first few
minutes a dim light will be shed by the small visible chord of the solar
disc, and this will give a proportionately modified degree of
illumination upon the prominent portion of the landscape, and impart to
it something of the weird aspect which so strikes an observer of a total
solar eclipse on earth when the scene is lit by the thin crescent of the
re-appearing sun. This impaired illumination constitutes the only dawn
that a lunar spectator could behold. And it must be of short duration;
for when, in the course of half an hour, the solar disc has risen half
into view the lighting would no doubt appear nearly as bright to the eye
as when the entire disc of the sun is above the horizon. In this lunar
sunrise, however, there is none of that gilding and glowing which makes
the phenomenon on earth so gorgeous. Those crimson sky-tints with which
we are familiar are due to the absorption of certain of the polychromous
rays of light by our atmosphere. The blue and violet components of the
solar beams are intercepted by our envelope of vapour, and only the red
portions are free to pass; while on the moon, as there is no atmosphere,
this selective absorption does not occur. If it did, an observer gazing
from the earth upon the regions of the moon upon which the sun is just
rising would see the surface tinted with rosy light. This, however, is
not the case; the faintest lunar features just catching the sun are seen
simply under white light diluted to a low degree of brightness. Only
upon rare occasions is the lunar scenery suffused with coloured
illumination, and these are when, as we shall presently have to
describe, the solar rays reach the moon after traversing the earth’s
atmosphere during an eclipse of the sun.

This atmosphere of ours is the most influential element in beautifying
our terrestrial scenery, and the absence of such an appendage from the
moon is the great modifying cause that affects lunar scenery as compared
with that of the earth. We are accustomed to the sun with its dazzling
brightness—overpowering though it be—subdued and softened by our
vaporous screen. Upon the moon there is no such modification. The sun’s
intrinsic brilliancy is undiminished, its apparent distance is
shortened, and it gleams out in fierce splendour only to be realised,
and then imperfectly, by the conception of a gigantic electric light a
few feet from the eye. And the brightness is rendered the more striking
by the blackness of the surrounding sky. Since there is no atmosphere
there can be no sky-light, for there is nothing above the lunar world to
diffuse the solar beams; not a trace of that moisture which even in our
tropical skies scatters some of the sun’s light and gives a certain
degree of opacity or blueness, deep though it be, to the heavens by day.
Upon the moon, with no light-diffusing vapour, the sky must be as dark
or even darker than that with which we are familiar upon the finest of
moonless nights. And this blackness prevails in the full blaze of the
lunar noon-day sun. If the eye (upon the moon) could bear to gaze upon
the solar orb (which would be less possible than upon earth) or could it
be screened from the direct beams, as doubtless it could by intervening
objects, it would perceive the nebulous and other appendages which we
know as the corona, the zodiacal light, and the red solar protuberances:
or if these appendages could not be viewed with the sun above the
horizon they would certainly be seen in glorious perfection when the
luminary was about to rise or immediately after it had set.

And, notwithstanding the sun’s presence, the planets and stars would be
seen to shine more brilliantly than we see them on the clearest of
nights; the constellations would have the same configurations, though
they would be differently situated with respect to the celestial pole
about which they would appear to turn, for the axis of rotation of the
moon is directed towards a point in the constellation Draco. The stars
would never twinkle or change colour as they appear to us to do, for
scintillation or twinkling is a phenomenon of atmospheric origin, and
they would retain their full brightness, down even to the horizon, since
there would be no haze to diminish their light. The planets, and the
brighter stars at least, would be seen even when they were situated very
near to the sun. The planet Mercury, so seldom detected by terrestrial
gazers, would be almost constantly in view during the lunar day,
manifesting his close attendance on the central luminary by making only
short excursions of about two (lunar) days’ length, first on one side
and then on the other. Venus would be nearly as continuously visible,
though her wanderings would be more extensive on either side. The
zodiacal light also, which in our English latitude and climate is but
rarely seen and in more favourable climes appears only when the sun
itself is hidden beneath the horizon, would upon the moon be seen as a
constant accompaniment to the luminary throughout his daily course
across the lunar sky. The other planets would appear generally as they
do to us on earth, but, never being lost in daylight, their courses
among the stars could be traced with scarcely any interruption.

One planet, however, that adorns the sky of the lunar hemisphere which
is turned towards us deserves special mention from the conspicuous and
highly interesting appearance it must present. We allude to the earth.
To nearly one-half of the moon (that which we never see) this imposing
object can never be visible; but to the half that faces us the
terrestrial planet must appear almost fixed in the sky. A lunar
spectator in (what is to us) the centre of the disc, or about the region
north of the lunar mountains Ptolemy and Hipparchus, would have the
earth in his zenith. From regions upon the moon a little out of what is
to us the centre, a spectator would see the earth a little declining
from the zenith, and this declination would increase as the regions
corresponding to the (to us) apparent edge of the moon were approached,
till at the actual edge it would be seen only upon the horizon. From the
phenomena of libration (explained in Chap. VI.) the earth would appear
from nearly all parts of the lunar hemisphere to which it is visible at
all to describe a small circle in the sky. To an observer, however, upon
the (to us) marginal regions of the lunar globe, it would appear only
during a portion of the lunar day—being visible in fact only in that
part of its small circular path which happened to lie above the
observer’s horizon: in some regions only a portion of the terrestrial
disc would make its brief appearance. From the lunar hemisphere beyond
this marginal line the earth can never be seen at all.

The lunar spectator whose situation enabled him to view the earth would
see it as a moon; and a glorious moon indeed it must be. Its diameter
would be four times as great as that of the moon itself as seen by us,
and the area of its full disc 13 times as great. It would be seen to
pass through its phases, just as does our satellite, once in a lunar day
or a terrestrial month, and during that cycle of phases, since 29 of our
days would be occupied by it, the axial rotation would bring all the
features of its surface configuration into view so many times in
succession. But the greatest beauty of this noble moon would be seen
during the lunar night, in considering which we shall again allude to
it; for when it is full-moon to the earth it is new-earth to the moon.
At lunar midnight this globe of ours is fully illuminated; as morning
nears, the earth-moon wanes, its disc slowly passing through the gibbous
phases until at sunrise it would be just half-illuminated. During the
long forenoon it assumes a crescent which narrows and narrows till at
midday the sun is in line with the earth and the latter is invisible,
save perhaps by a thin line of light marking its upper or lower edge,
accordingly as the sun is apparently above or below it. In the lunar
afternoon an illuminated crescent appears upon the opposite side of the
terrestrial globe, and this widens and widens till it becomes a half
disc by lunar sunset and a full disc by lunar midnight.

The sun in his daily course passes at various distances, sometimes above
and sometimes below, the nearly stationary earth. Obviously it will at
times pass actually behind it, and then the lunar spectator would behold
the sublime spectacle of a total solar eclipse, and that under
circumstances which render the phenomenon far more imposing than its
counterpart can appear from the earth; for whereas, when we see the moon
eclipse the sun, the nearly similar (apparent) diameters of the two
bodies renders the duration of totality extremely short—at most 7
minutes—a lunar spectator, the earth appearing to him four times the
diameter of the sun, and he and the earth being relatively stationary,
would enjoy a view of the totality extending over several hours. During
the passage of the solar disc behind that of the earth, a beautiful
succession of luminous phenomena would be observed to follow from the
refractions and dispersions which the sunbeams would suffer in passing
tangentially through those parts of our atmospheric envelope which lie
in their course; those, for instance, on the margin of the earth, as
seen from the moon. As the sun passed behind the earth, the latter would
be encircled upon the in-going side with a beautiful line of golden
light, deepening in places to glowing crimson, due to the absorption,
already spoken of, of all but the red and orange rays of the sun’s light
by the vapours of our atmosphere. As the eclipse proceeded and totality
came on, this ruddy glow would extend itself nearly, if not all, around
the black earth, and so bright would it be, that the whole lunar
landscape covered by the earth’s shadow would be illuminated with faint
crimson light,[18] save, perhaps, in some parts of the far distance,
upon which the earth had not yet cast its shadow, or off which the
shadow had passed. Although the crimson light would preponderate, it
would not appear bright and red alike all around the earth’s periphery.
The circle of light would be, in fact, _the ring of twilight_ round our
globe, and it would only appear red in those places where the atmosphere
chanced to be in that condition favourable for producing what on earth
we know as red sunset and sunrise. We know that the sun, even in clear
sky, does not always set and rise with the beautiful red glow, which may
be determined by merely local causes, and will therefore vary in
different parts of the earth. Now a lunar spectator watching the sun
eclipsed by the earth, would see, during totality and at a _coup d’œil_,
every point around our world upon which the sun is setting on one side
and rising upon the other. To every part of the earth around what is
then the margin, as seen from the moon, the sun is upon the horizon,
shining through a great thickness of atmosphere, reddening it, and being
reddened by it wherever the vaporous conditions conduce to that
colouration. And at all parts where these conditions obtain, the lunar
eclipse-observer would see the ring of light around the black
earth-globe brilliantly crimsoned; at other parts it would have other
shades of red and yellow, and the whole effect would be to make the
grand earth-ball, hanging in the lunar sky, like a dark sphere in a
circle of glittering gold and rubies.

During the early stages of the eclipse, this chaplet of
brilliant-coloured lights would be brightest upon the side of the
_disappearing_ sun; at the time of central eclipse the radiance
(supposing the sun to pass centrally behind the earth) would be equally
distributed, and during the later stages it would preponderate upon the
side of the _reappearing_ sun. We have endeavoured to give a pictorial
realization of this phenomenon and of the effect of the eclipse upon the
lunar landscape, but such a picture cannot but fall very, very far short
of the reality. (See Plate XXII.)

And now for a time let us turn attention from the lunar sky to the
scenery of the lunar landscape. Let us, in imagination, take our stand
high upon the eastern side of the rampart of one of the great craters.
Height, it must be remarked, is more essential on the moon to command
extent of view than upon the earth, for on account of the comparative
smallness of the lunar sphere the dip of the horizon is very rapid. Such
height, however, would be attained without great exercise of muscular
power, since equal amounts of climbing energy would, from the smallness
of lunar gravity, take a man six times as high on the moon as on the
earth. Let us choose, for instance, the hill-side of Copernicus. The day
begins by a sudden transition. The faint looming of objects under the
united illumination of the half-full earth and the zodiacal light is the
lunar precursor of day-break. Suddenly the highest mountain peaks
receive the direct rays of a portion of the sun’s disc as it emerges
from below the horizon. The brilliant lighting of these summits serves
but to increase, by contrast, the prevailing darkness, for they seem to
float like islands of light in a sea of gloom. At a rate of motion
twenty-eight times slower than we are accustomed to, the light tardily
creeps down the mountain-sides, and in the course of about twelve hours
the whole of the circular rampart of the great crater below us, and
towards the east, shines out in brilliant light, unsoftened by a trace
of mountain-mist. But on the opposite side, looking into the crater,
nothing but blackness is to be seen. As hour succeeds hour, the sunbeams
reach peak after peak of the circular rampart in slow succession, till
at length the circle is complete and the vast crater-rim, 50 miles in
diameter, glistens like a silver-margined abyss of darkness. By-and-by,
in the centre, appears a group of bright peaks or bosses. These are the
now illuminated summits of the central cones, and the development of the
great mountain cluster they form henceforth becomes an imposing feature
of the scene. From our high standpoint, and looking backwards to the
sunny side of our cosmorama, we glance over a vast region of the wildest
volcanic desolation. Craters from five miles diameter downwards crowd
together in countless numbers, so that the surface, as far as the eye
can reach, looks veritably frothed over with them. Nearer the base of
the rampart on which we stand, extensive mountain chains run to north
and to south, casting long shadows towards us; and away to southward run
several great chasms a mile wide and of appalling blackness and depth.
Nearer still, almost beneath us, crag rises on crag and precipice upon
precipice, mingled with craters and yawning pits, towering pinnacles of
rock and piles of scoria and volcanic _débris_. But we behold no sign of
existing or vestige of past organic life. No heaths or mosses soften the
sharp edges and hard surfaces: no tints of cryptogamous or lichenous
vegetation give a complexion of life to the hard fire-worn countenance
of the scene. The whole landscape, as far as the eye can reach, is a
realization of a fearful dream of desolation and lifelessness—not a
dream of death, for that implies evidence of preexisting life, but a
vision of a world upon which the light of life has never dawned.

[Illustration: PLATE XXII.
ASPECT OF AN ECLIPSE OF THE SUN BY THE EARTH, AS IT WOULD APPEAR AS
SEEN FROM THE MOON.]

Looking again, after some hours’ interval, into the great crateral
amphitheatre, we see that the rays of the morning sun have crept down
the distant side of the rampart, opposite to that on which we stand, and
lighted up its vast landslipped terraces into a series of seeming
hill-circles with all the rude and rugged features of a terrestrial
mountain view, and none of the beauties save those of desolate grandeur.
The plateau of the crater is half in shadow 10,000 feet below, with its
grand group of cones, now fully in sight, rising from its centre.
Although these last are twenty miles away and the base of the opposite
rampart fully double that distance, we have no means of judging their
remoteness, for in the absence of an atmosphere there can be no aërial
perspective, and distant objects appear as brilliant and distinct as
those which are close to the observer. Not the brightness only, but the
various colours also of the distant objects are preserved in their full
intensity; for colour we may fairly assume there must be. Mineral
chlorates and sublimates will give vivid tints to certain parts of the
landscape surface, and there must be all the more sombre colours which
are common to mineral matters that have been subjected to fiery
influence. All these tints will shine and glow with their greater or
less intrinsic lustres, since they have not been deteriorated by
atmospheric agencies, and far and near they will appear clear alike,
since there is no aërial medium to veil them or tarnish their pristine
brightness.

In the lunar landscape, in the line of sight, there are no means of
estimating distances; only from an eminence, where the intervening
ground can be seen, is it possible to realize _magnitude_ in a lunar
cosmorama and comprehend the dimensions of the objects it includes.

And with no air there can be no diffusion of light. As a consequence, no
illumination reaches those parts of the scene which do not receive the
direct solar rays, save the feeble amount reflected from contiguous
illuminated objects, and a small quantity shed by the crescent earth.
The shadows have an awful blackness. As we stand upon our chosen point
of observation, we see on the lighted side of the rampart almost
dazzling brightness, while beneath us, on the side away from the sun,
there is a region many miles in area impenetrable to the sight, for
there is no object within it receiving sufficient light to render it
discernible; and all around us, far and near, there is the violent
contrast between intense brightness of insolated parts and deep gloom of
those in equally intense shadow. The black though starlit sky helps the
violence of this contrast, for the bright mountains in the distance
around us stand forth upon a background formed by the darkness of
interplanetary space. The visible effects of these conditions must be in
every sense unearthly and truly terrible. The hard, harsh glowing light
and pitchy shadows; the absence of all the conditions that give
tenderness to an earthly landscape; the black noonday sky, with the
glaring sun ghastly in its brightness; the entire absence of vestiges of
any life save that of the long since expired volcanoes—all these
conspire to make up a scene of dreary, desolate grandeur that is
scarcely conceivable by an earthly habitant, and that the description we
have attempted but insufficiently pourtrays.

A legitimate extension of the imagination leads us to impressions of
lunar conditions upon other senses than that of sight, to which we have
hitherto confined our fancy. We are met at the outset with a difficulty
in this extension; for it is impossible to conceive the sensations which
the absence of an atmosphere would produce upon the most important of
our bodily functions. If we would attempt the task we must conjure up
feelings of suffocation, of which the thoughts are, however, too
horrible to be dwelt upon; we must therefore maintain the delusion that
we can exist without air, and attempt to realize some of the less
discomforting effects of the absence of this medium. Most notable among
these are the untempered heat of the direct solar rays, and the
influence thereof upon the surface material upon which we suppose
ourselves to stand. During a period of over three hundred hours the sun
pours down his beams with unmitigated ferocity upon a soil never
sheltered by a cloud or cooled by a shower, till that soil is heated, as
we have shown, to a temperature equal nearly to that of melting lead;
and this scorching influence is felt by everything upon which the sun
shines on the lunar globe. But while regions directly insolated are thus
heated, those parts turned from the sun would remain intensely cold, and
that scorching in sunshine and freezing in shade with which mountaineers
on the earth are familiar would be experienced in a terribly exaggerated
degree. Among the consequences, already alluded to, of the alternations
of temperature to which the moon’s crust is thus exposed, are doubtless
more or less considerable expansions and contractions of the surface
material, and we may conceive that a cracking and crumbling of the more
brittle constituents would ensue, together with a grating of contiguous
but disconnected masses, and an occasional dislocation of them. We refer
again to these phenomena to remark that if an atmospheric medium existed
they would be attended with noisy manifestations. There are abundant
causes for grating and crackling sounds, and such are the only sources
of noise upon the moon, where there is no life to raise a hum, no wind
to murmur, no ocean to boom and foam, and no brook to plash. Yet even
these crust-cracking commotions, though they might be felt by the
vibrations of the ground, would not manifest themselves audibly, for
without air there can be no communication between the grating or
cracking body and the nerves of hearing. Dead silence reigns on the
moon: a thousand cannons might be fired and a thousand drums beaten upon
that airless world, but no sound could come from them: lips might quiver
and tongues essay to speak, but no action of theirs could break the
utter silence of the lunar scene.

At a rate twenty-eight times slower than upon earth, the shadows shorten
till the sun attains his meridian height, and then, from the tropical
region upon which we have in imagination stood, nothing is to be seen on
any side, save towards the black sky, but dazzling light. The relief of
afternoon shadow comes but tardily, and the darkness drags its slow
length along the valleys and creeps sluggishly up the mountain-sides
till, in a hundred hours or more, the time of sunset approaches. This
phenomenon is but daybreak reversed, and is unaccompanied by any of the
gorgeous sky tints that make the kindred event so enrapturing on earth.
The sun declines towards the dark horizon without losing one jot of its
brilliancy, and darts the full intensity of its heat upon all it shines
on to the last. Its disc touches the horizon, and in half an hour dips
half-way beneath it, its intrinsic brightness and colour remaining
unchanged. The brief interval of twilight occurs, as in the morning,
when only a small chord of the disc is visible, and the long shadows now
sharpen as the area of light that casts them decreases. For a while the
zodiacal light vies with the earth-moon high in the heavens in
illuminating the scene; but in a few hours this solar appendage passes
out of view, and our world becomes the queen of the lunar night.

At this sunset time the earth, nearly in the zenith of us, will be at
its half-illuminated phase, and even then it will shed more light than
we receive upon the brightest of moonlight nights. As the night
proceeds, the earth-phase will increase through the gibbous stages until
at midnight it will be “full,” and our orb will be seen in its entire
beauty. It will perform at least one of its twenty-four-hourly rotations
during the time that it appears quite full, and the whole of its surface
features will in that time pass before the lunar spectator’s eye. At
times the northern pole will be turned towards our view, at times the
southern; and its polar ice-caps will appear as bright white spots,
marking its axis of rotation. If our lunar sojourn were prolonged we
should observe the northern ice-cap creep downwards to lower latitudes
(during our winter) and retreat again (during our summer); and this
variation would be perceptible in a less degree at the southern pole, on
account of the watery area surrounding it. The seas would appear (so far
as can be inferred) of pale blue-green tint; the continents
parti-coloured: and the tinted spots would vary with the changing
terrestrial seasons, as these are indicated by the positions and
magnitudes of the polar ice-caps. The permanent markings would be ever
undergoing apparent modification by the variations of the white
cloud-belts that encircle the terrestrial sphere. Of the nature of these
variations meteorological science is not as yet in a position to speak:
it would indeed be vastly to the benefit of that science if a view of
the distribution of clouds and vapours over the earth’s surface, as
comprehensive as that we are imagining, could really be obtained.

It might happen at “full-earth,” that a black spot with a fainter
penumbral fringe would appear on one side of the illuminated disc and
pass somewhat rapidly across it. This would occur when the moon passed
exactly between the sun and the earth, and the shadow of the moon was
cast upon the terrestrial disc. We need hardly say that these
shadow-transits would occur upon those astronomically important
occasions when an eclipse of the sun is beheld from the earth.

The other features of the sky during the long lunar night would not
differ greatly from those to which we alluded in speaking of its day
aspects. The stars would be the more brightly visible, from the greater
power of the eye-pupil to open in the absence of the glaring sun, and on
this account the milky-way would be very conspicuous and the brighter
nebulæ would come into view. The constellations would mark the night by
their positions, or the hours might be told off (in periods of
twenty-four each) by the successive reappearances of surface features on
certain parts of the terrestrial disc. The planets in opposition to the
sun would now be seen, and a comet might appear to vary the monotony of
the long lunar night. But a meteor would never flash across the sky,
though dark meteoric particles and masses would continually bombard the
lunar surface, sometimes singly, sometimes in showers. And these would
fall with a compound force due to their initial velocity added to that
of the moon’s attraction. As there is no atmosphere to consume the
meteors by frictional heat or break by its resistance the velocity of
their descent, they must strike the moon with a force to which that of a
cannon-ball striking a target is feeble indeed. A position on the moon
would be an unenviable stand-point from this cause alone.

The lunar landscape by night needs little description: it would be lit
by the earth-moon sufficiently to allow salient features, even at a
distance, to be easily made out, for its moon (_i.e._ the earth) has
thirteen times the light-reflecting area that our’s has. But the night
illumination will change in intensity, since the earth-moon varies from
half-full to full, and again to half-full, between sunset and the next
sunrise. The direction of the light, and hence the positions of the
shadows, will scarcely alter on account of the apparent fixity of the
earth in the lunar sky. A slight degree of warmth might possibly be felt
with the reflected earth-light; but it would be insufficient to mollify
the intensity of the prevailing cold. The heat accumulated by the ground
during the three hundred hours’ sunshine radiates rapidly into space,
there being no atmospheric coat to retain it, and a cooling process
ensues that goes on till, all warmth having rapidly departed, the
previously parched soil assumes a temperature approaching that of
celestial space itself, and which has been, as we have stated, estimated
at about 200° below the Fahrenheit zero. If moisture existed upon the
moon, its night-side would be bound in a grip of frost to which our
Arctic regions would be comparatively tropical. But since there is no
water, the aspect of the lunar scenery remains unmodified by effects of
changing temperature.

Such, then, are the most prominent effects that would manifest
themselves to the visual and other senses of a being transported to the
moon. The picture is not on the whole a pleasant one, but it is
instructive; and our rendering of it, imperfect though it be, may serve
to suggest other inferences that cannot but add to the interest which
always attaches to the contemplation of natural scenes and phenomena
from points of view different from those which we ordinarily occupy.

Apart from the recondite functions of the moon considered as one of the
interdependent members of the solar family, into which it would be
beyond our purpose to inquire, there are certain means by which it
subserves human interests and ministers to the wants of civilized man to
which we deem it desirable to call attention, especially as some of them
are not so self-apparent as to have attracted popular attention.

The most generally appreciated because the most evident of the uses of
the moon is that of a luminary. Popular regard for it is usually
confined to its service in that character, and in that character poets
and painters have never tired in their efforts to glorify it. And
obviously this service as a “lesser light” is sufficiently prominent to
excite our warmest admiration. But moonlight is, from the very
conditions of its production, of such a changeable and fugitive nature,
and it affords after all so partial and imperfect an alleviation of
night’s darkness, that we are fain to regard the light-giving office of
the moon as one of secondary importance. Far more valuable to mankind in
general, so estimable as to lead us to place it foremost in our category
of lunar offices, is the duty which the moon performs in the character
of a sanitary agent. We can conceive no direful consequences that would
follow from a withdrawal of the moon’s mere light; but it is easy to
imagine what highly dangerous results would ensue if the moon ceased to
produce the tides of the ocean. Motion and activity in the elements of
the terraqueous globe appear to be among the prime conditions in
creation. Rest and stagnation are fraught with mischief. While the sun
keeps the atmosphere in constant and healthy circulation through the
agency of the winds, the moon performs an analogous service to the
waters of the sea and the rivers that flow into them. It is as the chief
producer of the tides—for we must not forget that the sun exercises
_its_ tidal influences, though in much lesser degree—that we ought to
place the highest value on the services of the moon: but for its aid as
a mighty scavenger, our shores, where rivers terminate, would become
stagnant deltas of fatal corruption. Twice (to speak generally) a day,
however, the organic matter which rivers deposit in a decomposing state
at their embouchures is swept away by the tidal wave; and thus, thanks
to the moon, a source of direful pestilence is prevented from arising.
Rivers themselves are providentially cleansed by the same means, where
they are polluted by bordering towns and cities which, from the nature
of things, are sure to arise on river banks; and it seems to be also in
the nature of things that the river traversing a city must become its
main sewer. The foul additions may be carried down by the stream in its
natural course towards the ocean, but where the river is large there
will be a decrease in velocity of the current near the mouth or where it
joins the sea, thus causing partial stagnation and consequent deposition
of the deleterient matters. All this, however, is removed, and its
inconceivable evils are averted by our mighty and ever active “sanitary
commissioner,” the moon. We can scarcely doubt that a healthy influence
of less obvious degree is exerted in the wide ocean itself; but,
considering merely human interests, we cannot suppress the conviction
that man is more widely and immediately benefited by this purifying
office of the moon than by any other.

But the sanitary service is not the only one that the moon performs
through the agency of the tides. There is the work of tidal transport to
be considered. Upon tidal rivers and on certain coasts, notwithstanding
wind and the use of steam, a very large proportion of the heavy
merchandize is transported by that slow but powerful “tug” the
flood-tide; and a similar service, for which, however, the moon is not
to be entirely credited, is done by the down-flow of the ebb-tide. Large
ships and heavily-laden rafts and barges are quietly taken in tow by
this unobtrusive prime mover, and moved from the river’s mouth to the
far-up city, and from wharf to wharf along its banks; and a vast amount
of mechanical work is thus gratuitously performed which, if it had to be
provided by artificial means, would represent an amount of money value
which for such a city as London would have to be counted by thousands,
possibly millions, of pounds yearly. For this service we owe the moon
the gratitude that we ought to feel for a direct pecuniary benefactor.

In the existing state of civilization and prosperity, we do not,
however, utilize the power of the tides nearly to the extent of their
capabilities. Our coal mines, rich with “the light of other days”—for
coal was long ago declared by Stevenson to be “bottled sunshine”—at
present furnish us with so abundant a supply of power-generating
material that in our eagerness to use it upon all possible occasions we
are losing sight, or putting out of mind, many other valuable prime
movers, and amongst them that of the rise and fall of the waters, which
can be immediately converted into any form of mechanical power by the
aid of tide-mills. Such mills may be found in existence here and there,
but for the present they are generally out-rivalled by the steam-engine
with all its conveniences and adaptabilities; and hence they have not
shared the benefits of that inventive ingenuity which has achieved such
wonders of mechanical appliance while steam has been in the ascendant.
But it must be remembered that in our extravagant use of coal we are
drawing from a bank into which nothing is being paid. We are consuming
an exhaustive store, and the time must come when it will be needful to
look around in quest of “powers that may be.” Then an impetus may be
given to the application of the tides to mechanical purposes as a prime
mover.[19] For the people of the British Islands the problem would have
an especial importance, viewing the extent of our seaboard and the
number of our tidal rivers. The source of motion that offers itself is
of almost incalculable extent. There is not merely the onward flowing
motion of streams to be utilized, but also the _lift_ of water, which,
if small in extent, is stupendous in amount; and within certain limits
it matters little to the mechanician whether the “foot-pounds” of work
placed at his disposal are in the form of a great mass lifted to a small
height or a small mass lifted to a great height. There is no reason
either why the utilization of the tides should be confined to rivers.
The sea-side might well become the circle of manufacturing industry, and
the millions of tons of water lifted several feet twice daily on our
shores might be converted, even by schemes already proposed, to furnish
the prime movement of thousands of factories. And we must not forget how
completely modern science has demonstrated the inter-convertibility of
all kinds of force, and thus opened the way for the introduction of
systems of transporting power that, in such a state of things as we are
for the moment considering, might be of immense benefit. Gravity, for
instance, can be converted into electricity; and electricity gives us
that wonderful power of transmitting _force_ without transmitting (or
even moving) _matter_, which power we use in the telegraph, where we
generate a force at one end of a wire and _use_ it to ring bells or
deflect needles at the other end, which may be thousands of miles away.
What we do with the slight amount of force needful for telegraphy is
capable of being done with any greater amount. A tide-mill might convert
its mechanical energy by an electro-magnetic engine, and in the form of
electricity its force could be conveyed inland by proper wires and there
reconverted back to mechanical or moving power. True, there would be a
considerable loss of power, but that power would cost nothing for its
first production. Another means ready to hand for transporting power is
by compressed air, which has already done good service; another is the
system so admirably worked out by Sir W. Armstrong, of transmitting
water-power through the agency of an “accumulator,” now so generally
used at our Docks and elsewhere, for working cranes and such other uses.
And as the whole duty of the engineer is to _convert_ the forces of
nature, there is a rich field open for his invention, and upon which he
may one day have to enter, in adapting the pulling force of the moon to
his fellow man’s mechanical wants through the intermediation of the
tides.

Another of the high functions of the moon is that by which she subserves
the wants of the navigator, and enables him to track his course over the
pathless ocean. Of the two co-ordinates, Latitude and Longitude, that
are needful to determine the position of a ship at sea (or of any
standpoint upon the earth’s surface) the first is easily found, inasmuch
as it is always equal to the altitude of the celestial pole at the place
of observation. But the determination of the longitude has always been a
difficult problem, and one upon which a vast amount of ingenuity has
been expended. When it was first attacked it was soon discovered that
the moon was the object of all others by which it could be most
accurately and, all things considered, most readily determined. We must
premise that the longitude of one place from another is in effect the
difference between the local times at the two places, so that when we
say that a place or a ship is, for instance, seven hours, twenty-four
minutes, ten seconds, west of Greenwich, we mean that the time-o’-day at
the place or ship is seven hours twenty-four minutes ten seconds earlier
than that at Greenwich. Hence, finding the longitude at sea or at any
place and moment means finding what time it is at Greenwich at that
moment. Of course this could be most easily done if we could set a
timekeeper at Greenwich and rely upon its keeping time during a long sea
voyage; and this plan appeared so feasible that our Government long ago
offered a prize of £20,000 for a timekeeper which would perform to a
stated degree of accuracy after a certain sea voyage. One John Harrison
did make such a timekeeper, that actually satisfied the conditions, and
obtained the prize: and chronometers are now largely used for longitude,
their construction having been brought to great perfection, especially
in England, owing to a continuance (in a less liberal degree, however,)
of Government inducement. But chronometers are not entirely to be relied
on, even where several are carried, which in other than Government ships
is rarely the case: recourse must be had to the heavenly bodies for
check upon the timekeeper. And the moon is, as we have said, the body
that best serves the requirements of the problem.

The lunar method for longitude amounts practically to this. The stars
are fixed; the sun, moon, and planets move amongst them; the sun and
planets with very slow rates of apparent motion, the moon with a very
rapid one. If, then, it be predicted that at a certain instant of
Greenwich time the moon will be a certain distance from a fixed star,
and if the mariner at sea observes _when_ the moon has that exact
distance, he will know the Greenwich time at the instant of his
observation.[20] The moon thus becomes to him as the hand of a
timepiece, whereof the stars are the hour and minute marks, the whole
being, as it were, set to Greenwich time. Then if he knows (which he
does by other observations easily obtained) the local time at his ship,
he can take the difference between the Greenwich time and his time,
which difference is in fact his longitude from Greenwich. The requisite
predictions of the distance of the moon from several fixed stars near
her are given to the utmost exactness for every three hours of every day
and night (when the moon can possibly be seen) in the navigator’s _vade
mecum_, the “Nautical Almanac,” and from these given distances the
navigator can, by a simple process of differencing, obtain the distance,
and hence the Greenwich time, for any intermediate instant at which he
may chance to make his observation. Whenever he can see the moon he can
obtain Greenwich time. Of course the whole value of this method depends
upon the exactitude of the predicted distances corresponding to the
given Greenwich times. These distances are obtained by tables of the
moon’s motions, which must be found from observations. The motions in
question are of an intricacy almost past comprehension, on account of
the disturbing forces to which the moon is subjected by the sun and
planets. The powers of the profoundest mathematicians, from Newton
downwards, have been severely exercised in efforts to group them into a
theory, and represent them by tables capable of furnishing the requisite
exact predictions of lunar positions for nautical purposes. Accurate
observations of the moon’s place night after night have, from the dawn
of this lunar method for longitude, been in urgent request by
mathematicians for the purposes specified, and it was solely to procure
these observations that the Observatory at Greenwich was established,
and mainly for their continued prosecution (and for the stellar
observations necessary for their utilization) that it is sustained. For
two centuries the moon has been unremittingly observed at Greenwich, and
the tables at present used for making the “Nautical Almanac” (those
formed by Prof. Hansen) depend upon the observations there obtained. The
work still goes on, for even now the degree of exactitude is not what is
desired, and astronomers are looking forward with some interest to new
lunar tables which were left complete by the late M. Delaunay, formerly
the head of astronomy in France, based upon a theory which he evolved.
This use of the moon is the grandest of all in respect of the results to
which it has led.

Then, too, regarding the moon as a timekeeper, we must not forget the
service that it renders in furnishing a division of time intermediate
between the day—which is measured by the earth’s rotation—and the year,
which is defined by the earth’s orbital revolution. Notwithstanding the
survival of lunar reckoning in our religious services, we, in our time
and country, scarcely need a moon to mark our months; but we must not
forget that with many ancient people the moon was, and with some is
still, the chief timekeeper, the calendars of such people being lunar
ones, and all their events being reckoned and dated by “moons.” To us,
however, the moon is of great service in this department by enabling us
to fix dates to many historical events, the times of occurrence of which
are uncertain, by reason of defective records or by dependence upon such
uncertain data as “lives of emperors,” years of this or that king’s
reign, or generations of one or another family. The moon now and then
clears up a mystery, or decides a disputed point in chronology, by
furnishing the accurate date of an ancient eclipse, which was a
phenomenon that always inspired awe and secured for itself careful
record. The chronologer is continually applying to the astronomer for
the date and place of visibility of some total eclipse, of which he has
found an imperfect record, veritable as to the fact, but dated only by
reference to some year of a so-and-so’s reign, or by some battle or
other historical occurrence. The eclipses that occurred near the time
are then examined, and when one is found that tallies with recorded
conditions in other respects (such as the time of day and the place of
observation), its indisputable date becomes a starting-point from which
the chronologer works backwards and forwards in safety. There is one
famous eclipse—that predicted by Thales six centuries before Christ,
which put an end to the battle between the Medes and Lydians by the
terror its darkness created in both armies—which is most intimately
associated with ancient chronology, and has been used to rectify a
proximate date (the first year of Cyrus of Babylon) which forms the
foundation of all Scripture chronology. Sacred and profane history alike
are continually receiving assistance from the accurate dates which the
moon, by having caused eclipses of the sun, enables the astronomer to
fix beyond cavil or doubt.

The mention of eclipses reminds us, too, of the use which the moon has
been in increasing, through them, our knowledge of the physical
condition of the sun. If the moon had never intervened to cut off the
blinding glare of the solar disc, we should have been to this day left
to assume that the sun is all-contained by the dazzling globe that we
ordinarily see. But, thanks to the moon’s intervention, we now know that
the sun is by no means the mere naked sphere we should have suspected.
Eclipses have taught us that it is surrounded by an envelope of glowing
gases, and that it has a vast vaporous surrounding, beyond its glowing
atmosphere, which appears to be composed of matter streaming away from
the sun into surrounding space. With these discoveries still in their
infancy, it is impossible to foresee the knowledge to which they will
eventually lead, but they can hardly be barren of fruit, and whatever
they ultimately teach will be so much insight gained into the sublimest
problem that human science has before it—the determination of the source
and maintaining power of the light and heat and vivifying agency of the
sun. In according our thankful reflections to the moon for these
revelations, we must not forget that, should there be inhabitants upon
our neighbouring worlds, Mercury, Venus, and Mars, which have no
satellites, they, the supposed inhabitants, can gain no such knowledge
upon the surroundings of the ruler of the solar system. On the other
hand, any rational being who may be supposed to dwell upon Saturn or
Jupiter, would, through the intervention of their numerous moons, have,
in the latter case especially, far more abundant opportunities of
acquiring the knowledge in question than we have.

Finally, there is a use of the moon which touches us, author and reader,
very closely. It has taught us of a world in a condition totally
different from our own; of a planet without water, without air, without
the essentials to life development, but rather with the conditions for
life destruction; a planet left by the Creator—for wise purposes that we
cannot fully know—as it were but half-formed, with all the igneous
foundations fresh from the cosmical fire, and with its rough-cast
surface in its original state, its fire and mould-marks exposed to our
view. From these we have essayed to resolve some of the processes of
formation, and thus to learn something of the cosmical agencies that are
called forth in the purely igneous era of a planet’s history. We trust
that we, on our part, have shown that the study of the moon may be a
benefit not merely to the astronomer, but to the geologist; for we
behold in it a mighty “medal of creation” doubtless formed of the same
material and struck with the same die that moulded our earth; but while
the dust of countless ages and the action of powerful disintegrating and
denuding elements have eroded and obliterated the earthly impression,
the superscriptions on the lunar surface have remained with their
pristine clearness unsullied, every vestige sharp and bright as when it
left the Almighty Maker’s hands. The moon serves no second-rate or
insignificant service when it teaches us of the variety of creative
design in the worlds of our system, and exalts our estimation of this
peopled globe of ours by showing us that all the planetary worlds have
_not_ been deemed worthy to become the habitations of intelligent
beings.

Reflections upon the uses of the moon not unnaturally lead our thoughts
to some matters that may be regarded as abuses. These mainly take the
form of superstitions, erroneous beliefs in the moon’s influence over
terrestrial conditions, and occasionally of erroneous ideas upon the
moon’s functions as a luminary. The first-mentioned are almost beneath
notice, for they include such mythical suspicions as that the moon
influences human sanity and other affections of mind and body; that the
moon’s rays have a decomposing effect upon organic matter; that they
produce blindness by shining upon a sleeper’s eyes; that the moon
determines the hours of human death, which is supposed to occur with the
change of the tide, etc. All such, having no foundation on fact, are put
beyond our consideration. The third matter we have mentioned may also be
dismissed in a very few words. The erroneous ideas upon the moon’s
functions as a luminary, to which we allude, are those which are
manifested by poets and painters, and even historians, who do not
hesitate to bring the moon upon a scene in any form and at any time they
please without reference to actual lunar circumstances. It is no
uncommon thing to see, in a picture representing an evening scene, a
moon introduced which can only be seen in the morning—a waning moon
instead of a waxing one; and astronomical critics have, indeed, caught
artists so far tripping as to put a moon in a picture representing some
event that occurred upon a date when the moon was new, and therefore
invisible. Writers take the same liberties very frequently. A newspaper
correspondent, during the Franco-Prussian war, described the full moon
as shining upon a scene of desolation on a particular night, when really
there was no moon to be seen. One of the most flagrant cases of this
kind, however, occurs in Wolfe’s ballad on “The death of Sir John
Moore,” where it is written that the hero was buried “By the struggling
moonbeam’s misty light.” But the interment actually took place at a time
when the moon was out of sight. We mention these abuses of the moon in
the hope of promoting a better observance of the moon’s luminary office.
They who wish to bring the moon upon a scene, not knowing _ipso facto_
that it was there, should first take the advice of Nick Bottom in the
“Midsummer Night’s Dream,” and make sure of their object by consulting
an almanac.

The second of the specified abuses to which the moon is subject refers
to its supposed influence on the weather; and in the extent to which it
goes this is one of the most deeply rooted of popular errors. That there
is an infinitesimal influence exerted by the moon on our atmosphere will
be seen from the evidence we have to offer, but it is of a character and
extent vastly different from what is commonly believed. The popular
error is shown in its most absurd form when the mere _aspect_ of the
moon, the mere transition from one phase of illumination to another, is
asserted to be productive of a change of weather; as if the gradual
passage from first quarter to second quarter, or from that to third,
could of itself upset an existing condition of the atmosphere; or as if
the conjunction of the moon with the sun could invert the order of the
winds, generate clouds, and pour down rains. A moment’s reasoning ought
to show that the supposed cause and the observed effect have no
necessary connection. In our climate the weather may be said to change
at least every three days, and the moon changes—to retain the popular
term—every seven days; so that the probability of a coincidence of these
changes is very great indeed: when it occurs, the moon is sure to be
credited with causing it. But a theory of this kind is of no use unless
it can be shown to apply in every case; and, moreover, the change must
always be in the same direction: to suppose that the moon can turn a
fine day to a wet one, and a wet day to a fine morrow indiscriminately,
is to make our satellite blow hot and cold with the same mouth, and so
to reduce the supposition to an absurdity. If any marked connection
existed between the state of the air and the aspect of the moon, it must
inevitably have forced itself unsought upon the attention of
meteorologists. In the weekly return of Births, Deaths, and Marriages,
issued by the Registrar-General, a table is given, showing all the
meteorological elements at Greenwich for every day of the year, and a
column is set apart for noting the changes and positions of the moon.
These reports extend backwards nearly a quarter of a century. Here,
then, is a repertory of data that ought to reveal at a glance any such
connection, and would certainly have done so had it existed. But no
constant relation between the moon columns and those containing the
instrument readings has ever been traced. Our meteorological
observatories furnish continuous and unbroken records of atmospheric
variations, extending over long series of years: these afford still more
abundant means for testing the validity of the lunar hypothesis. The
collation has frequently been made for special points in the inquiry,
and certainly _some_ connection has been found to obtain between certain
positions of the moon in her orbit and certain instrumental averages;
but so small are the effects traceable to lunar influence, that they are
almost inappreciable among the grosser irregularities that arise from
other and as yet unexplained causes.

The lunar influences upon our atmosphere most likely to be detected are
those of a tidal character, and those due to the radiation of the heat
which the moon receives from the sun. The first would be shown by the
barometer, which may be called an “atmospheric tide gauge.” Some years
ago Sir Edward Sabine instituted a series of observations at St. Helena,
to determine the variations of barometric indications from hour to hour
of the lunar day. The greatest differences were found to occur between
the times when the moon was on the meridian, and when it was six hours
away from the meridian; in other words, between atmospheric high tide
and low tide. But the average of these differences amounted only to the
four-hundredth part of an inch on the instrument’s scale; a quantity
that no weather observer would heed, that none but the best barometers
would show, and that can have no perceptible effect on weather changes.
The distance of the moon from the earth varies, as is well known, in
consequence of the elliptical form of her orbit: this variation ought
also to produce an effect upon the instrument’s indications; but Colonel
Sabine’s analysis showed that it was next to insensible; the mean
reading at apogee differing from that at perigee by only the
two-thousandth part of an inch. Schubler, a German meteorologist, had
arrived at similarly negative results some years previously. Hence it
appears that the great index of the weather is not sensibly affected by
the state of the moon: the conclusion to be drawn with regard to the
weather itself is obvious enough. As regards the heat received from the
moon, we know, from the recent experiments of Lord Rosse in England, and
Marie Davy in France, elsewhere alluded to, that a degree of warmth
appreciable to the highly sensitive thermopile is exerted by the moon
upon the earth near to the time of full moon, when the sun’s rays have
been pouring their unmitigated heat upon the lunar surface continuously
for fourteen days. And as it is improbable that the whole of the heat
sent earthwards from the moon reaches the earth’s surface, we must infer
that a considerable amount is absorbed in the higher atmosphere, and
does work in evaporating the lighter clouds and thinning the denser
ones. The effect of this upon the earth is to facilitate the radiation
of its heat into space, and so to cool the lower atmospheric strata. And
this effect has been shown to be a veritable one by an exhaustive
tabulation of temperature records from various observatories, which was
undertaken by Mr. Park Harrison. The general conclusion from these was,
that the temperature at the earth’s surface is lower by about 2½ degrees
at moon’s last quarter than at first quarter; the paradoxical result
being what would naturally follow from the foregoing consideration. The
tendency of the full moon to clear the sky has been remarked by several
distinguished authorities, to wit, Sir John Herschel, Humboldt, and
Arago; and in general the clearing may be accepted as a meteorological
fact, though in one case of close examination it has been negatived. It
cannot be doubted that a full moon sometimes shows a night to be clear
that would in the absence of the moon be called cloudy.

When close comparisons are made between the moon’s positions and records
of rain-fall and wind-direction, dim indications of relation exhibit
themselves, which may be the feeble consequences of the change of
temperature just spoken of; but in every case where an effect has been
traced it has been of the most insignificant kind, and no apparent
connexion has been recognized between one effect and another. Certainly
there is nothing that can support the extensive popular belief in lunar
influence on weather, and nothing that can modify the conviction that
this belief as at present maintained is an absurd delusion. Yet its
acceptance is so general, and runs through such varied grades of
society, that we have felt it our duty to dwell upon it to the extent
that we have done.

CHAPTER XV.
CONCLUDING SUMMARY.

Having arrived at the conclusion of our subject, it appears to us
desirable that we should recall to the reader, by a rapid review, its
salient features.

Our main object being to attempt what we conceived to be a rational
explanation of the surface details of the moon which should be in
accordance with the generally received theory of planetary formation,
and with the peculiar physical conditions of the lunar globe—the opening
of our work was a summary of the nebular hypothesis as it was started by
the first Herschel and systemised by Laplace. Following these
philosophers we endeavoured to show how a chaotic mass of primordial
matter existing in space would, under the action of gravitation, become
transformed into a system of planetary bodies circulating about a common
centre of gravity; and further, how, in some cases, the circulating
planetary masses would themselves become sub-centres of satellitic
systems; our earth being one of these sub-centres with only one
satellitic attendant—to wit, the moon, the subject of our study.

The moon being thus considered as evolved from the parent nebulous mass,
and existing as an isolated and compact body, we had next to consider
what was the effect of the continued action of the gravitating force. By
the light of the beautiful “mechanical theory of heat” we argued that
this force, not being _destructible_, but being _convertible_, was
turned into heat; and that whatever may have been the original condition
of the parent nebulous mass, as regards temperature, its planetary
offspring became elevated to an intense degree of heat as they assumed
the form of spheres under the influence of gravitation.

The incandescent sphere having attained its maximum degree of heat by
the total conversion thereinto of the gravitating force it embodied, we
explained how there must have ensued a dispersion of that heat by
radiation into surrounding space, resulting in the cooling and
consequent solidification of the outermost stratum of the lunar sphere,
and subsequently in the continuation of the cooling process downwards or
inwards to the centre. And here we essayed to prove that in this second
stage of the cooling process, when the crust was solid and the subjacent
portion of the molten sphere was about to solidify, there would come
into operation a principle which appears to govern the behaviour of
certain fusible substances, and which may be concisely termed the
principle of pre-solidifying expansion. We adduced several examples of
the manifestation of this principle, soliciting for it the careful
consideration of physicists and geologists, and looking to it as
furnishing the key to the mystery of volcanic action upon the moon,
since, without needing recourse to aqueous or gaseous sources of
eruptive power, it afforded a rationale of the ejection of the fluid and
semifluid matter of the moon through the solidified crust thereof, and
also of the dislocations of that crust, unattended by actual ejection of
subsurface matter, of which our satellite presents a variety of
examples, and which the earth also appears to have experienced at some
period of its formative history.

Arrived at this stage of our subject we thought it needful to introduce
some pages of data and descriptive detail. Accordingly in one chapter we
discussed the form, magnitude, weight, and density of the moon, and the
force of gravity at its surface: and the more soundly to fix these data
in the mind, we devoted a few lines to explanation of the methods
whereby each has been ascertained. We then examined the question (so
important to our subject) of the existence or non-existence of a lunar
atmosphere, giving the evidence, which may be regarded as conclusive, in
proof of the absence of both air and water from the moon, and,
therefore, refuting the claim of these elements to be considered as
sources or influants of the moon’s volcanic manifestations. A general
_coup-d’œil_ of the lunar hemisphere facing the earth next engaged our
attention, and we considered the aspect of the disc as it is viewed by
the naked eye and with telescopes of various powers. From this general
survey we passed to the topography of the moon, tracing briefly the
admirable labours of those who have advanced this subject, and, by aid
of picture and skeleton maps and a table of position co-ordinates,
placing it within the reader’s power to become more than sufficiently
acquainted for the purposes of this work with the names and positions of
detail objects and features of interest. Special descriptions of
interesting and typical spots and regions were given in some few cases
where such appeared to be called for.

These descriptive matters disposed of, we proceeded to discuss the
various classes of surface features with a view to explaining the
precise actions which appear to us to have led to their formation.
Naturally the craters first demanded our attention. We pointed out the
reasons for regarding the great majority of the circular formations of
the moon as craters, as truly volcanic as those of which we have
examples, modified by obvious causes, upon the earth; and, tracing the
causative phenomena of terrestrial volcanoes, we showed how the
explanations which have been offered to account for them scarcely apply
to those of the moon: and thus, driven to other hypotheses, we
endeavoured to demonstrate the probability of the lunar craters having
been produced by eruptive force, generated by that pre-solidifying
expansion of successive portions of the moon’s molten interior, which we
enunciated in our third chapter. The precise course of phenomena which
resulted in the production of a crater of the normal lunar type, with or
without the significant central cone, were then illustrated by a series
of step-by-step diagrams with accompanying descriptive paragraphs. And
after treating of craters of the normal type we pointed out and
explained some variations thereupon that are here and there to be met
with, and likewise those curious complications of arrangement which
exhibit craters superimposed one upon another and intermingled in
strange confusion.

From craters manifestly volcanic we passed to the consideration of those
circular formations which, from their vastness of size, scarcely admit
of satisfactory explanation by a volcanic hypothesis. We summarized
several proffered theories of their origin, and pointed out what we
considered might be a possible key to the solution of the selenological
enigma which they constitute, without, however, expressing ourselves
entirely satisfied with the validity of our suggestion. The less
mysterious features presented by peaks and mountain ranges were then
discussed to the extent that we considered requisite, viewing their
comparatively simple character and the secondary position they occupy in
point of numerical importance upon the moon. At greater length we dealt
with the cracks and chasms and the allied phenomena of radiating
streaks, pointing out with regard to these latter the strikingly
beautiful correspondence in effect (and therefore presumably in cause)
between them and crack-systems of a glass globe “starred” by an
expanding internal medium.

The more notable objects and features of the lunar surface being
disposed of, we had next to say a few words upon some residual
phenomena, chiefly upon the colour of lunar surface details, and upon
their various degrees of brightness or reflective power. And, inasmuch
as varying brightness seemed to us to be related to varying antiquity,
we were thence led to the question of the chronology of selenological
formations, and to the disputation upon the continuance of volcanic
action upon the moon in recent years. We regarded this question from the
observational and the inferential points of view, and were led to the
conclusion that the moon’s surface arrived at its terminal condition
ages ago, and that it is next to hopeless to look for evidence of
existing change.

Thus far our work dealt with the moon as a planetary body merely. It
occurred to us, however, that we might add to the interest attaching to
our satellite were we to regard it for a time as a world, and consider
its conditions as respects fitness for habitation by beings like
ourselves. The arguments against the possibility of the moon being thus
fitted for human creatures, or, indeed, for any high organism, were
decisive enough to require little enforcing. It appeared to us,
nevertheless, that much might be learnt by imagining one’s self located
upon the moon during a period embracing one lunar day (a month of our
reckoning), with power to comprehend the peculiar circumstances and
conditions of such a situation. We therefore attempted a description of
an imaginary sojourn upon the moon, and pointed out some of the more
striking aspects and phenomena which we know by legitimate inference
would be there manifested. We trust, that while our modest efforts in
the chapter referring to this branch of our subject may prove in some
degree entertaining, they may be in a greater degree instructive,
inasmuch as certain facts are brought into prominence which would not
unnaturally be overlooked in contemplating the moon from the earth, the
only _real_ stand-point that is available to us.

In our final chapter we considered the moon as a satellite, and sought
to enhance popular regard for it on account of certain high functions
which it performs for man’s benefit on this earth; but which are in
great risk of being overlooked. We showed that, notwithstanding the
moon’s occasionally useful service as a nocturnal luminary, it fills a
far higher office as a sanitary agent by cleansing the shores of our
seas and rivers through the agency of the tides. We pointed out the vast
amount of absolutely mechanical work and commercial labour which the
same tidal agency executes in transporting merchandize up and down our
rivers—an amount that, to take the port of London alone, represents a
money value _per annum_ that may be reckoned in millions sterling,
seeing that if our river was tideless all transport would have to be
done by manual or steam power. We then hinted at the stupendous
reservoir of power that the tidal waters constitute, a form of power
which has not as yet been sufficiently called into operation, but which
may be invoked by-and-by, when we have begun to feel more acutely the
consequences of our present prodigal use of the fuel that was stored up
for us by bountiful nature ages upon ages ago. The moon’s services to
the navigator, in affording him a ready means of finding his longitude
at sea; to the chronologist and historian, as a timekeeper, counting
periods too vast for accurate reckoning by other means; to the
astronomer and student of nature, in revealing certain wonderful
surroundings of the solar globe, which, but for the phenomena of
eclipses caused by the moon’s interposition, would never have been
suspected to exist—these were other functions that we dwelt upon, all
too briefly for their deserts; and, lastly, we spoke of the moon as a
medal of creation fraught with instructive suggestions, which it has
been our endeavour to bring to notice in the course of this work. And
from uses we passed to abuses, directing attention to a few popular
errors and widespread illusions relating to lunar influence upon and in
connection with things terrestrial. This part of our work might have
been considerably expanded, for, in truth, the moon has been a
misunderstood and misjudged body. Some justice we trust we have done to
her: we have brought her face to the fireside; we have analysed her
features, and told of virtues that few of her admiring beholders
conceived her to possess. We have traced out her history, fraught with
wonderful interest, and doubtless typical of the history of other
spheres that in countless numbers pervade the universe: and now, having
done our best to make all these points familiar, we commend the moon to
still further study and still more intimate acquaintance, confident that
she will repay all attentions, be they addressed to her as

A PLANET, A WORLD, OR A SATELLITE.

Continue Reading

CRACKS AND RADIATING STREAKS

The lunar features next in order of conspicuity are the mountain ranges,
peaks, and hill-chains, a class of eminences more in common with
terrestrial formations than the craters and circular structures that
have engaged our notice in the preceding chapters.

In turning our attention to these features, we are at the outset struck
with the paucity on the lunar surface of extensive mountain systems as
compared with its richness in respect of crateral formations; and a
field of speculation is opened by the recognition of the remarkable
contrast which the moon thus presents to the earth, where mountain
ranges are the rule, and craters like the lunar ones are decidedly
exceptional. Another conspicuous but inexplicable fact is that the most
important ranges upon the moon occur in the northern half of the visible
hemisphere, where the craters are fewest and the comparatively
featureless districts termed “seas” are found. The finest range is that
named after our Apennines and which is included in our illustrative
Plate, No. IX. It extends for about 450 miles and has been estimated to
contain upwards of 3000 peaks, one of which—Mount Huyghens—attains the
altitude of 18,000 feet. The Caucasus is another lunar range which
appears like a diverted northward extension of the Apennines, and,
although a far less imposing group than the last named, contains many
lofty peaks, one of which approaches the altitude assigned to Mount
Huyghens while several others range between 11,000 and 14,000 feet high.
Another considerable range is the Alps, situated between the Caucasus
and the crater Plato, and reproduced on Plate XIV. It contains some 700
peaked mountains and is remarkable for the immense valley, 80 miles long
and about five broad, that cuts it with seemingly artificial
straightness; and that, were it not for the flatness of its bottom,
might set one speculating upon the probability of some extraneous body
having rushed by the moon at an enormous velocity, gouging the surface
tangentially at this point and cutting a channel through the impeding
mass of mountains. There are other mountain ranges of less magnitude
than the foregoing; but those we have specified will suffice to
illustrate our suggestions concerning this class of features.

[Illustration: PLATE XV.
MERCATOR & CAMPANUS.]

We remark, too, that there is a prevailing tendency of the ranges just
mentioned to present their loftiest constituents in abrupt terminal
lines, facing nearly the same direction, the reverse of that towards
which they are carried by the moon’s rotation; and as they recede from
the high terminal line, the mountains gradually fall off in height, so
that in bulk the ranges present the “crag and tail” contour which
individual hills upon the earth so frequently exhibit.

Isolated peaks are found in small numbers upon the moon; there are a few
striking examples of them nevertheless, and these are chiefly situated
in the mountainous region just alluded to. Several are seen to the east
(right hand) of the Alpine range depicted on Plate XIV. The best known
of these is Pico, which rises abruptly from a generally smooth plain to
a height of 7000 feet. It may be recognized as the lower of the two long
shadowing spots located almost centrally above the crater Plato in the
illustration just mentioned. Above it, at an actual distance of 40
miles, there is another peak (unnamed) about 4000 feet high; and away to
the west, beyond the small crater joined by a hill-ridge to Plato, is a
third pyramidal mountain nearly as high as Pico.

It seems natural to regard the great mountain chains as agglomerations
of those peaks of which we have isolated examples in Pico and its
compeers, and thus to consider that the formation of a mountain chain
has been a multiplication of the process that formed the single
pyramid-shaped eminences. At first thought it might appear that the
great mountain ranges were produced by bodily upthrustings of the crust
of the moon by some subsurface convulsions. But such an explanation
could hardly hold in relation to the isolated peaks, for it is
difficult, if not impossible, to conceive that these abrupt mountains,
almost resembling a sugarloaf in steepness, could have been protruded en
masse through a smooth region of the crust. On the contrary it is quite
consistent with probability to suppose that they were built up by a slow
process somewhat analogous to that to which we have ascribed the piling
of the central cones of the great craters. We believe they may be
regarded as true mountains of exudation, produced by the comparatively
gentle oozing of lava from a small orifice and its solidification around
it; the vent however remaining open and the summit or discharging
orifice continually rising with the growth of the mountain, as indicated
in the annexed cut, Fig. 36. This process is well exemplified in the
case of a water fountain playing during a severe frost; the water as it
falls around the lips of the orifice freezes into a hillock of ice,
through the centre of which, however, a vent for the fluid is preserved.
As the water trickles over the mound it is piled higher and higher by
accumulating layers of ice, till at length a massive cone is formed
whose height will be determined by the force or “head” of the water.
Substitute lava for water and we have at once a formative process which
may very fairly be considered as that which has given rise to the
isolated mountains of the moon.

[Illustration: Fig. 36.]

[Illustration: Fig. 37.]

[Illustration: Fig. 38.]

[Illustration: Fig. 39.]

There are upon the earth mountainous forms resembling the isolated peaks
of the moon, and which have been explained by a similar theory to the
above. We reproduce a figure of one observed by Dana at Hawaii (Fig.
37), and a sketch of another observed on the summit of the Volcano of
Bourbon, (Fig. 38); we also reproduce (Fig. 39) an ideal section of the
latter, given by Mr. Scrope, and showing the successive layers of lava
which would be disposed by just such an action as that manifested in the
case of the freezing fountain; and we quote that author’s words in
reference to this explanation of the formation of Etna and other
volcanic mountains. “On examining,” says Mr. Scrope,[11] “the structure
of the mountain (Etna) we find its entire mass, so far as it is exposed
to view by denudation or other causes (and one enormous cavity, the Val
de Bove penetrates deeply into its very heart), to be composed of beds
of lava-rock alternating more or less irregularly with layers of scoriæ,
lapillo and ashes, almost precisely identical in mineral character, as
well as in general disposition, with those erupted by the volcano at
known dates within the historical period. Hence we are fully justified
in believing the whole mountain to have been built up in the course of
ages in a similar manner by repeated intermittent eruptions. And the
argument applies by the rules of analogy to all other volcanic
mountains, though the history of their recent eruptions may not be so
well recorded, provided that their structure corresponds with, and can
be fairly explained by this mode of production. It is also further
applicable, under the same reservation, to all mountains composed
entirely, or for the most part, of volcanic rocks, even though they may
not have been in eruption within our time.”

To these illustrations furnished from Scrope’s work we add another,
copied from a photograph by Professor Piazzi Smyth, of a “blowing cone”
at the base of Teneriffe (Fig. 40), which is but one of many that are to
be found on that mountain and which has been formed by a process similar
to that we have been considering, but acting upon a comparatively small
scale. Professor Smyth describes this cone as about 70 feet high and of
parabolic figure, composed of hard lava and with an upper aperture still
yawning, “whence the burning breath of fires beneath once issued in fury
and with destruction.”

[Illustration: PLATE XVI
TYCHO,
AND ITS SURROUNDINGS.]

Reverting now to the moon, we remark that, if the foregoing explanation
of the isolated lunar peaks be tenable, it should hold equally for the
groups of them which we see in the lunar Apennines, Alps, Caucasus and
other ranges of like character. There occur in some places intermediate
groups which link the one to the other. Just above the crater
Archimedes, on Plate IX., for instance, we see several single peaks and
small clumps of them leading by successive multiple-peak examples to
what may be called chains of mountains like many that are included in
the contiguous Apennine system. And, in view of this connexion between
the single peaks and the mountain ranges formed of aggregations of such
peaks, it seems to us reasonable to conclude that the latter were formed
by the comparatively slow escape of lava through multitudinous openings
in a weak part of the moon’s crust, rather than to suppose that the
crust itself has been bodily upheaved and retained in its disturbed
position. The high peaks that many mountains in such a chain exhibit
accord better with the former than the latter explanation; for it is
difficult to imagine how such lofty eminences could be erected by an
upheaval, and we must remember that the moon has none of the denuding
elements which are at work upon the earth, weather-wearing its mountain
forms into sharpness and steepness.[12]

[Illustration: Fig. 40.
SMALL VOLCANIC MOUNTAIN AT THE END OF A STREET AT TENERIFFE.]

[Illustration: Fig. 41.]

And we have ground for believing the mountain-forming process on the
moon to have been a comparatively gentle one, in the fact that the
mountain systems appear in regions otherwise little disturbed, and where
craters, which have all the appearances of violent origin, are few and
far between. Evidently the mountain and crater-forming processes,
although both due to extrusive action, were in some measure different,
and it is reasonable to suppose that the difference was in degree of
intensity; so that while a violent ejection of volcanic material would
give rise to a crater, a more gradual discharge would pile up a
mountain. In this view craters are evidences of _eruptive_, and
mountains of comparatively gentle _exudative_ action.

We can hardly speculate with any degree of safety upon the cause of this
varying intensity of volcanic discharge. We may ascribe it to variation
of _depth_ of the initial disturbing force, or to suddenness of its
action; or it may be that different degrees of fluidity of the lava have
had modifying effects; or on the other hand different qualities of the
crust-material; or yet again differences of period—the quieter
extrusions having occurred at a time when the volcanic forces were dying
down. There is an alliance between lunar craters and mountains that goes
far to show that there has been no radical difference in their origins.
For instance, as we have previously pointed out, craters in some cases
run in linear groups, as if in those cases they had been formed along a
line of disruption or of least resistance of the crust; and the mountain
chains have a corresponding linear arrangement. Then we see craters and
mountain chains disposed in what seem obviously the same arcs of
disturbance. Thus Copernicus (No. 147), Erastothenes (No. 168), and the
Apennines appear to belong to one continuous line of eruption; and it
requires no great stretch of imagination to suppose that the Caucasus,
Eudoxus (No. 208) and Aristotle (No. 209) form a continuation of the
same line. Then around the Mare Serenetatis we see mountainous ridges
and craters alternating one with the other as though the exuding action
there, normally sufficient to produce the ridges, had at some points
become forcible enough to produce a crater. Again, upon the very
mountain ranges themselves, as for instance among the Apennines, we find
small craters occurring. We see, too, that the great craters are in many
cases surrounded by radiating systems of ridges which almost assume
mountainous proportions, and which are doubtless exuded matter from
“starred” cracks, the centres of which are occupied by the craters. The
same kind of ridges here and there occur apart from craters (see for
instance Plate XVIII., below Aristarchus and Herodotus) and sometimes
they occur in the neighbourhood of extensive cracks, to which they also
seem allied. We must indeed regard a linear crack as the origin either
of a ridge (if the exudation is slight) or of a mountain chain (if the
exudation is more copious) or a string of craters (if the extrusion
rises to eruptive violence). But the subject of cracks is important
enough to be treated in a separate chapter.

We alluded in Chap. III. to the phenomena of wrinkling or puckering as
productive of certain mountainous formations; and we pointed out the
striking similarity in character of configuration between a shrivelled
skin and a terrestrial mountain region. We do not perceive upon the moon
such a decided coincidence of appearances extending over any
considerable portion of her surface; but there are numerous limited
areas where we behold mountainous ridges which partake strongly of the
wrinkle character; and in some cases it is difficult to decide whether
the puckering agency or the exudative agency just discussed has produced
the ridges. The district bordering upon Aristarchus and Herodotus, above
referred to, is of this doubtful character; and a similar district is
that contiguous to Triesnecker (Plate XI.) There are, however, abundant
examples of less prominent lines of elevation, which may, with more
probability, be ascribed to a veritable wrinkling or puckering action;
they are found over nearly the whole lunar surface, some of them
standing out in considerable relief, and some merely showing gentle
lines of elevation, or giving the surface an undulating appearance. A
close examination of our picture-map (Plate IV.) will reveal very
numerous examples, especially in the south-east (right-hand-upper)
quadrant. Some of these lines of tumescence are so slightly prominent
that we may suppose them to have been caused by the action indicated by
Fig. 6 (p. 28), while others, from their greater boldness, appear to
indicate a formative action analogous to that represented by Fig. 9

We have hitherto confined our attention to those reactions of the moon’s
molten interior upon its exterior which have been accompanied by
considerable extrusions of sub-surface material in its molten or
semi-solid condition. We now pass to the consideration of some phenomena
resulting in part from that reaction and in part from other effects of
cooling, which have been accompanied by comparatively little ejection or
upflow of molten matter, and in some cases by none at all. Of such the
most conspicuous examples are those bright streaks that are seen, under
certain conditions of illumination, to radiate in various directions
from single craters, and some of the individual radial branches of which
extend from four to seven hundred miles in a great arc on the moon’s
surface.

There are several prominent examples of these bright streak systems upon
the visible hemisphere of the moon; the focal craters of the most
conspicuous are Tycho, Copernicus, Kepler, Aristarchus, Menelaus, and
Proclus. Generally these focal craters have ramparts and interiors
distinguished by the same peculiar bright or highly reflective material
which shows itself with such remarkable brilliance, especially at full
moon: under other conditions of illumination they are not so strikingly
visible. At or nearly full moon the streaks are seen to traverse over
plains, mountains, craters, and all asperities; holding their way
totally disregardful of every object that happens to lay in their
course.

The most remarkable bright streak system is that diverging from the
great crater Tycho. The streaks that can be easily individualized in
this group number more than one hundred, while the courses of some of
them may be traced through upwards of six hundred miles from their
centre of divergence. Those around Copernicus, although less remarkable
in regard to their extent than those diverging from Tycho, are
nevertheless in many respects well deserving of careful examination:
they are so numerous as utterly to defy attempts to count them, while
their intricate reticulation renders any endeavour to delineate their
arrangement equally hopeless.

The fact that these bright streaks are invariably found diverging from a
crater, impressively indicates a close relationship or community of
origin between the two phenomena: they are obviously the result of one
and the same causative action. It is no less clear that the actuating
cause or prime agency must have been very deep-seated and of enormous
disruptive power to have operated over such vast areas as those through
which many of the streaks extend. With a view to illustrate
experimentally what we conceive to have been the nature of this
actuating cause, we have taken a glass globe and, having filled it with
water and hermetically sealed it, have plunged it into a warm bath: the
enclosed water, expanding at a greater rate than the glass, exerts a
disruptive force on the interior surface of the latter, the consequence
being that at the point of least resistance, the globe is rent by a vast
number of cracks diverging in every direction from the focus of
disruption. The result is such a strikingly similar counterpart of the
diverging bright streak systems which we see proceeding from Tycho and
the other lunar craters before referred to, that it is impossible to
resist the conclusion that the disruptive action which originated them
operated in the same manner as in the case of our experimental
illustration; the disruptive force in the case of the moon being that to
which we have frequently referred as due to the expansion which precedes
the solidification of molten substances of volcanic character.

Our illustration, Plate XIX., is a photograph from one of many glass
globes which we have cracked in the manner described: a careful
comparison between the arrangement of the divergent cracks represented
in the photograph and those seen spreading from Tycho and other lunar
craters will, we trust, justify us in what we have stated as to the
similarity of the causes which have produced such identical results.

The accompanying figures will further illustrate our views upon the
causative origin of the bright streaks. The primary action rent the
solid crust of the moon and produced a system of radiating fissures
(Fig. 42): these immediately afforded egress for the molten matter
beneath to make its appearance on the surface simultaneously along the
entire course of every crack, and irrespective of all surface
inequalities or irregularities whatever (Fig. 43). We conceive that the
upflowing matter spread in both directions sideways and in this manner
produced streaks of very much greater width than the cracks or fissures
up through which it made its way to the surface.

[Illustration: Fig. 42.
ILLUSTRATIVE OF THE RADIATING CRACKS WHICH PRECEDE THE FORMATION OF
THE BRIGHT STREAKS.]

In further elucidation of this part of our subject we may refer to a
familiar but as we conceive cogent illustration of an analogous action
in the behaviour of water beneath the ice of a frozen pond, which, on
being fractured by some concentrated pressure, or by a blow, is well
known to “star” into radiating or diverging cracks, up through which the
water immediately issues, making its appearance on the surface of the
ice simultaneously along the entire course of every crack, and on
reaching the surface, spreading on both sides to a width much exceeding
that of the crack itself.

[Illustration: Fig. 43.
ILLUSTRATIVE OF THE RADIATING BRIGHT STREAKS.]

If this familiar illustration be duly considered, we doubt not it will
be found to throw considerable light on the nature of those actions
which have resulted in the bright streaks on the moon’s surface. Some
have attempted to explain the cause of these bright streaks by assigning
them to streams of lava, issuing from the crater at the centre of their
divergence and flowing over the surface, but we consider such an
explanation totally untenable, as any idea of lava, be it ever so fluid
at its first issue from its source, flowing in streams of nearly equal
width, through courses several hundred miles long, up hills, over
mountains, and across plains, appears to us beyond all rational
probability.

[Illustration: PLATE XVIII.
ARISTARCHUS & HERODOTUS.]

It may be objected to our explanation of the formation of these bright
streaks, that so far as our means of observation avail us, we fail to
detect any shadows from them or from such marginal edges as might be
expected to result from a sideway-spreading outflow of lava from the
cracks which afforded it exit in the manner described. Were the edges of
these streaks terminated by cliff-like or craggy margins of such height
as 30 or 40 feet, we might just be able at low angles of illumination
and under the most favourable circumstances of vision, to detect some
slight appearance of shadows; but so far as we are aware, no such
shadows have been observed. We are led to suppose that the impossibility
of detecting them is due not to their absence but to the height of the
margins being so moderate as not to cast any cognizable shadow, inasmuch
as an abrupt craggy margin of 10 or 15 feet high would, under even the
most favourable circumstances, fail to render such visible to us.
Reference to our ideal section of one of these bright streaks (Fig. 45),
will show how thin their edges may be in relation to their spreading
width.

The absence of cognizable shadows from the bright streaks has led some
observers to conclude that they have no elevation above the surface over
which they traverse, and it has therefore been suggested that their
existence is due to possible vapours which may have issued through the
cracks, and condensed in some sublimated or pulverulent form along their
courses, the condensed vapours in question forming a surface of high
reflective properties. That metallic or mineral substances of some kinds
do deposit on condensation very white powders, or sublimates, we are
quite ready to admit, and such explanation of the high luminosity of the
bright streaks, and of the craters situated at the foci or centres of
their divergence is by no means improbable, so far as concerns their
mere brightness. But as we invariably find a crater occupying the centre
of divergence, and such craters are possessed of all the characteristic
features and details which establish their true volcanic nature as the
results of energetic extrusions of lava and scoria, we cannot resist the
conclusion that the material of the crater, and that of the bright
streaks diverging from it, are not only of a common origin, but are so
far identical that the only difference in the structure of the one as
compared with the other is due to the more copious egress of the
extruded or erupted matter in the case of the crater, while the
restricted outflow or ejection of the matter up through the cracks would
cause its dispersion to be so comparatively gentle as to flood the sides
of the cracks and spread in a thin sheet more or less sideways
simultaneously along their courses. There are indeed evidences in the
wider of the bright streaks of their being the result of the outflow of
lava through _systems of cracks_ running parallel to each other, the
confluence of the lava issuing from which would naturally yield the
appearance of one streak of great width. Some of those diverging from
Tycho are of this class; many other examples might be cited, among which
we may name the wide streaks proceeding from the crater Menelaus and
also those from Proclus. Some of these occupy widths upwards of 25
miles—amply sufficient to admit of many concurrent cracks with confluent
lava outflows.

We are disposed to consider as related to the fore-mentioned radiating
streaks, the numerous, we may say the multitudinous, long and narrow
chasms that have been sometimes called “canals” or “rills,” but which
are so obviously _cracks_ or chasms, that it is desirable that this name
should be applied to them rather than one which may mislead by implying
an aqueous theory of formation. These cracks, singly and in groups, are
found in great numbers in many parts of the moon’s surface. As a few of
the more conspicuous examples which our plates exhibit we may refer to
the remarkable group west of Treisnecker (Plate XI.), the principal
members of which converge to or cross at a small crater, and thus point
to a continuity of causation therewith analogous to the evident relation
between the bright streaks and their focal craters. Less remarkable, but
no less interesting, are those individual examples that appear in the
region north of (below) the Apennines (Plate IX.), and some of which by
their parallelism of direction with the mountain-chain appear to point
to a causative relation also. There is one long specimen, and several
shorter in the immediate neighbourhood of Mercator and Campanus (Plate
XV.); and another curious system of them, presenting suggestive
contortions, occurs in connection with the mountains Aristarchus and
Herodotus (Plate XVIII.). Others, again, appear to be identified with
the radial excrescences about Copernicus (Plate VIII.). Capuanus,
Agrippa, and Gassendi, among other craters, have more or less notable
cracks in their vicinities.

Some of these chasms are conspicuous enough to be seen with moderate
telescopic means, and from this maximum degree of visibility there are
all grades downwards to those that require the highest optical powers
and the best circumstances for their detection. The earlier
selenographers detected but a few of them. Schroeter noted only 11;
Lohrman recorded 75 more; Beer and Maedler added 55 to the list, while
Schmidt of Athens raised the known number to 425, of which he has
published a descriptive catalogue. We take it that this increase of
successive discoveries has been due to the progressive perfection of
telescopes, or, perhaps, to increased education, so to speak, of the
eye, since Schmidt’s telescope is a much smaller instrument than that
used by Beer and Maedler, and is regarded by its owner as an inferior
one for its size. We doubt not that there are hundreds more of these
cracks which more perfect instruments and still sharper eyes will bring
to knowledge in the future.

While these chasms have all lengths from 150 miles (which is about the
extent of those near Treisnecker) down to a few miles, they appear to
have a less variable breadth, since we do not find many that at their
maximum openings exceed two miles across; about a mile or less is their
usual width throughout the greater part of their length, and generally
they taper off to invisibility at their extremities, where they do not
encounter and terminate at a crater or other asperity, which is,
however, sometimes the case. Of their depth we can form no precise
estimate, though from the sharpness of their edges we may conclude that
their sides approach perpendicularity, and, therefore, that their depth
is very great; we have elsewhere suggested ten miles as a possible
profundity. In a few cases, and under very favourable circumstances, we
have observed their generally black interiors to be interrupted here and
there with bright spots suggestive of fragments from the sides of the
cracks having fallen into the opening.

In seeking an explanation of these cracks, two possible causes suggest
themselves. One is the expansion of subsurface matter, already suggested
as explanatory of the bright streaks; the other, a contraction of the
crust by cooling. We doubt not that both causes have been at work, one
perhaps enhancing the other. Where, as in the cases we have pointed out,
there are cracks which are so connected with craters as to imply
relationship, we may conclude that an upheaving or expansive force in
the sublunar molten matter has given rise to the cracks, and that the
central craters have been formed simultaneously, by the release, with
ejective violence, of the matter from its confining crust. The nature of
the expansive force being assumed that of solidifying matter, the wide
extent of some chasms indicates a deep location of that force. And depth
in this matter implies lateness (in the scale of selenological time) of
operation, since the central portions of the globe would be the last to
cool. Now, we have evidence of comparative lateness afforded by the fact
that in many cases the cracks have passed through craters and other
asperities which thus obviously existed before the cracking commenced;
and thus, so far, the hypothesis of the expansion-cracking is supported
by absolute fact.

It may be objected that such an upheaving force as we are invoking,
being transitory, would allow the distended surface to collapse again
when it ceased to operate, and so close the cracks or chasms it
produced. But we consider it not improbable that in some cases, as a
consequence of the expansion of subsurface matter, an upflow thereof may
have partially filled the crack, and by solidifying have held it open;
and it is rational to suppose that there have been various degrees of
filling and even of overflow—that in some cases the rising matter has
not nearly reached the edge of the crack, as in Fig. 44, while in others
it has risen almost to the surface, and in some instances has actually
overrun it and produced some sort of elevation along the line of the
crack, like that represented sectionally in Fig. 45. It is probable that
some of the slightly tumescent lines on the moon’s surface have been
thus produced.

[Illustration: PLATE XIX.
GLASS GLOBE CRACKED BY INTERNAL PRESSURE.]

[Illustration: Fig. 44.]

[Illustration: Fig. 45.]

We have suggested shrinkage as a possible explanation of some cracks. It
could hardly have been the direct cause of those compound ones which are
distinguished by focal craters, though it may have been a co-operative
cause, since the contracting tendency of any area of the crust, by so to
speak weakening it, may have virtually increased the strength of an
upheaving force and thus have aided and localized its action. We see,
however, no reason why the inevitable ultimate contraction which must
have attended the cooling of the moon’s crust, even when all internal
reactions upon it had ceased, should not have created a class of cracks
without accompanying craters, while it would doubtless have a tendency
to increase the length and width of those already existing from any
other cause. Some of the more minute clefts, which presumably exist in
greater numbers than we yet know of, may doubtless be ascribed to this
effect of cooling contraction. In this view we should have to regard
such cracks as the latest of all lunar features. Whether the agency that
produced them is still at work—whether the cracks are on the increase—is
a question impossible of solution: for reasons to be presently adduced,
we incline to believe that all cosmical heat passed from the moon, and
therefore that it arrived at its present, and apparently final,
condition ages upon ages ago.

Besides the ridges spoken of on p. 140, and regarded as cracks up
through which matter has been extruded, there are numerous ridges of
greater or less extent, which we conceive are of the nature of wrinkles,
and have been produced by tangential compression due to the collapse of
the moon’s crust upon the shrunken interior, as explained and
illustrated in Chap. III. The distinguishing feature of the two classes
of phenomena we consider to be the presence of a serrated summit in
those of the extruded class, while those produced by “wrinkling” action
have their summits comparatively free from serration or marked
irregularity.

Speaking generally, the details of the lunar surface seem to us to be
devoid of colour. To the naked eye of ordinary sensitiveness the moon
appears to possess a silvery whiteness: more critical judges of colour
would describe it as presenting a yellowish tinge. Sir John Herschel,
during his sojourn at the Cape of Good Hope, had frequent opportunities
of comparing the moon’s lustre with that of the weathered sandstone
surface of Table Mountain, when the moon was setting behind it, and both
were illuminated under the same direction of sunlight; and he remarked
that the moon was at such times “scarcely distinguishable from the rock
in apparent contact with it.” Although his observations had reference
chiefly to brightness, it can hardly be doubted that similarity of
colour is also implied; for any difference in the tint of the two
objects would have precluded the use of the words “scarcely
distinguishable;” a difference of colour interfering with a comparison
of lustre in such an observation, though it must be remembered that he
observed through a dense stratum of atmosphere. Viewed in the telescope,
the same general yellowish-white colour prevails over all the moon, with
a few exceptions offered by the so-called seas. The _Mare Crisium_,
_Mare Serenetatis_, and _Mare Humorum_ have somewhat of a greenish tint;
the _Palus Somnii_ and the circular area of Lichtenberg incline to
ruddiness. These tints are, however, extremely faint, and it has been
suggested by Arago that they may be mere effects of contrast rather than
actual colouration of the surface material. This, however, can hardly be
the case, since all the “seas” are not alike affected; those that are
slightly coloured are, as we have said, some green and some red, and
contrast could scarcely produce such variations. The supposition of
vegetation covering these great flats and giving them a local colour is
in our view still more untenable, in the face of the arguments that we
shall presently adduce against the possibility of vegetable life
existing upon the moon.

It appears to us more rational to consider the tints due to actual
colour of the material (presumably lava or some once fluid mineral
substance) that has covered these areas; and it may well be conceived
that the variety of tint is due to different characters of material, or
even various conditions of the same material coming from different
depths below the lunar surface; and we may reasonably suppose that the
same variously-coloured substances occur in the rougher regions of the
lunar surface, but that they exist there in patches too small to be
recognized by us, or are “put out” by the brightness to which polyhedral
reflexion gives rise.

Seeing that volcanic action has had so large a share in giving to the
moon’s surface its structural character, analogy of the most legitimate
order justifies us in concluding not only that the materials of that
surface are of kindred nature to those of the unquestionably volcanic
portions of the earth, but also that the tints and colours that
characterize terrestrial volcanic and Plutonian products have their
counterparts on the moon. Those who have seen the interior and
surroundings of a terrestrial volcano after a recent eruption, and
before atmospheric agents have exercised their dimming influences, must
have been struck with the colours of the erupted materials themselves
and the varied brilliant tints conferred on these materials by the
sublimated vapours of metals and mineral substances which have been
deposited upon them. If, then, analogy is any guide in enabling us to
infer the appearance of the invisible from that which we know to be of
kindred nature and which we have seen, we may justly conclude that were
the moon brought sufficiently near to us to exhibit the minute
characteristics of its surface, we should behold the same bright and
varied colours in and around its craters that we behold in and about
those of the earth; and in all probability the coloured materials of
lunar volcanoes would be more fresh and vivid than those of the earth by
reason of the absence of those atmospheric elements which tend so
rapidly to impair the brightness of coloured surfaces exposed to their
influence.

Situated as we are, however, as regards distance from the moon, we have
no chance of perceiving these local colours in their smaller masses; but
it is by no means improbable, as we have suggested, that the faint tints
exhibited by the great plains are due to broad expanses of coloured
volcanic material.

But if we fail to perceive diversity of colour upon the lunar surface,
we are in a very different position in regard to diversity of brightness
or variable light-reflective power of different districts and details.
This will be tolerably obvious to those casual observers who have
remarked nothing more of the moon’s physiography than the resemblance to
a somewhat lugubrious human countenance which the full moon exhibits,
and which is due to the accidental disposition of certain large and
small areas of surface material which have less of the light-reflecting
property than other portions; for since all parts seen by a terrestrial
observer may be said to be equally shone upon by the sun, it is clear
that apparently bright and shaded parts must be produced by differences
in the nature of the surface as regards power of reflecting the light
received.

When we turn to the telescope and survey the full disc of the moon with
even a very moderate amount of optical aid, the meagre impression as to
variety of degree of brightness which the unassisted eye conveys is
vastly extended and enhanced, for the surface is seen to be diversified
by shades of brilliancy and dullness from almost glittering white to
sombre grey: and this variety of shading is rendered much more striking
by shielding the eye with a dusky glass from the excessive glare, which
drowns the details in a flood of light. Under these circumstances the
varieties of light and shade become almost bewildering, and defy the
power of brush or pencil to reproduce them.

We may, however, realize an imperfect idea of this characteristic of the
lunar surface by reference to the self-drawn portrait of the full moon
upon Plate III. This is, in fact, a photograph taken from the full moon
itself, and enlarged sufficiently to render conspicuous the spots and
large and small regions that are strikingly bright in comparison with
what may in this place be described as the “ground” of the disc. As an
example of a wide and irregularly extensive district of highly
reflective material, the region of which Tycho is the central object, is
very remarkable. We may refer also to the bright “splashes” of which
Copernicus and Kepler are the centres. So brilliant are these spots that
they can easily be detected by the unassisted eye about the time of full
moon. Still brighter but less conspicuous by its size is the crater
Aristarchus, which shines with specular brightness, and almost induces
the belief that its interior is composed of some vitreous-surfaced
matter: the highly reflective nature of this object has often caused it
to become conspicuous when in the dark hemisphere of the moon,
unilluminated by the sun, and lighted only by the light reflected from
the earth. At these times it appears so bright that it has been taken
for a volcano in actual eruption, and no small amount of popular
misconception at one time arose therefrom concerning the conditions of
the moon as respects existing volcanic activity—a misconception that
still clings to the minds of many.

The parts of the surface distinguished by deficiency of reflecting power
are conspicuous enough. We may cite, however, as an example of a detail
portion especially remarkable for its dingy aspect, the interior of the
crater Plato, which is one of the darkest spots (the darkest well
defined one) upon the hemisphere of the moon visible to us. For
facilitating reference to shades of luminosity, Schroeter and Lohrman
assorted the variously reflective parts into 10 grades, commencing with
the darkest. Grades 1 to 3 comprised the various deep greys; 4 and 5 the
light greys; 6 and 7 white; and 8 to 10 brilliant white. The spots
Grimaldi and Riccioli came under class 1 of this notation; Plato between
1 and 2. The “seas” generally ranged from 2 to 3; the brightest
mountainous portions mostly between degrees 4 and 6; the crater walls
and the bright streaks came between these and the bright peaks, which
fell under the 9th grade. The maximum brightness, the 10th grade, is
instanced only in the ease of Aristarchus and a point in Werner, though
Proclus nearly approaches it, as do many bright spots, chiefly the sites
of minute craters, which make their appearance at the time of full moon.

In photographic pictures produced by the moon of itself, there is always
an apparent exaggeration in the relation of light to dark portions of
the disc. The dusky parts look, upon the photograph, much darker than to
the eye directed to the moon itself, whether assisted or not by optical
appliances. It may be that the real cause of this discrepancy is that
the eye fails to discover the actual difference upon the moon itself,
being insensible to the higher degrees of brightness or not estimating
them at their proper brilliance with respect to parts less bright. On
the other hand, it is probable that the enhanced contrast in the
photograph is due to some peculiar condition of the darker surface
matter affecting its power of reflecting the actinic constituent of the
rays that fall upon it.

The study of the varying brightness or reflective power of different
regions and spots of the lunar disc leads us to the consideration of the
relative antiquity of the surface features; for it is hardly possible to
regard these variations attentively without being impressed with the
conviction that they have relation to some chronological order of
formation. We cannot, in the first place, resist the conviction that the
brightest features were the latest formed; this strikes us as evident on
_primâ facie_ grounds; but it becomes more clearly so when we remark
that the bright formations, as a rule, overlie the duller features. The
elevated parts of the crust are brighter than the “seas” and other
areas; and it is pretty clear that the former are newer than the latter,
upon which they appear to be super-imposed, or through which they seem
to have extruded.[13] The vast dusky plains are in every instance more
or less sprinkled with spots and minute craters, and these last were
obviously formed after the area that contains them. One is almost
disposed to place the order of formations in the order of relative
brightness, and so consider the dingiest parts the oldest and the
brightest spots and craters the newest features, though, in the absence
of an atmosphere competent to impair the reflective power of the surface
materials, we are unable to justify this classification by suggesting a
cause for such a deterioration by time as the hypothesis pre-supposes.

As we have entered upon the question of relative age of the lunar
features, we may remark that there are evidences of various epochs of
formation of particular classes of details, irrespective of their
condition in respect of brightness, or, as we may say, freshness of
material. As a rule, the large craters are older than the small ones.
This is proved by the fact that a large object of this class is never
seen to interfere with or overlap a small one. Those of nearly equal
size are, however, seen to overlap one another as though several
eruptions of equal intensity had occurred from the same source at
different points. This is strikingly instanced in the group of craters
situated in the position 35-141 on our map, the order of formation of
each of which is clearly apparent. The region about Tycho offers an
inexhaustible field for study of these phenomena of overlapping or
interpolating craters, and it will be found, with very few exceptions,
that the smaller crater is the impinging or parasitical one, and must
therefore have been formed after the larger, upon which it intrudes or
impinges. There are frequent cases in which a large crater has had its
rampart interrupted by a lesser one, and this again has been broken into
by one still smaller; and instances may be found where a fourth crater
smaller than all has intruded itself upon the previous intruder. The
general tendency of these examples is to show that the craters
diminished in size as the moon’s volcanic energy subsided: that the
largest were produced in the throes of its early violence, and that the
smallest are the results of expiring efforts possibly impeded through
the deep-seatedness of the ejective source.

Another general fact of this chronological order is that the mountain
chains are never seen to intrude upon formations of the crater order. We
do not anywhere find that a mountain chain runs absolutely into or
through a crater; but, on the other hand, we do find that craters have
formed on mountain chains. This leads unmistakably to the inference that
the craters were not formed _before_ their allied mountain chains; and
we might assume therefore that the mountains generally are the older
formations, but that there is nothing to prove that the two classes of
features, where they intermingle, as in the Apennines and Caucasus, were
not erupted cotemporaneously.

[Illustration: PLATE XX.
OVERLAPPING CRATERS.]

Upon the assumption that the latest ejected or extruded matter is that
which is brightest, we should place the bright streaks among the more
recent features. Be this as it may, it is tolerably certain that the
cracks, whose apparently close relation to the radiating streaks we have
endeavoured to point out, are relatively of a very late formative
period. We are indeed disposed to consider them as the most recent
features of all: the evidence in support of this consideration being the
fact that they are sometimes found intersecting small craters that, from
the way in which they are cut through by the cracks, must have been _in
situ_ before the cracking agency came into operation. It is in
accordance with our hypothesis of the moon’s transition from a fluid to
a solid body to consider that a cracking of the surface would be the
latest of all the phenomena produced by contraction in final cooling.

The foregoing remarks naturally lead us to the question whether changes
are still going on upon the surface of our satellite: whether there is
still left in it a spark of its volcanic activity, or whether that
activity has become totally extinct. We shall consider this question
from the observational and theoretical point of view. First as regards
observations. This much may be affirmed indisputably—that no object or
detail visible to the earliest selenographers (whose period may be dated
200 years back) has altered from the date of their maps to the present.
When we pass from the bolder features to the more minute details we find
ourselves at a loss for materials for forming an inference; the only map
pretending to accuracy even of the larger among small objects being that
of Beer and Maedler, which, truly admirable as it is, is not very safely
to be relied upon for settling any question of alleged change, on
account of the conventional system adopted for exhibiting the forms of
objects, every object being mapped rather than drawn, and shown as it
never is or can be presented to view on the moon itself. This difficulty
would present itself if a question of change were ever raised upon the
evidence of Beer and Maedler’s map: it may indeed have prevented such a
question being raised, for certainly no one has hitherto been bold
enough to assert that any portion or detail of the map fails to
represent the actual state of the moon at the present time.

In default of published maps, we are thrown for evidence on this
question upon observations and recollections of individual observers
whose familiarity with the lunar details extends over lengthy periods.
Speaking for ourselves, and upon the strength of close scrutinies
continued with assiduity through the past thirty years, we may say that
we have never had the suspicion suggested to our eye of any actual
change whatever having taken place in any feature or minute detail of
the lunar surface; and our scrutinies have throughout been made with
ample optical means, mostly with a 20-inch reflector. This experience
has made us not unnaturally in some slight degree sceptical concerning
the changes alleged to have been detected by others. Those asserted by
Schroeter and Gruithuisen were long ago rejected by Beer and Maedler,
who explained them, where the accuracy of the observer was not
questioned, by variations of illumination, a cause of illusory change
which is not always sufficiently taken into account. A notable instance
of this deception occurred a few years ago in the case of the minute
bright crater _Linné_, which was for a considerable period declared,
upon the strength of observations of very promiscuous character, to be
varying in form and dimensions almost daily, but the alleged constant
changes of which have since been tacitly regarded as due to varying
circumstances of illumination induced by combinations of libratory
effects with the ordinary changes depending upon the direction of the
sun’s rays as due to the age of the moon. This explanation does not,
however, dispose of the question whether the crater under notice
suffered any actual change before the hue and cry was raised concerning
it. Attention was first directed to it by Schmidt, of Athens, whose
powers of observation are known to be remarkable, and whose labours upon
the moon are of such extent and minuteness as to claim for his
assertions the most respectful consideration.[14] He affirmed in 1866
that the crater at that date presented an appearance decidedly different
from that which it had had since 1841: that whereas it had been from the
earlier epoch always easily seen as a very deep crater, in October 1866
and thenceforward it presented only a white spot, with at most but a
very shallow aperture, very difficult to be detected. Schmidt is one of
the very few observers whose long familiarity with the moon entitles him
to speak with confidence upon such a question as that before us upon the
sole strength of his own experience; and this case is but an isolated
one, at least it is the only one he has brought forward. He is, however,
still firmly convinced that it is an instance of actual change, and not
an illusion resulting from some peculiar condition of illumination of
the object. It should be added also on this side of the discussion that
an English observer, the Rev. T. W. Webb, while apparently indisposed to
concede the supposition of any notable changes in the lunar features,
has yet found from his own observations that, after all due allowance
for differences of light and shade upon objects at different times,
there is still a “residuum of minute variations not thus disposed of”
which seem to indicate that eruptive action in the moon has not yet
entirely died out, though its manifestation at present is very limited
in extent. It appears to us that, if evidence of continuing volcanic
action is to be sought on the moon, the place to look for it is around
the circumference of the disc, where eruption from any marginal orifice
would manifest itself in the form of a protruding haziness, somewhat as
illustrated to an exaggerated extent in the annexed cut.

[Illustration: Fig. 46.]

The theoretical view of the question, which we have now to consider, has
led us, however, to the strong belief that no vestige of its former
volcanic activity lingers in the moon—that it assumed its final
condition an inconceivable number of ages ago, and that the high
interest which would attach to the close scrutiny of our satellite if it
_were_ still the theatre of volcanic reactions cannot be hoped for. If
it be just and allowable to assume that the earth and the moon were
condensed into planetary form at nearly the same epoch (and the only
rational scheme of cosmogony justifies the assumption) then we may
institute a comparison between the condition of the two bodies as
respects their volcanic age, using the one as a basis for inference
concerning the state of the other. We have reason to believe that the
earth’s crust has nearly assumed its final state so far as volcanic
reactions of its interior upon its exterior are concerned: we may affirm
that within the historical period no igneous convulsions of any
considerable magnitude have occurred; and we may consider that the
volcanoes now active over the surface of the globe represent the last
expiring efforts of its eruptive force. Now in the earth we perceive
several conditions wherefrom we may infer that it parted with its
cosmical heat (and therefore with its prime source of volcanic agency)
at a rate which will appear relatively very slow when we come to compare
the like conditions in the moon. We may, we think, take for granted that
the surface of a planetary body generally determines its _heat
dispersing_ power, while its volume determines its _heat retaining_
power. Given two spherical bodies of similar material but of unequal
magnitude and originally possessing the same degree of heat, the smaller
body will cool more rapidly than the larger, by reason of the greater
proportion which the surface of the smaller sphere bears to its volume
than that of the larger sphere to its volume—this proportion depending
upon the geometrical ratio which the surfaces of spheres bear to their
volumes, the contents of spheres being as the _cubes_ and the surfaces
as the _squares_ of their diameters. The volume of the earth is 49 times
as great as that of the moon, but its surface is only 13 times as great;
there is consequently in the earth a power of retaining its cosmical
heat nearly four times as great as in the case of the moon; in other
words, the moon and earth being supposed at one time to have had an
equally high temperature, the moon would cool down to a given low
temperature in about one fourth the time that the earth would require to
cool to the same temperature. But the earth’s cosmical heat has without
doubt been considerably conserved by its vaporous atmosphere, and still
more by the ocean in its antecedent vaporous form. Yet notwithstanding
all this, the earth’s surface has nearly assumed its final condition so
far as volcanic agencies are concerned: it has so far cooled as to be
subject to no considerable distortions or disruptions of its surface.
What then must be the state of the moon, which, from its small volume
and large proportionate area, parted with its heat at the above
comparatively rapid rate? The matter of the moon is, too, less dense
than the earth, and hence doubtless from this cause disposed to more
rapid cooling; and it has no atmosphere or vaporous envelope to retard
its radiating heat. We are driven thus to the conclusion that the moon’s
loss of cosmical heat must have been so rapid as to have allowed its
surface to assume its final conformation ages on ages ago, and hence
that it is unreasonable and hopeless to look for evidence of change of
any volcanic character still going on.

We conceive it possible, however, that minute changes of a non-volcanic
character may be proceeding in the moon, arising from the violent
alternations of temperature to which the surface is exposed during a
lunar day and night. The sun, as we know, pours down its heat
unintermittingly for a period of fully 300 hours upon the lunar surface,
and the experimental investigations of Lord Rosse, essentially confirmed
by those of the French observer, Marie Davy, show that under this
powerful insolation the surface becomes heated to a degree which is
estimated at about 500° of Fahrenheit’s scale, the fusing point of tin
or bismuth. This heat, however, is entirely radiated away during the
equally long lunar night, and, as Sir John Herschel surmised, the
surface probably cools down again to a temperature as low as that of
interstellar space: this has been assumed as representing the absolute
zero of temperature, which has been calculated from experiments to be
250° below the zero of Fahrenheit’s scale. Now such a severe range of
heat and cold can hardly be without effect upon some of the component
materials of the lunar surface.[15] If there be any such materials as
the vitreous lavas that are found about our volcanoes, such as obsidian
for instance, they are doubtless cracked and shivered by these extreme
transitions of temperature; and this comparatively rapid succession of
changes continued through long ages would, we may suppose, result in a
disintegration of some parts of the surface and at length somewhat
modify the selenographic contour. It is, however, possible that the
surface matter is mainly composed of more crystalline and porous lavas,
and these might withstand the fierce extremes like the “fire-brick” of
mundane manufacture, to which in molecular structure they may be
considered comparable. Lavas as a rule are (upon the earth) of this
unvitreous nature, and if they are of like constitution on the moon,
there will be little reason to suspect changes from the cause we are
considering. Where, however, the material, whatever its nature, is piled
in more or less detached masses, there will doubtless be a grating and
fracturing at the points of contact of one mass with another, produced
by alternate expansions and contractions of the entire masses, which in
the long run of ages must bring about dislocations or dislodgments of
matter that might considerably affect the surface features from a close
point of view, but which can hardly be of sufficient magnitude to be
detected by a terrestrial observer whose best aids to vision give him no
perception of minute configurations. And it must always be borne in mind
that changes can only be _proved_ by reference to previous observations
and delineations of unquestionable accuracy.

Speaking by our own lights, from our own experience and reasoning, we
are disposed to conclude that in all visible aspects the lunar surface
is unchangeable, that in fact it arrived at its terminal condition
_eons_ of ages ago, and that in the survey of its wonderful features,
even in the smallest details, we are presented with the sight of objects
of such transcendent antiquity as to render the oldest geological
features of the earth modern by comparison.

Continue Reading

PEAKS AND MOUNTAIN RANGES

As we stated in our brief general description of the visible hemisphere
of the moon, and as a cursory glance at our map and plates will have
shown, the predominant features of the lunar surface are the circular or
amphitheatrical formations that, by their number, and from their almost
unnatural uniformity of design, induced the belief among early observers
that they must have been of artificial origin. In proceeding now to
examine the details of our subject with more minuteness than before,
these annular formations claim the first share of our attention.

By general acceptation the term “crater” has been used to represent
nearly all the circular hollows that we observe upon the moon; and
without doubt the word in its literal sense, as indicating a _cup_ or
circular cavity, is so far aptly applied. But among geologists it has
been employed in a more special sense to define the hollowing out that
is found at the summit of some extinct, and the majority of active,
volcanoes. In this special sense it may be used by the student of the
lunar surface, though in some, and indeed in the majority of cases, the
lunar crater differs materially in its form with respect to its
surroundings from those on the earth; for while, as we have said, the
terrestrial crater is generally a hollow on a mountain top with its flat
bottom high above the level of the surrounding country, those upon the
moon have their lowest points depressed more or less deeply below the
general surface of the moon, the external height being frequently only a
half or one-third of the internal depth. Yet are the lunar craters truly
volcanic; as Sir John Herschel has said, they offer the true volcanic
character _in its highest perfection_. We have upon the earth some few
instances in which the geological conditions which have determined the
surface-formation have been identical with those that have obtained upon
the moon; and as a result we have some terrestrial volcanic districts
that, could we view them under the same circumstances, would be
identical in character with what we see by telescopic aid upon our
satellite. The most remarkable case of this similarity is offered by a
certain tract of the volcanic area about Naples, known from classic
times as the _Campi Phlegræi_, or burning fields, a name given to them
in early days, either because they showed traces of ancient earth-fire,
or because there were attached to the localities traditions concerning
hot-springs and sulphurous exhalations, if not of actual fiery
eruptions. The resemblance of which we are speaking is here so close
that Professor Phillips, in his work on Vesuvius, which by the way
contains a historical description of the district in question, calls the
moon a grand Phlegreian field. How closely the ancient craters of this
famous spot resemble the generality of those upon the moon may be judged
from Plate VI., in which representations of two areas, terrestrial and
lunar, of the same extent, are exhibited side by side, the terrestrial
region being the volcanic neighbourhood of Naples, and the lunar a
portion of the surface about the crater Theophilus.

In comparing these volcanic circles together, we are however brought
face to face with a striking difference that exists between the lunar
and terrestrial craters. This is the difference of magnitude. None of
those Plutonian amphitheatres included in the terrestrial area depicted
exceed a mile in diameter, and few larger volcanic vents than these are
known upon the earth. Yet when we turn to the moon, and measure some of
the larger craters there, we are astonished to find them ranging from an
almost invisible minuteness to 74 miles in diameter. The same
disproportion exists between the depths of the two classes of craters.
To give an idea of relative dimensions, we would refer to our
illustration of Copernicus[8] and its hundreds of comparatively minute
surrounding craters. Our terrestrial Vesuvius would be represented by
one of these last, which upon the plate measures about the twentieth of
an inch in diameter! And this disproportion strikes us the more forcibly
when we consider that the lunar globe has an area only one-thirteenth of
that of the earth. In view of this great apparent discrepancy it is not
surprising that many should have been incredulous as to the true
volcanic character of the lunar mountains, and have preferred to
designate them by some “non-committal” term, as an American geologist
(Professor Dana) has expressed it. But there is a feature in the
majority of the ring-mountains that, as we conceive, demonstrates
completely the fact of volcanic force having been in full action, and
that seems to stamp the volcanic character upon the crater-forms. This
special feature is the central cone, so well known as a characteristic
of terrestrial volcanoes, accepted as the result of the last expiring
effort of the eruptive force, and formed by the deposit, immediately
around the volcanic orifice, of matter which there was not force enough
to project to a greater distance. Upon the moon we have the central cone
in small craters comparable to those on the earth, and we have it in
progressively larger examples, upon all scales, up to craters of 74
miles in diameter, as we have shown in Plate VII. Where, then, can we
draw the line? Where can we say the parallel action to that which placed
Vesuvius in or near the centre of the arc of Somma, or the cone figured
in our sectional drawing of Vesuvius (Fig. 3) in the middle of its
present crater—where can we say that the action in question ceased to
manifest itself on the moon, seeing that there is no break in the
continuity of the crater-and-cone system upon the moon anywhere between
craters of 1¾ miles and 74 miles in diameter? We have, it is true, many
examples of coneless craters, but these are of all sizes, down to the
smallest, and up to a largeness that _would_ almost seem to render
untenable the ejective explanation: of these we shall specially speak in
turn, but for the present we will confine ourselves to the normal class
of lunar craters, those that have central cones, and that are in all
reasonable probability truly volcanic.

[Illustration: Fig. 16.]

And in the first place let us take a passing glance at the probable
formative process of a terrestrial volcano. Rejecting the hypothesis of
Von Buch, which geologists have on the whole found to be untenable, and
which ascribes the formation of all mountains to the elevation of the
earth’s crust by some thrusting power beneath, we are led to regard a
volcano as a pyramid of ejected matter, thrown out of and around an
orifice in the external solid shell of the earth by commotions
engendered in its molten nucleus. What is the precise nature and source
of the ejective force geologists have not perfectly agreed upon, but we
may conceive that highly expanded vapour, in all probability steam, is
its primary cause. The escaping aperture may have been a weak place
since the foundations of the earth were laid, or it may have been formed
by a local expansion of the nucleus in the act of cooling, upon the
principle enunciated in our Third Chapter; or, again, the expansile
vapour may have forced its own way through that point of the confining
shell that offered it the least resistance. The vent once formed, the
building of the volcanic mountain commenced by the out-belching of the
lava, ashes, and scoria, and the dispersion of these around the vent at
distances depending upon the energy with which they were projected. As
the action continued, the ejected matter would accumulate in the form of
a mound, through the centre of which communication would be maintained
with the source of the ejected materials and the seat of the explosive
agency. The height to which the pile would rise must depend upon several
conditions: upon the steady sustenance of the matter, and upon the form
and weight of the component masses, which will determine the slope of
the mountain’s sides. Supposing the action to subside gradually, the
tapering form will be continued upwards by the comparatively gentle
deposition of material around the orifice, and a perfect cone will
result of some such form as that represented below, which is the outline
ascribed by Professor Phillips to Vesuvius in pre-historic, or even
pre-traditional times, and which may be seen in its full integrity in
the cases of Etna, Teneriffe, Fussi-Yamma, the great volcanic mountain
of Japan, and many others. The earliest recorded form of Vesuvius is
that of a truncated cone represented in Fig. 17, which shows its
condition, according to Strabo, in the century preceding the Christian
Era.

[Illustration: PLATE VII
DIAGRAM OF LUNAR CRATERS FORMING A SERIES RANGING FROM 1¾ MILES TO
78 MILES DIAMETER. ALL CONTAINING CENTRAL CONES.]

[Illustration: Fig. 17.]

[Illustration: Fig. 18.]

Now this form may have been assumed under two conditions. If, as
Phillips has surmised, the mountain originally had a peaked summit with
but a small crater-orifice at the point, then we must ascribe its
decapitation to a subsequent eruption which in its violence carried away
the upper portion, either suddenly, or through a comparatively slow
process of grinding away or widening out of the sides of the orifice by
the chafing or fluxing action of the out-going materials. But it is
probable that the mountain never had the perfect summit indicated in our
first outline. The violent outburst that caused the great crater-opening
of our second figure may have been but one paroxysmal phase of the
eruption that built the mountain: a sudden cessation of the eruptive
force when at its greatest intensity, and when the orifice was at its
widest, would leave matters in an opposite condition to that suggested
as the result of a slow dying out of the action: instead of the peak we
should have a wide crater-mouth. It is of small consequence for our
present purpose whether the crater was contemporaneous with the
primitive formation of the mountain, or whether it was formed centuries
afterwards by the blowing away of the mountain’s head; for upon the vast
scale of geological time, intervals such as those between successive
paroxysms of the same eruption, and those between successive eruptions,
are scarcely to be discriminated, even though the first be days and the
second centuries. We may remark that the widening of a crater by a
subsequent and probably more powerful eruption than that which
originally produced it is well established. We have only to glance at
the sketch, Fig. 18, of the outline of Vesuvius as it appeared between
the years A.D. 79 and 1631 to see how the old crater was enlarged by the
terrible Pompeian eruption of the first-mentioned year. Here we have a
crater ground and blown away till its original diameter of a mile and
three-quarters has been increased to nearly three miles. Scrope had no
hesitation in expressing his conviction that the external rings, such as
those of Santorin, St. Jago, St Helena, the Cirque of Teneriffe, the
Curral of Madeira, the cliff range that surrounds the island of Bourbon,
and others of similar form and structure, however wide the area they
enclose, are truly the “basal wrecks” of volcanic mountains that have
been blown into the air each by some eruption of peculiar paroxysmal
violence and persistence; and that the circular or elliptical basins
which they wholly or in part surround are in all cases true craters of
eruption.

When the violent outburst that produces a great crater in a volcanic
mountain-top more or less completely subsides, the funnel or escaping
orifice becomes choked with débris. Still the vent strives to keep
itself open, and now and then gives out a small delivery of cindery
matter, which, being piled around the vent, after the manner of its
great prototype, forms the inner cone. This last may in its turn bear an
open crater upon its summit, and a still smaller cone may form within
_it_. As the action further dies away, the molten lava, no longer
seething and boiling, and spirting forth with the rest of the ejected
matter, wells upwards slowly, and cooling rapidly as it comes in contact
with the atmosphere, solidifies and forms a flat bottom or floor to the
crater.

[Illustration: Fig. 19.]

It may happen that a subsequent eruption from the original vent will be
comparable in violence to the original one, and then the inner cone
assumes a magnitude that renders it the principal feature of the
mountain, and reduces the old crater to a secondary object. This has
been the case with Vesuvius. During the eruption of 1631 the great cone
which we now call Vesuvius was thrown up, and the ancient crater now
distinguished as Monte Somma became a subsidiary portion of the whole
mountain. Then the appearance was that shown in Fig. 19, and which does
not differ greatly from that presented in the present day. The summit of
the Vesuvian cone, however, has been variously altered; it has been
blown away, leaving a large crateral hollow, and it has rebuilt itself
nearly upon its former model.

When we transfer our attention to the volcanoes of the moon, we find
ourselves not quite so well favoured with means for studying the process
of their formation; for the sight of the building up of a volcanic
mountain such as man has been permitted to behold upon the earth has not
been allowed to an observer of the moon. The volcanic activity,
enfeebled though it now be, of which we are witnesses from time to time
on the earth, has altogether ceased upon our satellite, and left us only
its effects as a clue to the means by which they were produced. If we in
our time could have seen the actual throwing up of a lunar crater, our
task of description would have been simple; as it is we are compelled to
infer the constructive action from scrutiny of the finished structure.

We can scarcely doubt that where a lunar crater bears general
resemblance to a terrestrial crater, the process of formation has been
nearly the same in the one case as in the other. Where variations
present themselves they may reasonably be ascribed to the difference of
conditions pertaining to the two spheres. The greatest dissimilarity is
in the point of dimensions; the projection of materials to 20 or more
miles distance from a volcanic vent appears almost incredible, until we
realize the full effect of the conditions which upon the moon are so
favourable to the dispersive action of an eruptive force. In the first
place, the force of gravity upon our satellite is only one-sixth of that
to which bodies are subject upon the earth. Secondly, by reason of the
small magnitude of the moon and its proportionally much larger surface
in ratio to its magnitude, the rate at which it parted with its cosmical
heat must have been much more rapid than in the case of the earth,
especially when enhanced by the absence of the heat-conserving power of
an atmosphere of air or water vapour; and the disruptive and eruptive
action and energy may be assumed to be greater in proportion to the more
rapid rate of cooling; operating, too, as eruptive action would on
matter so much reduced in weight as it is on the surface of the moon, we
thus find in combination conditions most favourable to the display of
volcanic action in the highest degree of violence. Moreover, as the
ejected material in its passage from the centre of discharge had not to
encounter any atmospheric resistance, it was left free to continue the
primary impulse of its ejection without other than gravitative
diminution, and thus to deposit itself at distances from its source
vastly greater than those of which we have examples on the earth.

We can of course only conjecture the source or nature of the moon’s
volcanic force. If geologists have had difficulty in assigning an origin
to the power that threw up our earthly volcanoes, into whose craters
they can penetrate, whose processes they can watch, and whose material
they can analyze, how vastly more difficult must be the inquiry into the
primary source of the power that has been at work upon the moon, which
cannot be virtually approached by the eye within a distance of six or
eight hundred miles, and the material of which we cannot handle to see
if it be compacted by heat, or distended by vapours. Steam is the agent
to which geologists have been accustomed to look for explanation of
terrestrial volcanoes; the contact of water with the molten nucleus of
our globe is accepted as a probable means whereby volcanic commotions
are set up and ejective action is generated. But we are debarred from
referring to steam as an element of lunar geology, by reason of the
absence of water from the lunar globe. We might suppose that a small
proportion of water once existed; but a small proportion would not
account for the immense display of volcanic action which the whole
surface exhibits. If we admitted a Neptunian origin to the disturbances
of the moon’s crust, we should be compelled to suppose that water had
existed nearly in as great quantity, area for area, there as upon our
globe; but this we cannot reasonably do.

[Illustration: PLATE VIII.
COPERNICUS.]

Aqueous vapour being denied us, we must look in other directions for an
ejective force. Of the nature of the lunar materials we can know
nothing, and we might therefore assume anything; some have had recourse
to the supposition of expansive vapours given off by some volatile
component of the said material while in a state of fusion, or generated
by chemical combinations. Professor Dana refers to sulphur as probably
an important element in the moon’s geology, suggesting this substance
because of the part which it appears to play in the volcanic or igneous
operations of our globe, and on account of its presence in cosmical
meteors that have come within range of our analysis. Any matter
sublimated by heat in the substrata of the moon would be condensed upon
reaching the cold surrounding space, and would be deposited in a state
of fine powder, or otherwise in a solid form. Maedler has attributed the
highly reflective portions of some parts of the surface, such as the
bright streams that radiate from some of the craters, Copernicus and
Tycho for instance, to the vitrification of the surface matter by
gaseous currents. But in suppositions like these we must remember that
the probability of truth diminishes as the free ground for speculation
widens. It does not appear clear how expansive vapours could have lain
dormant till the moon assumed a solid crust, as all such would doubtless
make their escape before any shell was formed, and at an epoch when
there was ample facility for their expansion.

While we are not insensible of the value of an expansive vapour
explanation, if it could be based on anything beyond mere conjecture, we
are disposed to attach greater weight to that afforded by the principle
sketched in our third chapter, viz., of expansion upon solidification.
We gave, as we think, ample proof that molten matter of volcanic nature,
when about passing to the solid state, increases its bulk to a
considerable degree, and we suggested that the lunar globe at one period
of its history must have been, what our earth is now, a solid shell
encompassing a molten nucleus; and further, that this last, in
approaching its solid condition, expanded and burst open or rent its
confining crust. At first sight it may seem that we are ascribing too
great a degree of energy to the expansive force which molten substances
exhibit in passing to the solid condition, seeing that in general such
forces are slow and gradual in their action; but this anomaly disappears
when we consider the vast bulk of the so expanding matter, and the
comparatively small amount that in its expansion it had to displace. It
is true that there are individual mountains on the moon covering many
square miles of surface, that as much as a thousand cubic miles of
material may have been thrown up at a single eruption; but what is this
compared to the entire bulk of the moon itself? A grain of mustard-seed
upon a globe three feet in diameter represents the scale of the loftiest
of terrestrial mountains; a similar grain upon a globe one foot in
diameter, would indicate the proportion of the largest upon the moon. A
model of our satellite with the elevations to scale would show nothing
more than a little roughness, or superficial blistering. Turn for a
moment to our map (Plate IV.), upon which the shadows give information
as to the heights of the various irregularities, and suppose it to
represent the actual size of some sphere whose surface has been broken
up by reactions of some kind of the interior upon the exterior—suppose
it to have been a globe of fragile material filled with some viscous
substance, and that this has expanded, cracked its shell, oozed out in
the process of solidification, and solidified: the irregularity of
surface which the small sphere, roughened by the out-leaking matter,
would present, would not be less than that exhibited in the map under
notice. When we say that a lunar crater has a diameter of 30 miles, we
raise astonishment that such a structure could result from an eruption
by the expansive force of solidifying matter; but when we reflect that
this diameter is less than the two-hundredth part of the circumference
of the moon, we need have no difficulty in regarding the upheaval as the
result of a force slight in comparison to the bulk of the material
giving rise to it. We have upon the moon evidence of volcanic eruptions
being the final result of most extensive dislocations of surface, such
as could only be produced by some widely diffused uplifting force. We
allude to the frequent occurrence of chains of craters lying in a nearly
straight line, and of craters situated at the converging point of
visible lines of surface disturbance. Our map will exhibit many examples
of both cases. An examination of the upper portion (the southern
hemisphere of the moon) will reveal abundant instances of the linear
arrangement, three, four, five or even more crateral circles will be
found to lie with their centres upon the same great-circle track,
proving almost undoubtedly a connexion between them so far as the
original disturbing force which produced them is concerned. Again, in
the craters Tycho (30), Copernicus (147), Kepler (146), and Proclus
(162), we see instances of the situation of a volcanic outburst at an
obvious focus of disturbance. These manifest an up-thrusting force
covering a large sub-surface area, and escaping at the point of least
resistance. Such an extent of action almost precludes the gaseous
explanation, but it is compatible with the expansion on consolidation
theory, since it is reasonable to suppose that in the process of
consolidation the viscous nucleus would manifest its increase of bulk
over considerable areas, disturbing the superimposed crust either in one
long crack, out of the wider opening parts of which the expanded
material would find its escape, or “starring” it with numerous cracks,
from the converging point of which the confined matter would be ejected
in greatest abundance and, if ejected there with great energy and
violence, would result in the formation of a volcanic crater.

The actual process by which a lunar crater would be formed would differ
from that pertaining to a terrestrial crater only to the extent of the
different conditions of the two globes. We can scarcely accept Scrope’s
term “basal wrecks” (of volcanic mountains that have had the summits
blown away) as applicable to the craters of the moon, for the reason
that the lunar globe does not offer us any instance of a mountain
comparable in extent to the great craters and whose summit has _not_
been blown away. Scrope’s definition implies a double, or divided
process of formation: first the building up of a vast conical hill and
then the decapitation and “evisceration” of it at some later period.
There are grounds for this inferred double action among the terrestrial
volcanoes, since both the perfect cone and its summitless counterpart
are numerously exemplified. But upon the moon we have no perfect cone of
great size, we have no exception whereby the rule can be proved. It is
against probability, supposing every lunar crater to have once been a
mountain, that in every case the mountain’s summit should have been
blown away; and we are therefore compelled to consider that the moon’s
volcanic craters were formed by one continuous outburst, and that their
“evisceration” was a part of the original formative process. We do not,
however, include the central cone in this consideration: that may be
reasonably ascribed to a secondary action or perhaps, better, to a
weaker or modified phase of the original and only eruption.

[Illustration: Fig. 20.]

[Illustration: Fig. 21.]

[Illustration: PLATE IX.
THE LUNAR APENNINES, ARCHIMEDES &c., &c.]

Under these circumstances we conceive the upcasting and excavating of a
normal lunar crater to have been primarily caused by a local
manifestation of the force of expansion upon solidification of the
subsurface matter of the moon, resulting in the creation of a mere
“star” or crack in and through the outermost and solid crust. As we
shall have to rely upon diagrams to explain the more complicated
features, we give one of this elementary stage also as a commencement of
the series; and Fig. 20 therefore represents a probable section of the
lunar surface at a point which was subsequently the location of a
crater. From the vent thus formed we conceive the pent-up matter to have
found its escape, not necessarily at a single outburst, but in all
probability in a paroxysmal manner, as volcanic action manifests itself
on our globe. The first outflow of molten material would probably
produce no more than a mere hill or tumescence as shewn sectionally in
Fig. 21; and if the ejective force were small this might increase to the
magnitude of a mountain by an exudative process to be alluded to
hereafter. But if the ejective force were violent, either at the moment
of the first outburst or at any subsequent paroxysm, an action
represented in Fig. 22 would result: the unsupported edges or lips of
the vent-hole would be blown and ground or fluxed away, and a
funnel-formed cavity would be produced, the ejected matter (so much of
it as in falling was not caught by the funnel) being deposited around
the hollow and forming an embryo circular mountain. The continuance of
this action would be accompanied by an enlargement of the conical cavity
or crater, not only by the outward rush of the violently discharged
material, but also by the “sweating” or grinding action of such of it as
in descending fell within the hollow. And at the same time that the
crater enlarged the rampart would extend its circumference, for it would
be formed of such material as did not fall back again into the crater.
Upon this view of the crater-forming process we base the sketch, Fig.
23, of the probable section of a lunar crater at one period of its
development.

[Illustration: Fig. 22.]

So long as each succeeding paroxysm was greater than its predecessor,
this excavating of the hollow and widening of its mouth and mound would
be extended. But when a weaker outburst came, or when the energy of the
last eruption died away, a process of slow piling up of matter close
around the vent would ensue. It is obvious that when the ejective force
could no longer exert itself to a great distance it must merely have
lifted its burden to the relieving vent and dropped it in the immediate
neighbourhood. Even if the force were considerable, the effect, so long
as it was insufficient to throw the ejecta beyond the rim of the crater,
would be to pile material in the lowermost part of the cavity; for what
was not cast over the edge would roll or flow down the inner slope and
accumulate at the bottom. And as the eruption died away, it would add
little by little to the heap, each expiring effort leaving the out-given
matter nearer the orifice, and thus building up the central cone that is
so conspicuous a feature in terrestrial volcanoes, and which is also a
marked one in a very large proportion of the craters of the moon. This
formation of the cone is pictorially described by Fig. 24.

[Illustration: Fig. 23.]

[Illustration: Fig. 24.]

In the volcanoes of the earth we observe another action either
concurrent with or immediately subsequent to the erection or formation
of the cone: this is the outflow or the welling forth of fluid lava,
which in cooling forms the well-known plateau. We have this feature
copiously represented upon the moon and it is presumable that it has in
general been produced in a manner analogous to its counterparts upon the
earth. We may conceive that the fluid matter was either spirted forth
with the solid or semisolid constituents of the cone, in which case it
would drain down and fill the bottom of the crater; or we may suppose
that it issued from the summit of the cone and ran down its sides, or
that, as we see upon the earth, it found its escape before reaching the
apex, by forcing its way through the basal parts. These actions are
indicated hypothetically for the moon in Fig. 25; and the parallel
phenomena for the earth are shewn by the actual case (represented in
Fig. 26 and on Plate I.) of Vesuvius as it was seen by one of the
authors in 1865, when the principal cone was vomiting forth ashes,
stones, and red-hot lava, while a vent at the side emitted very fluid
lava which was settling down and forming the plateau.

[Illustration: Fig. 25.]

Although we cannot, obviously, see upon the moon evidence of a cone
actually overtopped by the rising lake of lava, yet it is not
unreasonable to suppose that such a condition of things actually
occurred in many of those instances in which we observe craters without
central cones, but with plateaux so smooth as to indicate previous
fluidity or viscosity. From the state of things exhibited in Fig. 25 the
transition to that shewn in Fig. 27 is easily, and to our view
reasonably, conceivable. We are in a manner led up to this idea by a
review of the various heights of central cones above their surrounding
plateaux. For instance, in such examples as Tycho or Theophilus, we have
cones high above the lava floor; in Copernicus, Arzachael and Alphonsus
they are comparatively lower; the lava in these and some other craters
does not appear to have risen so high; while in Aristotle and Eudoxus
among others, we have only traces of cones, and it is supposable that in
these cases the lava rose so high as nearly to overtop the central
cones. Why should it not have risen so far as to overtop and therefore
conceal some cones entirely? We offer this as at least a feasible
explanation of some coneless craters: it is not necessary to suppose
that it applies to all such, however: there may have been many craters,
the formation of which ceased so abruptly that no cone was produced,
though the welling forth of lava occurred from the vent, which may have
been left fully open, as in Fig. 28, or so far choked as to stay the
egress of solid ejecta and yet allow the fluid material to ooze upwards
through it, and so form a lake of molten lava which on consolidation
became the plateau. As most of the examples of coneless craters exhibit
on careful examination minute craters on the surface of the otherwise
smooth plateaux, we may suppose that such minute craters are evidences
of the upflow of lava which resulted in the plateaux.

[Illustration: PLATE X.
ARISTOTLE & EUDOXUS.]

[Illustration: Fig. 26.]

[Illustration: Fig. 27.]

[Illustration: Fig. 28.]

We have strong evidence in support of this up-flow of lava offered by
the case of the crater Wargentin, (No. 26, 57·5—140·2) situated near the
south-east border of the disc, and of which we give a special plate.
(Plate XVII.) It appears to be really a crater in which the lava has
risen almost to the point of overflowing, for the plateau is nearly
level with the edge of the rampart. This edge appears to have been
higher on one side than the other, for on the portion nearest the centre
of the visible disc we may, under favourable circumstances, detect a
segment of the basin’s brim rising above the smooth plateau as indicated
in our illustration. Upon the opposite side there is no such feature
visible, the plateau forms a sharp table-like edge. It is just possible
that an actual overflow of lava took place at this part of the crater,
but from the unfavourable situation of this remarkable object it is
impossible to decide the point by observation. There is no other crater
upon the visible hemisphere of the moon that exhibits this filled-up
condition; but, unique as it is, it is sufficient to justify our
conclusion that the plateau-forming action upon the moon has been a
flowing-up of fluid matter from below subsequent to the formation of the
crater-rampart, and not, as a casual glance at the great smooth-bottom
craters might lead us to suspect, a result of some sort of diluvial
deposit which has filled hollows and cavities and so brought up an even
surface. The elevated basin of Wargentin could not have been filled thus
while the surrounding craters with ramparts equally or less high
remained empty: its contained matter must have been supplied from
within, we must conjecture by the upflow of lava from the orifice which
gave forth the material to form the crateral rampart in the first
instance. We are free to conjecture that at some period of this
table-mountain’s formation it was a crater with a central cone, and that
the rising lava over-topped this last feature in the manner shewn by the
above figure (Fig. 29).

[Illustration: Fig. 29.]

The question occurs whether other craters may not have been similarly
filled and have emptied themselves by the bursting of the wall under the
pressure of the accumulated lake of lava within. We know that this
breaching is a common phenomenon in the volcanoes of our globe; the
district of Auvergne furnishing us with many examples; and there are
some suspicious instances upon the moon. Copernicus exhibits signs of
such disruption, as also does the smaller crater intruding upon the
great circle of Gassendi. (See Frontispiece.) But the existence of such
discharging breaches implies the outpouring of a body of fluid or
semi-fluid material, comparable in cubical content to the capacity of
the crater, and of this we ought to see traces or evidence in the form
of a bulky or extensive lava stream issuing from the breach. But
although there are faint indications of once viscous material lying in
the direction that escaping fluid would take, we do not find anything of
the extent that we should expect from the mass of matter that would
constitute _a craterfull_. It is true that if the escaping fluid had
been very limpid it might have spread over a large area and have formed
a stratum too thin to be detected. Such a degree of limpidity as would
be required to fulfil this condition we are hardly, however, justified
in assuming.

To return to the subject of central cones. Although there are cases in
which the simple condition of a single cone exists, yet in the majority
we see that the cone-forming process has been divided or interrupted,
the consequence being the production of a group of conical hills instead
of a single one. Copernicus offers an example of this character, six,
some observers say seven, separate points of light, indicating as many
peaks tipped with sunshine, having been seen when the greater part of
the crater has been buried in shadow. Erastothenes, Bulialdus,
Maurolicus, Petavius, Langreen, and Gassendi, are a few among many
instances of craters possessing more than a central single cone. This
multiplication of peaks upon the moon doubtless arose from similar
causes to those which produce the same feature in terrestrial volcanoes.
Our sketch of Vesuvius in 1865 (Fig. 26) shows the double cone and the
probable source of the secondary one in the diverted channel of the
out-coming material. A very slight interruption in the first instance
would suffice to divert the stream and form another centre of action, or
a choking of the original vent would compel the issuing matter to find a
less resisting thoroughfare into open space, and the process of
cone-building would be continued from the new orifice, perhaps to be
again interrupted after a time and again driven in another direction. In
this manner, by repeated arrests and diversions of the ejecta, cone has
grown upon the side of cone, till, ere the force has entirely spent
itself, a cluster of peaks has been produced. It may have been that this
action has taken place after the formation of the plateau, in the manner
indicated by Fig. 30; a spasmodic outburst of comparatively slight
violence having sought relief from the original vent, and the flowing
matter, finding the one orifice not sufficiently open to let it pass,
having forced other exit through the plateau.

[Illustration: PLATE XI.
TRIESNECKER.]

[Illustration: Fig. 30.]

In frequent instances we observe the state of things represented in Fig.
31, in which the plateau is studded with few or many small craters. This
is the case with Plato, with Arzachael, Hipparchus, Clavius (which
contains about 15 small internal craters), and many others. It is
probable that these subsidiary craters were produced by an after-action
like that which has produced duplicated cones, but in which the
secondary eruption has been of somewhat violent character, for it may
almost be regarded as an axiom that violent eruptions excavate craters
and weak ones pile up cones. In the cases referred to it seems
reasonable to suppose that the main vent has been the channel for an
up-cast of material, but that at some depth below the surface this
material met with some obstruction or cause of diversion, and that it
took a course which brought it out far away from the cone upon the floor
of the plateau. It might even be carried so far as to be upon the
rampart, and it is no uncommon thing to see small craters in such a
situation, though when they appear at such a distance from the primary
vent, it seems more reasonable to suppose that they do not belong to it
but have arisen from a subsequent and an independent action.

[Illustration: Fig. 31.]

We find scarcely an instance of a small crater occurring just in the
centre of a large one, or taking the place of the cone. This is a
curious circumstance. Whenever we have any central feature in a great
crater, that feature is a cone. The tendency of this fact is to prove
that cones were produced by very weak efforts of this expiring force,
for had there been any strength in the last paroxysm it is presumable
that it would have blown out and left a crater. No very violent
eruptions have therefore taken place from the vents that were connected
with the great craters of the moon, nothing more powerful than could
produce a cone of exudation or a cinder-heap. And with regard to cones,
it is noteworthy that whether they be single or multiple, they never
rise so high as the circular ramparts of their respective craters. This
supports the inferred connexion between the crater origin and the cone
origin, for supposing the two to have been independent, a supposition
untenable in view of the universality of the central position of the
cone, it is scarcely conceivable that the mountains should have always
been located within ramparts higher than themselves. The less height
argues less power in the upcasting agency, and the diminished force may
well be considered as that which would almost of necessity precede the
expiration of the eruption.

Occasionally a crater is met with that has a double rampart, and the
concentricity suggests that there have been two eruptions from the same
vent: one powerful, which formed the exterior circle, and a second
rather less powerful which has formed the interior circle. It is not,
however evident that this duplication of the ring has always been due to
a double eruption. In many cases there is duplication of only a portion:
a terrace exhibits itself around a part of the circular range, sometimes
upon the outside and sometimes upon the inside. These terraces are not
likely to have been formed by any freak of the eruption, and we are led
to ascribe them in general to landslip phenomena. When, in the course of
a volcano’s formation, the piling-up of material about the vent has
continued till the lower portions have been unable to support the upper,
or when from any cause, the material composing the pile has lost its
cohesiveness, the natural consequence has been a breaking away of a
portion of the structure and its precipitation down the inclined sides
of the crater. Vast segments of many of the lunar mountain-rings appear
to have been thus dislodged from their original sites and cast down the
flanks to form crescent ranges of volcanic rocks either within or
without the circle. Nearly every one of our plates contains craters
exhibiting this feature in more or less extensive degree. Sometimes the
separated portion has been very small in proportion to the circumference
of the crater: Plato is an instance in which a comparatively small mass
has been detached. In other cases very large segments have slid down and
lie in segmental masses on the plateaux or form terraces around the
rampart. Aristarchus, Treisnecker and Copernicus exhibit this larger
extent of dislocation.

It is possible that these landslips occurred long after the formation of
the craters that have been subject to them. They are probably
attributable to recent disintegration of the lunar rocks, and we have a
powerful cause for this in the alternations of temperature to which the
lunar crust is exposed. We shall have occasion to revert to this subject
by-and-bye; at present it must suffice to point out that the extremes of
cold and heat, between which the lunar soil varies, are, with reasonable
probability, assumed to be on the one hand the temperature of space
(which is supposed to be about 200° below zero), and, on the other hand,
a degree of heat equal to about twice that of boiling water. A range of
at least 500° must work great changes in such heterogeneous materials as
we may conjecture those of the lunar crust to be, by the alternate
contractions and expansions which it must engender, and which must tend
to enlarge existing fissures and create new ones, to grind contiguous
surfaces and to dislodge unstable masses. This cause of change, it is to
be remarked, is one which is still exerting itself.

In a few cases we have an entirely opposite interruption of the
uniformity of a crater’s contour. Instead of the breaking away of the
ring in segments we see the entire circuit marked with deep ruts that
run down the flanks in a radial direction, giving us evidence of a
downward _streaming_ of semi-fluid matter, instead of a disruption of
solid masses. We cannot doubt that these ruts have been formed by lava
currents, and they indicate a condition of ejected material different
from that which existed in the cases where the landslip character is
found. In these last the ejecta appears to have been in the form of
masses of solidified or rapidly solidifying matter, which remained where
deposited for a time and then gave way from overloading or loss of
cohesiveness, whereas the substances thrown out in the case of the
rutted banks were probably mixed solid and fluid, the former remaining
upon the flanks while the latter trickled away. Nothing so well
represents, upon a small scale, this radial channelling as a heap of
wetted sand left for a while for the water to drain off from it. The
solid grains in such a heap sustain its general mass-form, but the
liquid in passing away cuts the surface into fissures running from the
summit to the base, and forms it into a model of a volcanic mountain
with every feature of peak, crag, and chasm reproduced, This similarity
of effect leads us to suspect a parallelism of cause, and thus to the
inference that the material which originally formed such a
crater-mountain as Aristillus (which is a most prominent example of this
rutted character, and appears in Plate IX., side by side with a crater
that has its banks segmentally broken), must have been of the compound
nature indicated; and that an action analogous to that which ruts a damp
sand-heap, rutted also the banks of the lunar crater.

[Illustration: PLATE XII.
THEOPHILUS CYRILLUS & CATHARINA.
SUNSET ASPECT.]

Before passing from the subject of craters it behoves us to say a few
words upon the curious manner in which these formations are complicated
by intermingling and superposition. Yet, upon this point, we may be
brief, for in the way of description our plates speak more forcibly than
is possible by words. In particular we would refer to Plate XII., which
represents the conspicuous group of craters of which the three largest
members have been respectively named Theophilus, Cyrillus, and
Catherina. But the area included in this plate is by no means an
extraordinary one; there are regions about Tycho wherein the craters so
crowd and elbow each other that, in their intricate combinations, they
almost defy accurate depiction. Our map and Plate XVI. will serve to
give some idea of them. This intermingling of craters obviously shows
that all the lunar volcanoes were not simultaneously produced, but that
after one had been formed, an eruption occurred in its immediate
neighbourhood and blew a portion of it away; or it may have been that
the same deep-seated vent at different times gave forth discharges of
material the courses of which were more or less diverted on their way to
the surface.

We have before alluded to the frequent occurrence of lines of craters
upon the moon. In these lines the overlapping is frequently visible; it
is seen in Plate XII. before referred to, where the ring mountains are
linked into a chain slightly curved, and upon the map, Plate IV., the
nearly central craters Ptolemy and Alphonsus, the latter of which
overlaps the former, are seen to form part of a line of craters marking
a connection of primary disturbance. An extensive crack suggests itself
as a favourable cause for the production of this overlaying of craters,
for it would serve as a sort of “line of fire” from various points at
which eruptions would burst forth, sometimes weak or far apart, when the
result would be lines of isolated craters, and sometimes near together,
or powerful, when the consequence would be the intrusion of one upon the
other, and the perfect production of the latest formed at the expense or
to the detriment of those that had been formed previously. The linear
grouping of volcanoes upon the earth long ago struck observant minds.
The fable of the _Typhon_ lying under Sicily and the Phlegreian fields
and disturbing the earth by its writhings, is a mythological attempt to
explain the particular case in that region.

The capricious manner in which these intrusions occur is very curious.
Very commonly a small crater appears upon the very rampart of a greater
one, and a more diminutive one still will appear upon the rampart of the
parasite. Stoeffler presents us with one example of this character,
Hipparchus with another, Maurolycus with a third, and these are but a
few cases of many. Here and there we observe several craters ranged in a
line with their rims in one direction all perfect, and the whole
appearing like a row of coins that have fallen from a heap. There is an
example near to Tycho which we reproduce in Plate XX. In this case one
is led to conjecture that the ejective agency, after exerting itself in
one spot, travelled onward and renewed itself for a time; that it ceased
after forming crater number two, and again journeyed forward in the same
line, recommencing action some miles further, and again subsiding; yet
again pushing forward and repeating its outburst, till it produced the
fourth crater, when its power became expended. In each of these
successive eruptions the centre of discharge has been just outside the
crater last formed; and the close connexion of the members of the group,
together with the fact of their nearly similar size, appears to indicate
a community of origin. For it seems feasible that as a general rule the
size of a crater may be taken as a measure of the depth of force that
gave rise to the eruption producing it. This may not be true for
particular cases, but it will hold where a great number are collectively
considered; for if we assume the existence of an average disturbing
force, it is apparently clear that it will manifest itself in disturbing
greater or less surface-areas in proportion as it acts from greater or
less depths. Or, _mutatis mutandis_, if we assume an uniform depth for
the source of action, the greater or less surface disturbance will be a
measure of greater or less eruptive intensity.

Perhaps the most remarkable case of a vast number of craters, which,
from their uniform dimensions, suggest the idea of community of
source-power or source-depth, is that offered by the region surrounding
Copernicus, which, as will be seen by our plate of that object, is a
vast Phlegreian field of diminutive craters. So countless are the minute
craters that a high magnifying power brings into view when atmospheric
circumstances are favourable, and so closely are they crowded together,
that the resulting appearance suggests the idea of froth, and we should
be disposed to christen this the “frothy region” of the moon, did not a
danger exist in the tendency to connect a name with a cause. The craters
that are here so abundant are doubtless the remains of true volcanoes
analogous to the parasitical cones that are to be found on several
terrestrial mountains, and not such accidental formations as the
_Hornitos_ described by Humboldt as abounding in the neighbourhood of
the Mexican volcano, Jurillo, but which the traveller did not consider
to be true cones of eruption.[9] Although upon our plate, and in
comparison with the great crater that is its chief feature, these
countless hollows appear so small as at first sight to appear
insignificant, we must remember that the minutest of them must be grand
objects, each probably equal in dimensions to Vesuvius. For since, as we
have shown in an early chapter, the smallest discernible telescopic
object must subtend an angle to our eye of about a second, and since
this angle extended to the moon represents a mile of its surface, it
follows that these tiny specks of shadow that besprinkle our picture,
are in the reality craters of a mile diameter. This comparison may help
the conception of the stupendous magnitude of the moon’s volcanic
features; for it is a conception most difficult to realize. It is hard
to bring the mind to grasp the fact that that hollow of Copernicus is
fifty miles in diameter. We read of an army having encamped in the once
peaceful crater of Vesuvius, and of one of the extinct volcanoes of the
_Campi Phlegræi_ being used as a hunting preserve by an Italian king.
These facts give an idea of vastness to those who have not the good
fortune to see the actual dimensions of a volcanic orifice themselves.
But it is almost impossible to conjure up a vision of what that
fifty-mile crater would look like upon the moon itself; and for want of
a terrestrial object as a standard of comparison, our picture, and even
the telescopic view of the moon itself, fails to render the imagination
any help. We may try to realize the vastness by considering that one of
our average English counties could be contained within its ramparts, or
by conceiving a mountainous amphitheatre whose opposite sides are as far
apart as the cathedrals of London and Canterbury, but even these
comparisons leave us unimpressed with the true majesty which the object
would present to a spectator upon the surface of our satellite.

In our previous chapter we have given a reason for regarding as true
volcanic craters all those circular formations, of whatever size, that
exhibit that distinctive feature _the central cone_. Between the
smallest crater with a cone that we can detect under the best telescopic
conditions, namely, the companion to Hell, 1¾ mile diameter, and the
great one called Petavius, 78 miles in diameter, we find no break in the
continuity of the crater-cum-cone system that would justify us in saying
that on the one side the volcanic or eruptive cause ceased, and on the
other side some other causative action began. But there are numerous
circular formations that surpass the magnitude of Petavius and its
peers, but that have no central cone, and are, therefore, not so
manifestly volcanic as those which possess this feature. Our map will
show many striking examples of this class at a glance. We may in
particular refer _inter alia_ to Ptolemy near the centre of the moon, to
Grimaldi (No. 125), Shickard (No. 28), Schiller (No. 24), and Clavius
(No. 13), all of which exceed 100 miles in diameter. Even the great
_Mare Crisium_, nearly 300 miles in diameter, appears to be a formation
not distinct from those which we have just named. These present little
of the generic crater character in their appearance; and they have been
distinguished therefrom by the name of _Walled_ or _Ramparted Plains_.
Their actual origin is beyond our explanation, and in attempting to
account for them we must perforce allow considerable freedom to
conjecture. They certainly, as Hooke suggested, present a “broken
bubble”-like aspect; but one cannot reasonably imagine the existence of
any form of mineral matter that would sustain itself in bubble form over
areas of many hundreds of square miles. And if it were reasonable to
suppose the great rings to be the foundations of such vast volcanic
domes, we must conclude these to have broken when they could no longer
sustain themselves, and in that case the surface beneath should be
strewed with _débris_, of which, however, we can find no trace.
Moreover, we might fairly expect that some of the smaller domes would
have remained standing: we need hardly say that nothing of the kind
exists.

[Illustration: Fig. 32.]

The true circularity of these objects appears at first view a remarkable
feature. But it ceases to be so if we suppose them to have been produced
by some very concentrated sublunar force of an upheaving nature, and if
only we admit the homogeneity of the moon’s crust. For if the crust be
homogeneous, then _any_ upheaving force, deeply seated beneath it, will
exert itself _with equal effects at equal distances from the source_:
the lines of equal effect will obviously be radii of a sphere with the
source of the disturbance for its centre, and they will meet a surface
over the source in a circle. This will be evident from Fig. 32, in which
a force is supposed to act at F below the surface s s s s. The matter
composing s s being homogeneous, the action of F will be equal at equal
distances in all directions. The lines of equal force, F_ f_, F _f_,
will be of equal length, and they will form, so to speak, radii of a
sphere of force. This sphere is cut by the plane at _s s s s_, and as
the intersection necessarily takes place everywhere at the extremity of
these radii, the figure of intersection is demonstrably a circle (shown
in perspective as an ellipse in the figure). Thus we see that an intense
but extremely confined explosion, for instance, beneath the moon’s crust
must disturb a _circular_ area of its surface, if the intervening
material be homogeneous. If this be not homogeneous there would be,
where it offered _less_ than the average resistance to the disturbance,
an outward distortion of the circle; and an opposite interruption to
circularity if it offers _more_ than the average resistance. This
assumed homogeneity may possibly be the explanation of the general
circularity of the lunar surface features, small and great.

[Illustration: Fig. 33.]

[Illustration: Fig. 34.]

We confess to a difficulty in accounting for such a very local
generation of a deep-seated force; and, granting its occurrence, we are
unprepared with a satisfactory theory to explain the resultant effect of
such a force in producing a raised ring at the limit of the circular
disturbance. We may indeed, suppose that a vast circular cake or conical
frustra would be temporarily upraised as in Fig. 33, and that upon its
subsidence a certain extrusion of subsurface matter would occur around
the line or zone of rupture as in Fig. 34. This supposition, however,
implies such a peculiarly cohesive condition of the matter of the
uplifted cake, that it is doubtful whether it can be considered tenable.
We should expect any ordinary form of rocky matter subjected to such an
upheaval to be fractured and distorted, especially when the original
disturbing force is greater in the centre than at the edge, as,
according to the above hypothesis, it would be; and in subsiding, the
rocky plateau would thus retain some traces of its disturbance; but in
the circular areas upon the moon there is nothing to indicate that they
have been subjected to such dislocations.

[Illustration: Fig. 35. A A. Fissures gaping downwards and injected
by intumescent lava beneath. B B B. Fissures gaping upwards and
allowing wedges of rock to drop below the level of the intervening
masses, C C. Wedges forced upwards by horizontal compression. E F.
Neutral plane or pivot axis, above and below which the directions of
the tearing strain and horizontal compression are severally
indicated by the smaller arrows; the larger arrows beneath represent
the direction of the primary expansive force.]

Mr. Scrope in his work on volcanoes has given a hypothetical section of
a portion of the earth’s crust, which presents a bulging or tumescent
surface in some measure resembling the effect which such a cause as we
have been considering would produce. We give a slightly modified version
of his sketch in Fig. 35, showing what would be the probable phenomena
attending such an upheaval as regards the behaviour of the disturbed
portion of the crust, and also that of the lava or semifluid matter
beneath: and, as will be seen by the sketch, a possible phase of the
phenomena is the production of an elevated ridge or rampart at the
points of disruption _c c_; and where there is a ring of disruption, as
by our hypothesis there would be, the ridge or rampart _c c_ would be a
circle. In this drawing we see the cracking and distortion to which the
elevated area would be subjected, but of which, as previously remarked,
the circular areas of the moon present no trace of residual appearance.

[Illustration: PLATE XIV.
PLATO.]

Those who have offered other explanations of these vast ring-formed
mountain ranges, have been no more happy in their conjectures. M. Rozet,
who communicated a paper on selenology to the French Academy in 1846,
put forth the following theory. He argued that during the formation of
the solid scoriaceous pelicules of the moon, circular or tourbillonic
movements were set up; and these, by throwing the scoria from the centre
to the circumference, caused an accumulation thereof at the limit of the
circulation. He considered that this phenomenon continued during the
whole process of solidification, but that the amplitude of the whirlpool
diminished with the decreasing fluidity of the surface material.
Further, he suggested that when many vortices were formed, and the
distances of their centres, taken two and two, were less than the sums
of their radii, there resulted closed spaces terminated by arcs of
circles; and when for any two centres the distance was greater than the
sum of the radii of action, two separate and complete rings were formed.
We have only to remark on this, that we are at a loss to account for the
origination of such vorticose movements, and M. Rozet is silent on the
point. If the great circles are to be referred to an original sea of
molten matter, it appears to us more feasible to consider that wherever
we see one of them there has been, at the centre of the ring, a great
outflow of lava that has flooded the surrounding surface. Then, if from
any cause, and it is not difficult to assign one, the outflow became
intermittent, or spasmodic, or subject to sudden impulses, concentric
waves would be propagated over the pool and would throw up the scoria or
the solidifying lava in a circular bank at the limit of the fluid area.

This hypothesis does not differ greatly from the _ebullition_ theory
proposed by Professor Dana, the American geologist, to explain these
formations. He considered that the lunar ring-mountains were formed by
an action analogous to that which is exemplified on the earth in the
crater of Kilauea, in the Hawaiian islands. This crater is a large open
pit exceeding three miles in its longer diameter, and nearly a thousand
feet deep. It has clear bluff walls round a greater part of its circuit,
with an inner ledge or plain at their base, raised 340 feet above the
bottom. This bottom is a plain of solid lavas, entirely open to day,
which may be traversed with safety (we are quoting Professor Dana’s own
statement written in 1846, and therefore not correctly applying to the
present time): over it there are pools of boiling lava in active
ebullition, and one is more than a thousand feet in diameter. There are
also cones at times, from a few yards to two or three thousand feet in
diameter, and varying greatly in angle of inclination. The largest of
these cones have a circular pit or crater at the summit. The great pit
itself is oblong, owing to its situation on a fissure, but the lakes
upon its bottom are round, and in them, says Professor Dana, “the
circular or slightly elliptical form of the moon’s craters is
exemplified to perfection.”

Now Dana refers this great pit crater and its contained lava-lakes to
“the fact that the action at Kilauea is simply _boiling_, owing to the
extreme fluidity of the lavas. The gases or vapours which produce the
state of active ebullition escape freely in small bubbles, with little
commotion, like jets over boiling water; while at Vesuvius and other
like cones they collect in immense bubbles before they accumulate force
enough to make their way through; and consequently the lavas in the
latter case are ejected with so much violence that they rise to a height
often of many thousand feet and fall around in cinders. This action
builds up the pointed mountain, while the simple boiling of Kilauea
makes no cinders and no cinder cones.”

Professor Dana continues, “If the fluidity of lavas, then, is sufficient
for this active ebullition, we may have boiling going on over an area of
an indefinite extent; for the size of a boiling lake can have no limits
except such as may arise from a deficiency of heat. The size of the
lunar craters is therefore no mystery. Neither is their circular form
difficult of explanation; for a boiling pool necessarily, by its own
action, extends itself circularly around its centre. The combination of
many circles, and the large sea-like areas are as readily
understood.”[10]

In justice to Professor Dana it should be stated that he included in
this theory of formation all lunar craters, even those of small size and
possessing central cones; and he put forth his views in opposition to
the eruptive theory which we have set forth, and which was briefly given
to the world more than twenty-five years ago. As regards the smallest
craters with cones, we believe few geologists will refuse their
compliance with the supposition that they were formed as our
crater-bearing volcanoes were formed: and we have pointed out the
logical impossibility of assigning any limit of size beyond which the
eruptive action could not be said to hold good, so long as the central
cone is present. But when we come to ring-mountains having no cones, and
of such enormous size that we are compelled to hesitate in ascribing
them to ejective action, we are obliged to face the possibility of some
other causation. And, failing an explanation of our own that satisfied
us, we have alluded to the few hypotheses proffered by others, and of
these Professor Dana’s appears the most rational, since it is based upon
a parallel found on the earth. In citing it, however, we do necessarily
not endorse it.

Continue Reading

A LUNAR ATMOSPHERE

At the close of the preceding chapter we stated that any force acting in
opposition to that of gravity would be six times more effective on the
moon than on the earth. But, in fact, it would in many cases be still
more so; at all events, so far as projectile forces are concerned; for
the reason that “the powerful coercer of projectile range,” as the
earth’s atmosphere has been termed, has no counterpart, or at most a
very disproportionate one, upon the moon.

The existence of an atmosphere surrounding the moon has been the subject
of considerable controversy, and a great deal of evidence on both sides
of the question has been offered from time to time, and is to be found
scattered through the records of various classes of observations. Some
of the more important items of this evidence it is our purpose to set
forth in the course of the present chapter.

With the phenomena of the terrestrial atmosphere, with the effects that
are attributable to it, we are all well familiar, and our best course
therefore is to examine, as far as we are able, whether counterparts of
any of these effects are manifested upon the moon. For instance, the
clouds that are generated in and float through our air would, to an
observer on the moon, appear as ever changing bright or dusky spots,
obliterating certain of the permanent details of the earth’s surface,
and probably skirting the terrestrial disc, like the changing belts we
perceive on the planet Jupiter, or diversifying its features with less
regularity, after the manner exhibited by the planet Mars. If such
clouds existed on the moon it is evident that the details of its surface
must be, from time to time, similarly obscured; but no trace of such
obscuration has ever been detected. When the moon is observed with high
telescopic powers, all its details come out sharp and clear, without the
least appearance of change or the slightest symptoms of cloudiness other
than the occasional want of general definition, which may be proved to
be the result of unsteadiness or want of homogeneity in our own
atmosphere; for we must tell the uninitiated that nights of pure, good
definition, such as give the astronomer opportunity of examining with
high powers the minute details of planetary features, are very few and
far between. Out of the three hundred and sixty-five nights of a year
there are probably not a dozen that an astronomer can call really fine:
usually, even on nights that are to all common appearance superbly
brilliant, some strata of air of different densities or temperatures, or
in rapid motion, intervene between the observer and the object of his
observation, and through these, owing to the ever-changing refractions
which the rays of light coming from the object suffer in their course,
observation of the delicate markings of a planet is impossible: all is
blurred and confused, and nothing but bolder features can be recognized.
It has in consequence sometimes happened that a slight indistinctness of
some minute detail of the moon has been attributed to clouds or mists at
the lunar surface, whereas the real cause has been only a bad condition
of our own atmosphere. It may be confidently asserted that when all
indistinctness due to terrestrial causes is taken account of or
eliminated, there remain no traces whatever of any clouds or mists upon
the surface of the moon.

This is but one proof against the existence of a lunar atmosphere, and,
it may be argued, not a very conclusive one; because there may still be
an atmosphere, though it be not sufficiently aqueous to condense into
clouds and not sufficiently dense to obscure the lunar details. The
probable existence of an atmosphere of such a character used to be
inferred from a phenomenon seen during total eclipses of the sun. On
these occasions the black body of the moon is invariably surrounded by a
luminous halo, or glory, to which the name “corona” has been applied;
and, further, besides this corona, apparently floating in it and
sometimes seemingly attached to the black edge of the moon, are seen
masses of cloud-like matter of a bright red colour, which, from the form
in which they were first seen and from their flame-like tinge, have
become universally known as the “red-flames.” It used to be said that
this corona could only be the consequence of a lunar atmosphere lit up
as it were by the sun’s rays shining through it, after the manner of a
sunbeam lighting up the atmosphere of a dusty chamber; and the red
flames were held by those who first observed them to be clouds of denser
matter floating in the said atmosphere, and refracting the red rays of
solar light as our own clouds are seen to do at sunrise and sunset. But
the evidence obtained, both by simple telescopic observation and by the
spectroscope, from recent extensively observed eclipses of the sun has
set this question quite at rest; for it has been settled finally and
indisputably that both the above appearances pertain to the sun, and
have nothing whatever to do with the moon.

[Illustration: Fig. 11.]

The occurrence of a solar eclipse offers other means in addition to the
foregoing whereby a lunar atmosphere would be detected. We know that all
gases and vapours absorb some portion of any light which may shine
through them. If then our satellite had an atmosphere, its black nucleus
when seen projected against the bright sun in an eclipse would be
surrounded by a sort of penumbra, or zone of shadow, in contact with its
edge, somewhat like that we have shown in an exaggerated degree in the
annexed cut (Fig. 11), and the passage of this penumbra over solar spots
and other features of the solar photosphere would to some extent obscure
the more minute details of such features. No such dusky band has however
been at any time observed. On the contrary, a band somewhat brighter
than the general surface of the sun has frequently been seen in contact
with the black edge of the moon: this in its turn was held to indicate
an atmosphere about the moon; but Sir George Airy has shown that a lunar
atmosphere, if it really did exist, could not produce such an
appearance, and that the cause of it must be sought in other directions.
If this effect were really due to the passage of the solar rays through
a lunar atmosphere a similar effect ought to be produced by the passage
of the sun’s rays through the terrestrial atmosphere: and we might hence
expect to see the shadow of the earth projected on the moon during a
lunar eclipse surrounded by a sort of bright zone or halo: we need
hardly say such an appearance has never manifested itself. Similarly as
we stated that the delicate details of solar spots would be obscured by
a lunar atmosphere, small stars passing behind the moon would suffer
some diminution in brightness as they approached apparent contact with
the moon’s edge: this fading has been watched for on many occasions, and
in a few cases such an appearance has been suspected, but in by far the
majority of instances nothing like a diminution of brightness or change
of colour of the stars has been seen; stars of the smallest magnitude
visible under such circumstances retain their feeble lustre unimpaired
up to the moment of their disappearance behind the moon’s limb.

Again, in a solar eclipse, even if there were an atmosphere about the
moon not sufficiently dense to form a hazy outline or impair the
distinctness of the details of a solar spot, it would still manifest its
existence in another way. As the moon advances upon the sun’s disc the
latter assumes, of course, a crescent form. Now if air or vapour
enveloped the moon, the exceedingly delicate cusps of this crescent
would be distorted or turned out of shape. Instead of remaining
symmetrical, like the lower one in the annexed drawing (Fig. 12), they
would be bent or deformed after the manner we have shown in the upper
one. The slightest symptom of a distortion like this could not fail to
obtrude itself upon an observer’s eye; but in no instance has anything
of the kind been seen.

Reverting to the consequences of the terrestrial atmosphere: one of the
most striking of these is the phenomenon of diffused daylight, which we
need hardly remind the reader is produced by the scattering or diffusion
of the sun’s rays among the minute particles of vapour composing or
contained in that atmosphere. Were it not for this reflexion and
diffusion of the sun’s light, those parts of our earth not exposed to
direct sunshine would be hidden in darkness, receiving no illumination
beyond the feeble amount that might be reflected from proximate
terrestrial objects actually illuminated by direct sunlight. Twilight is
a consequence of this reflexion of light by the atmosphere when the sun
is below the horizon. If, then, an atmosphere enveloped the moon, we
should see by diffused light those parts of the lunar details that are
not receiving the direct solar beams; and before the sun rose and after
it had set upon any region of the moon, that region would still be
partially illuminated by a twilight. But, on the contrary, the shadowed
portions of a lunar landscape are pitchy black, without a trace of
diffused-light illumination, and the effects that a twilight would
produce are entirely absent from the moon. Once, indeed, one observer,
Schroeter, noticed something which he suspected was due to an effect of
this kind: when the moon exhibited itself as a very slender crescent, he
discovered a faint crepuscular light, extending from each of the cusps
along the circumference of the unenlightened part of the disc, and he
inferred from estimates of the length and breadth of the line of light
that there was an atmosphere about the moon of 5376 feet in height. This
is the only instance on record, we believe, of such an appearance being
seen.

[Illustration: Fig. 12.]

Spectrum analysis would also betray the existence of a lunar atmosphere.
The solar rays falling on the moon are reflected from its surface to the
earth. If, then, an atmosphere existed, it is plain that the solar rays
must first pass through such atmosphere to reach the reflecting surface,
and returning from thence, again pass through it on their way to the
earth; so that they must in reality pass through virtually twice the
thickness of any atmosphere that may cover the moon. And if there be any
such atmosphere, the spectrum formed by the moon’s light, that is, by
the sun’s light reflected from the moon, would be modified in such a
manner as to exhibit absorption-lines different from those found in the
spectrum of the direct solar rays, just as the absorption-lines vary
according as the sun’s rays have to pass through a thinner or a denser
stratum of the terrestrial atmosphere. Guided by this reasoning, Drs.
Huggins and Miller made numerous observations upon the spectrum of the
moon’s light, which are detailed in the “Philosophical Transactions” for
the year 1864; and their result, quoting the words of the report, was
“that the spectrum analysis of the light reflected from the moon is
wholly negative as to the existence of any considerable lunar
atmosphere.”

Upon another occasion, Dr. Huggins made an analogous observation of the
spectrum of a star at the moment of its occultation, which observation
he records in the following words:—“When an observation is made of the
spectrum of a star a little before, or at the moment of its occultation
by the dark limb of the moon, several phenomena characteristic of the
passage of the star’s light through an atmosphere might possibly present
themselves to the observer. If a lunar atmosphere exist, which either by
the substances of which it is composed, or by the vapours diffused
through it, can exert a selective absorption upon the star’s light, this
absorption would be indicated to us by the appearance in the spectrum of
new dark lines immediately before the star is occulted by the moon.”

“If finely divided matter, aqueous or otherwise, were present about the
moon, the red rays of the star’s light would be enfeebled in a smaller
degree than the rays of higher refrangibilities.”

“If there be about the moon an atmosphere free from vapour, and
possessing no absorptive power, but of some density, then the spectrum
would not be extinguished by the moon’s limb at the same instant
throughout its length. The violent and blue rays would lay behind the
red rays.”

“I carefully observed the disappearance of the spectrum of η Piscium at
its occultation of January 4, 1865, for these phenomena; but no signs of
a lunar atmosphere were detected.”

But perhaps the strongest evidence of the non-existence of any
appreciable lunar atmosphere is afforded by the non-refraction of the
light of a star passing behind the edge of the lunar disc. Refraction,
we know, is a bending of the rays of light coming from any object,
caused by their passage through strata of transparent matter of
different densities; we have a familiar example in the apparent bending
of a stick when half plunged into water. There is a simple schoolboy’s
experiment which illustrates refraction in a very cogent manner, but
which we should, from its very simplicity, hesitate to recall to the
reader’s mind did it not very aptly represent the actual case we wish to
exemplify. A coin is placed on the bottom of an empty basin, and the eye
is brought into such a position that the coin is just hidden behind the
basin’s rim. Water is then poured into the basin and, without the eye
being moved from its former place, as the depth of water increases, the
coin is brought by degrees fully into view; the water refracting or
turning out of their course the rays of light coming from the coin, and
lifting them, as it were, over the edge of the basin. Now a perfectly
similar phenomenon takes place at every sunrise and sunset on the earth.
When the sun is really below the horizon, it is nevertheless still
visible to us because it is _brought up_ by the refraction of its light
by the dense stratum of atmosphere through which the rays have to pass.
The sun is, therefore, exactly represented by the coin at the bottom of
the basin in the boy’s experiment, the atmosphere answers to the water,
and the horizon to the rim or edge of the basin. If there were no
atmosphere about the earth, the sun would not be so brought up above the
horizon, and, as a consequence, it would set earlier and rise later by
about a minute than it really does. This, of course, applies not merely
to the sun, but to all celestial bodies that rise and set. Every planet
and every star remains a shorter time below the horizon than it would if
there were no atmosphere surrounding the earth.

To apply this to the point we are discussing. The moon in her orbital
course across the heavens is continually passing before, or occulting,
some of the stars that so thickly stud her apparent path. And when we
see a star thus pass behind the lunar disc on one side and come out
again on the other side, we are virtually observing the setting and
rising of that star upon the moon. If, then, the moon had an atmosphere,
it is clear, from analogy to the case of the earth, that the star must
disappear later and reappear sooner than if it has no atmosphere: just
as a star remains too short a time below the earth’s horizon, or behind
the earth, in consequence of the terrestrial atmosphere, so would a star
remain too short a time behind the moon if an atmosphere surrounded that
body. The point is settled in this way:—The moon’s apparent diameter has
been measured over and over again and is known with great accuracy; the
rate of her motion across the sky is also known with perfect accuracy:
hence it is easy to calculate how long the moon will take to travel
across a part of the sky exactly equal in length to her own diameter.
Supposing, then, that we observe a star pass behind the moon and out
again, it is clear that, if there be no atmosphere, the interval of time
during which it remains occulted ought to be exactly equal to the
computed time which the moon would take to pass over the star. If,
however, from the existence of a lunar atmosphere, the star disappears
too late and reappears too soon, as we have seen it would, these two
intervals will not agree; the computed time will be greater than the
observed time, and the difference, if any there be, will represent the
amount of refraction the star’s light has sustained or suffered, and
hence the extent of atmosphere it has had to pass through.

Comparisons of these two intervals of time have been repeatedly made,
the most recent and most extensive was executed under the direction of
the Astronomer-Royal several years ago, and it was based upon no less
than 296 occultation observations. In this determination the measured or
telescopic semidiameter of the moon was compared with the semidiameter
deduced from the occultations, upon the above principle, and it was
found that the telescopic semidiameter was greater than the occultation
semidiameter by two seconds of angular measurement or by about a
thousandth part of the whole diameter of the moon. Sir George Airy,
commenting on this result, says that it appears to him that the origin
of this difference is to be sought in one of two causes. “Either it is
due to irradiation[3] of the telescopic semidiameter, and I do not doubt
that a part at least of the two seconds is to be ascribed to that cause;
or it may be due to refraction by the moon’s atmosphere. If the whole
two seconds were caused by atmospheric refraction this would imply a
horizontal refraction of one second, which is only 1/2000 part of the
earth’s horizontal refraction. It is possible that an atmosphere
competent to produce this refraction would not make itself visible in
any other way.” This result accords well, considering the relative
accuracy of the means employed, with that obtained a century ago by the
French astronomer Du Séjour, who made a rigorous examination of the
subject founded on observations of the solar eclipse of 1764. He
concluded that the horizontal refraction produced by a possible lunar
atmosphere amounted to 1″·5—a second and a half—or about 1/1400 of that
produced by the earth’s atmosphere. The greater weight is of course to
be allowed to the more recent determination in consideration of the
large number of accurate observations upon which it was based.

But an atmosphere 2,000 times rarer than our air can scarcely be
regarded as an atmosphere at all. The contents of an air-pump receiver
can seldom be rarefied to a greater extent than to about 1/1000 of the
density of air at the earth’s surface, with the best of pneumatic
machines; and the lunar atmosphere, if it exist at all, is thus proved
to be twice as attenuated as what we are accustomed to recognise as a
vacuum. In discussing the physical phenomena of the lunar surface, we
are, therefore, perfectly justified in omitting all considerations of an
atmosphere, and adapting our arguments to the non-existence of such an
appendage.

And if there be no air upon the moon, we are almost forced to conclude
that there can be no water; for if water covered any part of the lunar
globe it must be vapourised under the influence of the long period of
uninterrupted sunshine (upwards of 300 hours) that constitutes the lunar
day, and would manifest itself in the form of clouds or mists obscuring
certain parts of the surface. But, as we have already said, no such
obliteration of details ever takes place; and, as we have further seen,
no evidence of aqueous vapour is manifested upon the occasion of
spectrum observations. Since, then, the effects of watery vapour are
absent, we are forced to conclude that the cause is absent also.

Those parts of the moon which the ancient astronomers assumed, from
their comparatively smooth and dusky appearance, to be seas, have long
since been discovered to be merely extensive regions of less reflective
surface material; for the telescope reveals to us irregularities and
asperities covering well nigh the whole of them, which asperities could
not be seen if they were covered with water; unless, indeed, we admit
the possibility of seeing to the bottom of the water, not only
perpendicularly, but obliquely. Some observers have noticed features
that have led them to suppose that water was at one time present upon
the moon, and has left its traces in the form of appearances of erosive
action in some parts. But if water ever existed, where is it now? One
writer, it is true, has suggested as possible, that whatever air, and we
presume he would include whatever water also, the moon may possess, is
hidden away in sublunarean caves and hollows; but even if water existed
in these places it must sometimes assume the vapoury form, and thus make
its presence known.

[Illustration: Fig. 13.]

[Illustration: Fig. 14.]

Sir John Herschel pointed out that if any moisture exists upon the moon,
it must be in a continual state of migration from the illuminated or
hot, to the unilluminated or cold side of the lunar globe. The
alternations of temperature, from the heat produced by the unmitigated
sunshine of 14 days’ duration, to the intensity of cold resulting from
the absence of any sunshine whatever for an equal period, must, he
argued, produce an action similar to that of the _cryophorus_ in
transporting the lunar moisture from one hemisphere to the other. The
cryophorus is a little instrument invented by the late Dr. Wollaston; it
consists of two bulbs of glass connected by a bent tube, in the manner
shown in the annexed illustration, fig. 13. One of the bulbs, A, is
half-filled with water, and, all air being exhausted, the instrument is
hermetically sealed, leaving nothing within but the water and the
aqueous vapour which rises therefrom in the absence of atmospheric
pressure. When the empty bulb, B, is placed in a freezing mixture, a
rapid condensation of this vapour takes place within it, and as a
consequence the water in the bulb A gives off more vapour. The
abstraction of heat from the water, which is a natural consequence of
this evaporation, causes it to freeze into a solid mass of ice. Now upon
the moon the same phenomenon would occur did the material exist there to
supply it. In the accompanying diagram let A represent the illuminated
or heated hemisphere of the moon, and B the dark or cold hemisphere; the
former being probably at a temperature of 300° above, and the latter
200° below Fahrenheit’s zero. Upon the above principle, if moisture
existed upon A it would become vapourised, and the vapour would migrate
over to B, and deposit itself there as hoarfrost; it would, therefore,
manifest itself to us while in the act of migrating by clouding or
dimming the details about the boundary of the illuminated hemisphere.
The sun, rising upon any point upon the margin of the dark hemisphere,
would have to shine through a bed of moisture, and we may justly
suppose, if this were the case, that the tops of mountains catching the
first beams of sunlight would be tinged with colour, or be lit up at
first with but a faint illumination, just as we see in the case of
terrestrial mountains whose summits catch the first, or receive the last
beams of the rising or setting sun. Nothing of this kind is, however,
perceptible: when the solar rays tip the lofty peaks of lunar mountains,
these shine at once with brilliant light, quite as vivid as any of those
parts that receive less horizontal illumination, or upon which the sun
is almost perpendicularly shining.

All the evidence, then, that we have the means of obtaining, goes to
prove that neither air nor water exist upon the moon. Two complicating
elements affecting all questions relating to the geology of the
terraqueous globe we inhabit may thus be dismissed from our minds while
considering the physical features of the lunar surface. Fire on the one
hand and water or the other, are the agents to which the configurations
of the earth’s surface are referrable: the first of these produced the
igneous rocks that form the veritable foundations of the earth, the
second has given rise to the superstructure of deposits that constitute
the secondary and tertiary formations: were these last removed from the
surface of our planet, so as to lay bare its original igneous crust,
that crust, so far as reasoning can picture it to us, would probably not
differ essentially from the visible surface of the moon. In considering
the causes that have given birth to the diversified features of that
surface, we may, therefore, ignore the influence of air and water action
and confine our reasoning to igneous phenomena alone: our task in this
matter, it is hardly necessary to remark, is materially simplified
thereby.

We have now reached that stage of our subject at which it behoves us to
repair to the telescope for the purpose of examining and familiarising
ourselves with the various classes of detail that the lunar surface
presents to our view.

That the moon is not a smooth sphere of matter is a fact that manifested
itself to the earliest observers. The naked eye perceives on her face
spots exhibiting marked differences of illumination. These variations of
light and shade, long before the invention of the telescope, induced the
belief that she possessed surface irregularities like those that
diversify the face of the earth, and from analogy it was inferred that
seas and continents alternated upon the lunar globe. It was evident,
from the persistence and invariability of the dusky markings, that they
were not due to atmospheric peculiarities, but were veritable variations
in the character or disposition of the surface material. Fancy made
pictures of these unchangeable spots: untutored gazers detected in them
the indications of a human countenance, and perhaps the earliest map of
the moon was a rough reproduction of a man’s face, the eyes, nose and
mouth representing the more salient spots discernible upon the lunar
disc. Others recognised in these spots the configuration of a human
form, head, arms and legs complete, which a French superstition that
lingers to the present day held to be the image of Judas Iscariot
transported to the moon in punishment for his treason. Again, an Indian
notion connects the lunar spots with a representation of a roebuck or a
hare, and hence the Sanskrit names for the moon, _mrigadhara_, a
roebuck-bearer, and _’sa’sabhrit_, a hare-bearer. Of these similitudes
the one which has the best pretensions to a rude accuracy is that first
mentioned; for the resemblance of the full moon to a human countenance,
wearing a painful or lugubrious expression, is very striking. Our
illustration of the full moon (Plate III.) is derived from an actual
photograph;[4] the relative intensities of light and shade are hence
somewhat exaggerated; otherwise it represents the full moon very nearly
as the naked eye sees it, and by gazing at the plate from a short
distance,[5] the well-known features will manifest themselves, while
they who choose may amuse themselves by arranging the markings in their
imagination till they conform to the other appearances alluded to.

We may remark in passing that by one sect of ancient writers the moon
was supposed to be a kind of mirror, receiving the image of the earth
and reflecting it back to terrestrial spectators. Humboldt affirmed that
this opinion had been preserved to his day as a popular belief among the
people of Asia Minor. He says, “I was once very much astonished to hear
a very well educated Persian from Ispahan, who certainly had never read
a Greek book, mention when I showed him the moon’s spots in a large
telescope in Paris, this hypothesis as a widely diffused belief in his
country: ‘What we see in the moon,’ said the Persian, ‘is ourselves; it
is the map of our earth.’” Quite as extravagant an idea, though perhaps
a more excusable one, was that held by some ancient philosophers, to the
effect that the spots on the moon were the shadows of opaque bodies
floating in space between it and the sun.

[Illustration: PLATE III.
FULL MOON.]

An observer watching the forms and positions of the lunar face-marks,
from night to night and from lunation to lunation, cannot fail to notice
the circumstance that they undergo no easily perceptible change of
position with respect to the circular outline of the disc; that in fact
the face of the moon presented to our view is always the same, or very
nearly so. If the moon had no orbital motion we should be led from the
above phenomenon to conclude that she had no axial motion, no movement
of rotation; but when we consider the orbital motion in connection with
the permanence of aspect, we are driven to the conclusion—one, however,
which superficial observers have some difficulty in recognising—that the
moon has an axial rotation equal in period to her orbital revolution.
Since the moon makes the circuit of her orbit in twenty-seven days and
one-third (more exactly 27d. 7h. 43m. 11s.) it follows that this is the
time of her axial rotation, as referred to the stars, or as it would be
made out by an observer located at a fixed position in space outside the
lunar orbit. But if referred to the sun this period appears different;
because the moon while revolving round the earth is, with the earth,
circulating around the sun. Suppose the three bodies, moon, earth, and
sun, to be in a line at a certain period of a lunation, as they are at
full moon: by the time the moon has completed her twenty-seven days’
journey around the earth, the latter will have moved along twenty-seven
days’ march of its orbit, which is about twenty-seven degrees of
celestial longitude: the sun will apparently be that much distant from a
straight line passing through earth and moon, and the moon must
therefore move forward to overtake the sun before she can assume the
full phase again. She will take something over two days to do this;
hence the solar period of her revolution becomes more than twenty-nine
days (to be exact, 29d. 12h. 44m. 2s. ·87). This is the length of a
solar day upon the moon—the interval from one sunrise to another at any
spot upon the equator of our satellite, and the interval between
successive reappearances of the same phase to observers on the earth.
The physical cause of the coincidence of times of rotation and
revolution was touched upon in a previous chapter.

We have said that the moon continuously presents to us the same
hemisphere. This is generally true, but not entirely so. Galileo, by
long scrutiny, familiarised himself with every detail of the lunar-disc
that came within the limited grasp of his telescopes, and he recognised
the fact that according as the position of the moon varied in the sky,
so the aspect of her face altered to a slight degree; that certain
regions at the edge of her disc, alternately came in sight and receded
from his view. He perceived, in fact, an _apparent_ rocking to and fro
of the globe of the moon; a sort of balancing or _libratory_ motion.
When the moon was near the horizon he could see spots upon her uppermost
edge, which disappeared as she approached the zenith, or highest point
of her nightly path; and as she neared this point, other spots, before
invisible, came into view, near to what had been her lower edge. Galileo
was not long in referring this phenomenon to its true cause. The centre
of motion of the moon being the centre of the earth, it is clear that an
observer on the surface of the latter, looks down upon the rising moon
as from an eminence, and thus he is enabled to see more or less over or
around her. As the moon increases in altitude, the line of sight
gradually becomes parallel to the line joining the observer and the
centre of the earth, and at length he looks her full in the face: he
loses the full view and catches another side face view as she nears the
horizon in setting. This phenomenon, occurring as it does, with a daily
period, is known as the _diurnal libration_.

But a kindred phenomenon presents itself in another period, and from
another cause. The moon rotates upon her axis at a speed that is
rigorously uniform. But her orbital motion is not uniform, sometimes it
is faster, and at other times slower than its average rate. Hence, the
angle through which she moves along her orbit in a given time, now
exceeds, and now falls short of the angle through which she turns upon
her axis. Her visible hemisphere thus changes to an extent depending
upon the difference between these orbital and axial angles, and the
apparent balancing thus produced is called the _libration in longitude_.
Then there is a _libration in latitude_ due to the circumstance that the
axis of the moon is not exactly perpendicular to the plane of her orbit;
the effect of this inclination being, that we sometimes see a little
more of the north than of the south polar regions of our satellite, and
_vice versâ_.[6]

The extent of the moon’s librations, taking them all and in combination
into account, amounts to about seven degrees of arc of latitude or
longitude upon the moon, both in the north-south and east-west
directions. And taking into account the whole effect of them, we may
conclude that our view of the moon’s surface, instead of being confined
to one half, is extended really to about four-sevenths of the whole area
of the lunar globe. The remaining three-sevenths must for ever remain a
_terra incognita_ to the habitants of this earth, unless, indeed, from
some catastrophe which it would be wild fancy to anticipate, a period of
rotation should be given to the moon different from that which it at
present possesses. Some highly fanciful theorists have speculated upon
the possible condition of the invisible hemisphere, and have propounded
the absurd notion that the opposite side of the moon is hollow, or that
the moon is a mere shell; others again have urged that the hidden half
is more or less covered with water, and others again that it is peopled
with inhabitants. There is, however, no good reason for supposing that
what we may call the back of the moon has a physical structure
essentially different from the face presented towards us. So far as can
be judged from the peeps that libration enables us to obtain, the same
characteristic features (though of course with different details)
prevail over the whole lunar surface.

The speculative ideas held by the philosophers of the pre-telescopic
age, touching the causes which produced the inequalities of light and
shade upon the moon, received their _coup de grâce_ from the revelations
of Galileo’s glasses. Our satellite was one of the earliest objects, if
not actually the first, upon which the Florentine turned his telescope;
and he found that the inequalities upon her surface were due to
differences in its configuration analogous to the continents and
islands, and (as might then have been thought) the seas of our globe. He
could trace, even with his moderate means, the semblance of
mountain-tops upon which the sun shone while their lower parts were in
shadow, of hills that were brightly illuminated upon their sides towards
the sun, of brightly shining elevations, and deeply shadowed
depressions, of smooth plains, and regions of mountainous ruggedness. He
saw that the boundary of sunlight upon the moon was not a clearly
defined line, as it would be if the lunar globe were a smooth sphere, as
the Aristotelians had asserted, but that the terminator was uneven and
broken into an irregular outline. From these observations the Florentine
astronomer concluded that the lunar world was covered not only with
mountains like our globe, but with mountains whose heights far surpassed
those existing upon the earth, and whose forms were strangely limited to
circularity.

Galileo’s best telescopes magnified only some thirty times, and the
views which he thus obtained, must have been similar to those exhibited
by the smaller photographs of the moon produced in late years by Mr. De
la Rue and now familiar to the scientific public. Of course there is in
the natural moon as viewed with a small telescope a vivid brilliancy
which no art can imitate, and in photographs especially there is a
tendency to exaggeration of the depths of shade in a lunar picture. This
arises from the circumstance that various regions of the moon do not
impress a chemically sensitized plate as they impress the retina of the
eye. Some portions, notably the so-called “seas” of the moon, which to
the eye appear but slightly duller than the brighter parts, give off so
little _actinic_ light that they appear as nearly black patches upon a
photograph, and thus give an undue impression of the relative brightness
of various parts of the lunar surface. Doubtless by sufficient exposure
of the plate in the camera-telescope the dark patches might be rendered
lighter, but in that case the more strongly illuminated portions, which
after all are those most desirable to be preserved, would be lost by the
effect which photographers understand as “solarization.”

In speaking of a view of the moon with a magnifying power of thirty, it
is necessary to bear in mind that the visible features will differ
considerably with the diameter of the object-glass of the telescope to
which this power is applied. The same details would not be seen alike
with the same power upon an object-glass of 10 inches diameter and one
of 2 inches. The superior illumination of the image in the former case
would bring into view minute details that could not be perceived with
the smaller aperture. He who would for curiosity wish to see the moon,
or any other object, as Galileo saw it, must use a telescope of the same
size and character in all respects as Galileo’s: it will not do to put
his magnifying power upon a larger telescope. With large telescopes, and
low powers used upon bright objects like the moon, there is a blinding
flood of light which tends to contract the pupil of the eye and prevent
the passage of the whole of the pencil of rays coming through the
eye-piece. Although this last result may be productive of no
inconvenience, it is clearly a waste of light, and it points to a rule
that the lowest power that a telescope should bear is that which gives a
pencil of light equal in diameter to the pupil of the eye under the
circumstances of brightness attendant upon the object viewed. In
observing faint objects this point assumes more importance, since it is
then necessary that all available light should enter the pupil. The
thought suggests itself that an artificial enlargement of the pupil, as
by a dose of belladonna, might be of assistance in searching for faint
objects, such as nebulæ and comets: but we prefer to leave the
experiment for those to try who pursue that branch of astronomical
observation.

A merely cursory examination of the moon with the low power to which we
have alluded is sufficient to show us the more salient features. In the
first place we cannot help being struck with the immense preponderance
of circular or craterform asperities, and with the general tendency to
circular shape which is apparent in nearly all the lunar surface
markings; for even the larger regions known as the “seas” and the
smaller patches of the same character seem to repeat in their outlines
the round form of the craters. It is at the boundary of sunlight on the
lunar globe that we see these craterform spots to the best advantage, as
it is there that the rising or setting sun casts long shadows over the
lunar landscape, and brings elevations and asperities into bold relief.
They vary greatly in size, some are so large as to bear an estimable
proportion to the moon’s diameter, and the smallest are so minute as to
need the most powerful telescopes and the finest conditions of
atmosphere to perceive them. It is doubtful whether the smallest of them
have ever been seen, for there is no reason to doubt that there exist
countless numbers that are beyond the revealing powers of our finest
telescopes.

From the great number and persistent character of these
circumvallations, Kepler was led to think that they were of artificial
construction. He regarded them as pits excavated by the supposed
habitants of the moon to shelter themselves from the long and intense
action of the sun. Had he known their real dimensions, of which we shall
have to speak when we come to describe them more in detail, he would
have hesitated in propounding such a hypothesis; nevertheless it was, to
a certain extent, justified by the regular and seemingly unnatural
recurrence of one particular form of structure, the like of which is,
too, so seldom met with as a structural feature of the surface of our
own globe.

The next most striking features, revealed by a low telescopic power upon
the moon, are the seemingly smooth plains that have the appearance of
dusky spots, and that collectively cover a considerable portion—about
two-thirds—of the entire disc. The larger of these spots retain the name
of _seas_, the term having been given when they were supposed to be
watery expanses, and having been retained, possibly to avoid the
confusion inevitable from a change of name, after the existence of water
upon the moon was disproved. Following the same order of nomenclature,
the smaller spots have received the appellations of _lakes_, _bays_ and
_fens_. We see that many of these “seas” are partially surrounded by
ramparts or bulwarks which, under closer examination, and having regard
to their real magnitude, resolve themselves into immense mountain
chains. The general resemblance in form which the bulwarked plains thus
exhibit to the circular craters of large size, would lead us to suppose
that the two classes of objects had the same formative origin, but when
we take into account the immense size of the former, and the process by
which we infer the latter to have been developed, the supposition
becomes untenable.

Another of the prominent features which we notice as highly curious, and
in some phases of the moon—at about the time of full—the most remarkable
of all, are certain bright lines that appear to radiate from some of the
more conspicuous craters, and extend for hundreds of miles around. No
selenological formations have so sorely puzzled observers as these
peculiar streaks, and a great deal of fanciful theorizing has been
bestowed upon them. As we are now only glancing at the moon, we do not
enter upon explanations concerning them or any other class of details;
all such will receive due consideration in their proper order in
succeeding chapters.

We thus see that the classes of features observable upon the moon are
not great in number: they may be summed up as _craters_ and their
central cones, _mountain chains_, with occasional isolated peaks,
_smooth plains_, with more or less of irregularity of surface, and
_bright radiating streaks_. But when we come to study with higher powers
the individual examples of each class we meet with considerable
diversity. This is especially the case with the craters, which appear
under very numerous variations of the one order of structure, viz., the
ring-form. A higher telescopic power shows us that not only do these
craters exist of all magnitudes within a limit of largeness, but
seemingly with no limit of smallness, but that in their structure and
arrangement they present a great variety of points of difference. Some
are seen to be considerably elevated above the surrounding surface,
others are basins hollowed out of that surface and with low surrounding
ramparts; some are merely like walled plains or amphitheatres with flat
plateaux, while the majority have their lowest point of hollowness
considerably below the general level of the surrounding surface; some
are isolated upon the plains, others are aggregated into a thick crowd,
and overlapping and intruding upon each other; some have elevated peaks
or cones in their centres, and some are without these central cones,
while the plateaux of others again contain several minute craters
instead; some have their ramparts whole and perfect, others have them
breached or malformed, and many have them divided into terraces,
especially on their inner sides.

In the plains, what with a low power appeared smooth as a water surface
becomes, under greater magnification, a rough and furrowed area, here
gently undulated and there broken into ridges and declivities, with now
and then deep rents or cracks extending for miles and spreading like
river-beds into numerous ramifications. Craters of all sizes and classes
are scattered over the plains; these appear generally of a different
tint to the surrounding surface, for the light reflected from the plains
has been observed to be slightly tinged with colour, The tint is not the
same in all cases: one large sea has a dingy greenish tinge, others are
merely grey and some others present a pale reddish hue. The cause of
this diversity of colour is mysterious; it has been supposed to indicate
the existence of vegetation of some sort; but this involves conditions
that we know do not exist.

The mountains, under higher magnification, do not present such diversity
of formation as the craters, or at least the points of difference are
not so apparent; but they exhibit a plentiful variety of combinations.
There are a few perfectly isolated examples that cast long shadows over
the plains on which they stand like those of a towering cathedral in the
rising or setting sun. Sometimes they are collected into groups, but
mostly they are connected into stupendous chains. In one of the grandest
of these chains, it has been estimated that a good telescope will show
3000 mountains clustered together, without approach to symmetrical
order. The scenery which they would present, could we get any other than
the “bird’s eye view” to which we are confined, must be imposing in the
extreme, far exceeding in sublime grandeur anything that the Alps or the
Himalayas offer; for while on the one hand the lunar mountains equal
those of the earth in altitude, the absence of an atmosphere, and
consequently of the effects produced thereby, must give rise to
alternations of dazzling light and black depths of shade combining to
form panoramas of wild scenery that, for want of a parallel on earth, we
may well call unearthly. But we are debarred the pleasure of actually
contemplating such pictures by the circumstance that we look _down_ upon
the mountain tops and into the valleys, so that the great height and
close aggregation of the peaks and hills are not so apparent. To compare
the lunar and terrestrial mountain scenery would be “to compare the
different views of a town seen from the car of a balloon, with the more
interesting prospects by a progress through the streets.” Some of the
peculiarities of the lunar scenery we have, however, endeavoured to
realize in a subsequent Chapter.

A high power gives us little more evidence than a low one upon the
nature of the long bright streaks that radiate from some of the more
conspicuous craters, but it enables us to see that those streaks do not
arise from any perceptible difference of level of the surface—that they
have no very definite outline, and that they do not present any sloping
sides to catch more sunlight, and thus shine brighter, than the general
surface. Indeed, one great peculiarity of them is that they come out
most forcibly where the sun is shining perpendicularly upon them; hence
they are best seen where the moon is at full, and they are not visible
at all at those regions upon which the sun is rising or setting. We also
see that they are not diverted by elevations in their path, as they
traverse in their course craters, mountains, and plains alike, giving a
slight additional brightness to all objects over which they pass, but
producing no other effect upon them. To employ a commonplace simile,
they look as though, after the whole surface of the moon had assumed its
final configuration, a vast brush charged with a whitish pigment had
been drawn over the globe in straight lines radiating from a central
point, leaving its trail upon everything it touched, but obscuring
nothing.

Whatever may be the cause that produces this brightness of certain parts
of the moon without reference to configuration of surface, this cause
has not been confined to the formation of the radiating lines, for we
meet with many isolated spots, streaks and patches of the same bright
character. Upon some of the plains there are small areas and lines of
luminous matter possessing peculiarities similar to those of the
radiating streaks, as regards visibility with the high sun, and
invisibility when the solar rays fall upon them horizontally. Some of
the craters also are surrounded by a kind of aureole of this highly
reflective matter. A notable specimen is that called _Linné_, concerning
which a great hue and cry about change of appearance and inferred
continuance of volcanic action on the moon was raised some years ago.
This object is an insignificant little crater of about a mile or two in
diameter, in the centre of an ill-defined spot of the character referred
to, and about eight or ten miles in diameter. With a low sun the crater
alone is visible by its shadow; but as the luminary rises the shadow
shortens and becomes all but invisible, and then the white spot shines
forth. These alternations, complicated by variations of atmospheric
condition, and by the interpretations of different observers, gave rise
to statements of somewhat exaggerated character to the effect that
considerable changes, of the nature of volcanic eruptions, were in
progress in that particular region of the moon.

In the foregoing remarks we have alluded somewhat indefinitely to high
powers; and an enquiring but unastronomical reader may reasonably demand
some information upon this point. It might have been instructive to have
cited the various details that may be said to come into view with
progressive increases of magnification. But this would be an all but
impossible task, on account of the varying conditions under which all
astronomical observations must necessarily be made. When we come to
delicate tests, there are no standards of telescopic power and
definition. Assuming the instrument to be of good size and high optical
character, there is yet a powerful influant of astronomical definition
in the atmosphere and its variable state. Upon two-thirds of the clear
nights of a year the finest telescopes cannot be used to their full
advantage, because the minute flutterings resulting from the passage of
the rays of light through moving strata of air of different densities
are magnified just as the image in the telescope is magnified, till all
minute details are blurred and confused, and only the grosser features
are left visible. And supposing the telescope and atmosphere in good
state, there is still an important point, the state of the observer’s
eye, to be considered. After all it is the eye that sees, and the best
telescopic assistance to an untrained eye is of small avail. The eye is
as susceptible of education and development as any other organ; a
skilful and acute observer is to a mere casual gazer, what a watchmaker
would be to a ploughman, a miniature painter to a whitewasher. This fact
is not generally recognized; no man would think of taking in hand an
engraver’s burin, and expecting on the instant to use it like an adept,
or of going to a smithy and without previous preparation trying to forge
a horse-shoe. Yet do folks enter observatories with uneducated eyes, and
expect at once to realize all the wonderful things that their minds have
pictured to themselves from the perusal of astronomical books. We have
over and over again remarked the dissatisfaction which attends the first
looks of novices through a powerful telescope. They anticipate
immediately beholding wonders, and they are disappointed at finding how
little they can see, and how far short the sight falls of what they had
expected. Courtesy at times leads them to express wonder and surprise,
which it is easy to see is not really felt, but sometimes honesty
compels them to give expression to their disappointment. This arises
from the simple fact that their eyes are not fit for the work which is
for the moment imposed upon them; they know not what to look for, or how
to look for it. The first essay at telescopic gazing, like first essays
generally, serves but to teach us our incapability.

To a tutored eye a great deal is visible with a comparatively low power,
and practised observers strive to use magnifying powers as low as
possible, so as to diminish, as far as may be, the evils arising from an
untranquil atmosphere. With a power so small as 30 or 40, many
exceedingly delicate details on the moon are visible to an eye that is
familiar with them under higher powers. With 200 we may say that every
ordinary detail will come out under favourable conditions; but when
minute points of structure, mere nooks and corners as it were, are to be
scrutinised, 300 may be used with advantage. Another hundred diameters
almost passes the practical limit. Unless the air be not merely fine,
but superfine, the details become “clothy” and tremulous; the extra
points brought out by the increased power are then only caught by
momentary glimpses, of which but a very few are obtained during a
lengthy period of persistent scrutiny. We may set down 250 as the most
useful, and 350 the utmost effective power that can be employed upon the
particular work of which we are treating. Could every detail on the moon
be thoroughly and reliably represented as this amount of magnification
shows it, the result would leave little to be wished for.

But it may be asked by some, what is the absolute effect of such powers
as those we have spoken of, in bringing the moon apparently nearer to
our eyes? and what is the actual size of the smallest object visible
under the most favourable circumstances? A linear mile upon the moon
corresponds to an angular interval of 0·87 of a second; this refers to
regions about the centre of the disc; near the circumference the
foreshortening makes a difference, very great as the edge is approached.
Perhaps the smallest angle that the eye can without assistance
appreciate is half a minute; that is to say, an object that subtends to
the eye an arc of less than a half a minute can scarcely be seen.[7]
Since there are 60 seconds in a minute, it follows that we must magnify
a spot a second in diameter upon the moon thirty times before we can see
it; and since a second represents rather more than a mile, really about
2000 yards, on the moon, as seen from the earth, the smallest object
visible with a power of 30 will be this number of yards in diameter or
breadth. To see an object 200 yards across, we should require to magnify
it 300 times, and this would only bring it into view as a point; 20
yards would require a power of 3000, and 1 yard 60,000 to effect the
same thing. Since, as we have said, the highest practicable power with
our present telescopes, and at ordinary terrestrial elevations, is 350,
or for an extreme say 400, it is evident that the minutest lunar object
or detail of which we can perceive as a point must measure about 150
yards: to see the form of an object, so as to discriminate whether it be
round or square, it would require to be probably twice this size; for it
may be safely assumed that we cannot perceive the outline of an object
whose average breadth subtends a less angle than a minute.

Arago put this question into another shape:—The moon is distant from us
237,000 miles (mean). A magnifying power of a thousand would show us the
moon as if she were distant 237 miles from the naked eye.

2000 would bring her within 118 miles.
4000 ” ” ” 59 ”
6000 ” ” ” 39 ”

Mont Blanc is visible to the naked eye from Lyons, at the distance of
about 100 miles; so that to see the mountains of the moon as Mont Blanc
is seen from Lyons would require the impracticable power of 2500.

It is scarcely necessary to seek the reasons which prompted astronomers,
soon after the invention of the telescope, to map the surface features
of the moon. They may have considered it desirable to record the
positions of the spots upon her disc, for the purpose of facilitating
observations of the passage of the earth’s shadow over them in lunar
eclipses; or they may have been actuated by a desire to register
appearances then existing, in order that if changes took place in after
years these might be readily detected. Scheiner was one of the earliest
of lunar cartographers; he worked about the middle of the seventeenth
century; but his delineations were very rough and exaggerated. Better
maps—the best of the time, according to an old authority—were engraved
by one Mellan, about the years 1634 or 1635. At about the same epoch,
Langreen and Hevelius were working upon the same subject. Langreen
executed some thirty maps of portions of the moon, and introduced the
practice of naming the spots after philosophers and eminent men.
Hevelius spent several years upon his task, the results of which he
published in a bulky volume containing some 50 maps of the moon in
various phases, and accompanied by 500 pages of letter-press. He
rejected Langreen’s system of nomenclature, and called the spots after
the seas and continents of the earth to which he conceived they bore
resemblance. Riccioli, another selenographer, whose map was compiled
from observations made by Grimaldi, restored Langreen’s nomenclature,
but he confined himself to the names of eminent astronomers, and his
system has gained the adhesion of the map-makers of later times. Cassini
prepared a large map from his own observations, and it was engraved
about the year 1692. It appears to have been regarded as a standard
work, for a reduced copy of it was repeatedly issued with the yearly
volumes of the _Connaissance des Temps_, (the “Nautical Almanac” of
France) some time after its publication. These small copies have no
great merit: the large copper plate of the original was, we are told by
Arago, who received the statement from Bouvard, sold to a brazier by a
director of the French Government Printing-Office, who thought proper to
disembarrass the stores of that establishment, by ridding them of what
he considered lumber! La Hire, Mayer, and Lambert, followed during the
succeeding century, in this branch of astronomical delineation. At the
commencement of the present century, the subject was very earnestly
taken up by the indefatigable Schroeter, who, although he does not
appear to have produced a complete map, produced a topograph of the moon
in a large series of partial maps and drawings of special features.
Schroeter was a fine observer, but his delineations show him to have
been an indifferent draughtsman. Some of his drawings are but the rudest
representations of the objects he intended to depict; many of the bolder
features of conspicuous objects are scarcely recognizable in them. A bad
artist is as likely to mislead posterity as a bad historian, and it
cannot be surprising if observers of this or future generations,
accepting Schroeter’s drawings as faithful representations, should infer
from them remarkable changes in the lunar details. It is much to be
regretted that Schroeter’s work should be thus depreciated. Lohrman of
Dresden, was the next cartographer of the moon; in 1824 he put forth a
small but very excellent map of 15 inches diameter, and published a book
of descriptive text, accompanied by sectional charts of particular
areas. His work, however, was eclipsed by the great one which we owe to
the joint energy of MM. Beer and Maedler, and which represents a
stupendous amount of observing work carried on during several years
prior to 1836, the date of their publication. The long and patient
labour bestowed upon their map and upon the measures on which it
depends, deserve the highest praise which those conversant with the
subject can bestow, and it must be very long before their efforts can be
superseded.

Beer and Maedler’s map has a diameter of 37 inches: it represents the
phase of the moon visible in the condition of mean libration. The
details were charted by a careful process of triangulation. The disc was
first divided into “triangles of the first order,” the points of which
(conspicuous craters) were accurately laid down by reference to the
edges of the disc: one hundred and seventy-six of these triangles,
plotted accurately upon an orthographic projection of the hemisphere,
formed the reliable basis for their charting work. From these a great
number of “points of the second order” were laid down, by measuring
their distance and angle of position with regard to points first
established. The skeleton map thus obtained was filled up by drawings
made at the telescope: the diameters of the measureable craters being
determined by the micrometer.

Beer and Maedler also measured the heights of one thousand and
ninety-five lunar mountains and crater-summits: the resulting measures
are given in a table contained in the comprehensive text-book which
accompanies their map. These heights are found by one of two methods,
either by measuring the length of the shadow which the object casts
under a known elevation of the sun above its horizon, or by measuring
the distance between the illuminated point of the mountain and the
“terminator” in the following manner. In the annexed figure (Fig. 15)
let the circle represent the moon and M a mountain upon it: let S A be
the line of direction of the sun’s rays, passing the normal surface of
the moon at A and just tipping the mountain top. A will be the
terminator, and there will be darkness between it and the star-like
mountain summit M. The distance between A and M is measured: the
distance A B is known, for it is the moon’s radius. And since the line S
M is a tangent to the circle the angle B A M is a right angle. We know
the length of its two sides AB, AM, and we can therefore by the known
properties of the right-angled triangle find the length of the
hypothenuse BM: and since BM is made up of the radius BA plus the
mountain height, we have only to subtract the moon’s radius from the
ascertained whole length of the hypothenuse and we have the height of
the mountain. MM. Beer and Maedler exhibited their measures in French
toises: in the heights we shall have occasion to quote, these have been
turned into English feet, upon the assumption that the toise is equal to
6·39 English feet. The nomenclature of lunar features adopted by Beer
and Maedler is that introduced by Riccioli: mountains and features
hitherto undistinguished were named by them after ancient and modern
philosophers, in continuance of Riccioli’s system, and occasionally
after terrestrial features. Some minute objects in the neighbourhood of
large and named ones were included under the name of the large one and
distinguished by Greek or Roman letters.

[Illustration: Fig. 15.]

[Illustration: PLATE IV.
PICTURE MAP OF THE MOON.]

[Illustration: PLATE V.
Skeleton Map of Moon
To Accompany Picture Map, Chap. VII]

The excellent map resulting from the arduous labours of these
astronomers is simply a map: it does not pretend to be a picture. The
asperities and depressions are symbolized by a conventional system of
shading and no attempt is made to exhibit objects as they actually
appear in the telescope. A casual observer comparing details on the map
with the same details on the moon itself would fail to identify or
recognize them except where the features are very conspicuous. Such an
observer would be struck by the shadows by which the lunar objects
reveal themselves: he would get to know them mostly by their shadows,
since it is mainly by those that their forms are revealed to a
terrestrial observer. But such a map as that under notice indicates no
shadows, and objects have to be identified upon it rather by their
positions with regard to one another or to the borders of the moon than
by any notable features they actually present to view. This
inconvenience occurred to us in our early use of Beer and Maedler’s
chart, and we were induced to prepare for ourselves a map in which every
object is shown somewhat, if imperfectly, as it actually appears at some
period of a lunation. This was done by copying Beer and Maedler’s
outlines and filling them up by appropriate shading. To do justice to
our task we enlarged our map to a diameter of six feet. Upon a circle of
this diameter the positions and dimensions of all objects were laid down
from the German original. Then from our own observations we depicted the
general aspect of each object: and we so adjusted the shading that all
objects should be shown under about the same angle of illumination—a
condition which is never fulfilled upon the moon itself, but which we
consider ourselves justified in exhibiting for the purpose of conveying
a fair impression of how the various lunar objects actually appear at
some one or other part of a lunation.

The picture-map thus produced has been photographed to the size
convenient for this work: and in order to make it available for the
identification of such objects as we may have occasion to refer to, we
have placed around it a co-ordinate scale of arbitrary divisions by
which any object can be found as by the latitude and longitude divisions
upon a common geographical map. We have also prepared a skeleton map
which includes the more conspicuous objects, and which faces the picture
map (Plates IV. and V.) The numbers on the skeleton map are those given
in the second column of the accompanying table. The table also gives the
co-ordinate positions of the various craters, the names of which are,
for convenience of reference, printed in alphabetical order.

Name. Number. Map Ordinates.

Abulfeda 107 30·0 120·7
Agrippa 151 31·2 110·0
Airy 93 34·7 123·0
Albategnius 109 35·5 119·7
Aliacensis 61 35·8 131·0
Almanon 94 29·0 122·3
Alpetragius 92 40·8 122·4
Alphonsus 110 39·6 120·9
Apianus 62 33·6 129·3
Apollonius 154 6·5 109·5
Arago 152 24·7 108·7
Archimedes 191 40·3 95·8
Aristarchus 176 62·3 99·2
Aristillus 190 37·0 93·3
Aristotle 209 30·0 84·6
Arzachael 84 39·5 124·0
Atlas 228 20·7 86·6
Autolycus 189 36·8 95·5
Azophi 76 30·7 126·8
Bacon 17 32·5 142·0
Baily 207 26·0 85·4
Barocius 34 31·8 138·5
Bessel 179 27·4 100·1
Bettinus 11 48·8 144·9
Bianchini 215 51·6 86·3
Billy 121 64·3 121·4
Blancanus 12 43·7 144·8
Bonpland 110 48·5 117·6
Borda 56 15·2 131·0
Boscovich 160 31·1 106·8
Bouvard 40 66·6 134·3
Briggs 196 68·0 97·2
Bullialdus 86 50·1 125·5
Burg 206 25·5 87·5
Calippus 199 32·4 90·3
Campanus 71 52·3 129·0
Capella 104 17·8 118·0
Capuanus 43 50·5 132·8
Casatus 7 43·7 147·0
Cassini 200 35·5 89·7
Catherina 95 24·7 124·0
Cavalerius 144 71·2 109·5
Cavendish 88 63·5 127·4
Cichus 44 47·3 132·8
Clavius 13 41·8 143·5
Cleomides 183 10·7 97·0
Colombo 98 12·8 122·7
Condamine 214 48·7 84·2
Condorcet 164 4·5 104·7
Copernicus 147 49·8 107·0
Cyrillus 96 23·5 121·3
Damoiseau 124 69·2 117·2
Davy 113 43·2 119·8
Deambrel 129 26·8 113·5
Delisle 195 55·7 95·2
Descartes 106 28·5 119·3
Diophantus 194 55·5 96·3
Doppelmayer 70 58·6 129·6
Encke 140 59·7 110·6
Endymion 227 20·6 83·8
Epigenes 223 39·0 79·5
Erastothenes 168 44·6 104·0
Eudoxus 208 29·7 88·0
Fabricius 35 20·0 136·8
Fernelius 37 35·1 134·8
Firmicus 156 5·8 107·7
Flamsteed 126 62·8 114·5
Fontana 122 65·9 123·0
Fontenelle 221 43·0 81·3
Fourier 67 62·5 130·7
Fracastorius 78 20·5 127·0
Furnerius 52 11·7 133·0
Gambart 138 47·2 112·2
Gartner 224 26·5 82·3
Gassendi 90 59·7 123·3
Gauricus 46 43·5 132·5
Gauss 201 10·3 90·3
Gay Lussac 169 50·1 103·8
Geber 83 29·6 124·8
Geminus 187 13·0 93·0
Gérard 218 63·7 88·8
Goclenius 101 11·8 118·5
Godin 135 31·3 111·7
Grimaldi 125 70·8 116·3
Gruemberger 6 41·4 145·8
Gueriké 114 46·5 119·6
Guttemberg 102 13·9 118·3
Inghirami 27 61·3 138·9
Isidorus 103 16·7 118·0
Kant 105 25·8 118·5
Kepler 146 60·0 108·0
Kies 72 49·7 128·8
Kircher 10 47·5 145·8
Klaproth 8 43·5 146·7
La Caille 74 37·5 126·8
Lagrange 68 67·0 131·3
La Hire 177 54·3 99·3
Lalande 117 43·4 115·3
Lambert 193 49·6 97·8
Landsberg 127 54·0 113·0
Langreen 100 6·3 117·7
Letronne 120 62·0 119·0
Licetus 21 34·1 139·6
Lichtenberg 197 66·5 94·9
Linnæus 188 31·7 95·7
Littrow 185 20·5 99·4
Lohrman 143 71·3 112·8
Longomontanus 23 45·7 140·6
Lubiniezky 91 51·3 123·5
Macrobius 182 13·7 100·2
Maginus 22 40·0 140·4
Mairan 217 56·7 89·5
Manilius 167 32·2 103·9
Manzinus 4 31·3 146·0
Maraldi 181 18·6 100·8
Marius 171 65·0 105·5
Maskelyne 132 19·5 111·0
Mason 204 23·7 88·8
Maupertius 213 48·7 85·8
Maurolycus 33 31·8 137·0
Menelaus 165 28·3 103·0
Mercator 65 51·4 130·2
Mersenius 89 61·7 125·7
Messala 202 14·0 90·5
Messier 131 10·8 114·0
Metius 36 18·8 105·9
Moretus 5 39·5 146·5
Moesting 128 41·6 113·2
Neander 57 18·7 131·0
Nearchus 18 26·8 142·0
Newton 1 41·0 147·7
Nonius 49 36·5 133·2
Olbers 172 73·0 107·7
Pallas 149 38·6 109·5
Parrot 108 35·8 121·6
Petavius 80 9·5 127·5
Phocylides 25 55·5 141·6
Piazzi 41 65·0 133·5
Picard 163 8·3 104·7
Piccolomini 58 21·7 131·0
Pico 211 41·9 87·3
Pitatus 63 44·1 130·2
Plana 205 24·8 88·8
Plato 210 41·8 84·8
Playfair 75 33·5 127·5
Pliny 165 24·2 103·4
Poisson 60 32·8 131·0
Polybius 82 24·5 125·6
Pontanus 59 29·0 130·2
Posidonius 186 22·2 94·3
Proclus 162 11·4 104·5
Ptolemy 111 39·5 118·2
Purbach 73 38·7 128·4
Pythagoras 220 53·0 81·2
Pytheas 178 49·7 100·4
Ramsden 42 52·9 132·5
Reamur 118 37·3 114·6
Reiner 145 67·3 108·5
Reinhold 139 51·5 111·2
Repsold 219 60·2 85·7
Rheita 51 16·1 134·2
Riccioli 142 72·7 113·8
Riccius 50 23·7 133·5
Ritter 134 26·0 111·6
Roemer 184 18·3 97·6
Ross 161 25·0 105·3
Sabine 133 25·0 112·0
Sacrobosco 77 27·5 127·7
Santbech 79 15·7 126·8
Saussure 31 39·6 137·7
Scheiner 14 45·5 143·5
Schickard 28 59·0 137·5
Schiller 24 51·3 141·0
Schroeter 137 42·3 110·7
Schubert 155 2·3 110·8
Segner 16 51·3 143·5
Seleucus 174 69·0 99·8
Sharp 216 54·2 87·7
Short 2 39·7 147·4
Silberschlag 157 32·0 108·1
Simpelius 3 35·8 147·7
Snell 55 11·3 129·6
Soemmering 136 42·8 112·2
Stadius 148 45·6 107·0
Stevinus 53 11·9 130·7
Stoefler 32 35·6 136·8
Strabo 226 23·2 81·6
Struve 203 18·3 88·7
Taruntius 153 11·7 109·0
Taylor 130 27·6 116·2
Thales 225 24·3 81·8
Thebit 85 40·8 126·8
Theophilus 97 22·3 120·0
Timæus 222 38·3 80·8
Timocharis 192 45·1 97·0
Tobias Mayer 170 54·5 103·0
Triesnecker 150 35·5 109·8
Tycho 30 43·0 142·3
Ukert 159 37·1 107·5
Vasco de Gama 173 72·8 104·9
Vendelinus 99 6·8 121·6
Vieta 69 64·3 129·7
Vitello 66 55·8 130·7
Vitruvius 180 20·1 102·0
Vlacq 19 25·0 140·1
Zuchius 15 50·7 144·2

The strong family likeness pervading the craters of the moon renders it
unnecessary that we should attempt a description of each one of them or
even of one in twenty. We have, however, thought that a few remarks upon
the salient features of a few of the most important may be acceptable in
explanation of our illustrative plates; and what we have to say of the
few may be taken as representative of the many.

COPERNICUS, 147—(49·8—107·0). Plate VIII.

This may deservedly be considered as one of the grandest and most
instructive of lunar craters. Although its vast diameter (46 miles) is
exceeded by others, yet, taken as a whole, it forms one of the most
impressive and interesting objects of its class. Its situation, near the
centre of the lunar disc, renders all its wonderful details, as well as
those of its immediately surrounding objects, so conspicuous as to
establish it as a very favourite object. Its vast rampart rises to
upwards of 12,000 feet above the level of the plateau, nearly in the
centre of which stands a magnificent group of cones, three of them
attaining the height of upwards of 2400 feet.

The rampart is divided by concentric segmental terraced ridges, which
present every appearance of being enormous landslips, resulting from the
crushing of their over-loaded summits, which have slid down in vast
segments and scattered their débris on to the plateau. Corresponding
vacancies in the rampart may be observed from whence these prodigious
masses have broken away. The same may be noticed, although in a somewhat
modified degree, around the exterior of the rampart. In order to
approach a realization of the sublimity and grandeur of this magnificent
example of a lunar volcanic crater, our reader would do well to
endeavour to fix his attention on its enormous magnitude and attempt to
establish in his mind’s eye a correct conception of the scale of its
details as well as its general dimensions, which, as they so
prodigiously transcend those of the largest terrestrial volcanic
craters, require that our ideas as to magnitude of such objects should
be, so to speak, educated upon a special standard. It is for this reason
we are anxious our reader, when examining our illustrations, should
constantly refer the objects represented in them to the scale of miles
appended to each plate, otherwise a just and true conception of the
grandeur of the objects will escape him.

Copernicus is specially interesting, as being evidently the result of a
vast discharge of molten matter which has been ejected at the focus or
centre of disruption of an extensively upheaved portion of the lunar
crust. A careful examination of the crater and the district around it,
even to the distance of more than 100 miles on every side, will supply
unmistakable evidence of the vast extent and force of the original
disruption, manifested by a wonderfully complex reticulation of bright
streaks which diverge in every direction from the crater as their common
centre. These streaks do not appear on our plate, nor are they seen upon
the moon except at and near the full phase. They show conspicuously,
however, by their united lustre on the full moon, Plate III. Every one
of those bright streaks, we conceive, is a record of what was originally
a crack or chasm in the solid crust of the moon, resulting from some
vastly powerful upheaving agency over the site of whose focus of energy
Copernicus stands. The cracking of the crust must have been followed by
the ejection of subjacent molten matter up through the reticulated
cracks; this, spreading somewhat on either side of them, has left these
bright streaks as a visible record of the force and extent of the
upheaval; while at the focus of disruption from whence the cracks
diverge, the grand outburst appears to have taken place, leaving
Copernicus as its record and result.

Many somewhat radial ridges or spurs may be observed leading away from
the exterior banks of the great rampart. These appear to be due to the
more free egress which the extruded matter would find near the focus of
disruption. The spur-ridges may be traced fining away for fully 100
miles on all sides, until they become such delicate objects as to
approach invisibility. Several vast open chasms or cracks may be
observed around the exterior of the rampart. They appear to be due to
some action subsequent to the formation of the great crater—probably the
result of contraction on the cooling of the crust, or of a deep-seated
upheaval long subsequent to that which resulted in the formation of
Copernicus itself, as they intersect objects of evidently prior
formation.

Under circumstances specially favourable for “fine vision,” for upwards
of 70 miles on all sides around Copernicus, myriads of comparatively
minute but perfectly-formed craters may be observed. The district on the
south-east side is specially rich in these wonderfully thickly-scattered
craters, which we have reason to suppose stand over or upon the
reticulated bright streaks; but, as the circumstances of illumination
which are requisite to enable us to detect the minute craters are widely
adverse to those which render the bright streaks visible, namely, nearly
full moon for the one and gibbous for the other, it is next to
impossible to establish the fact of coincidence of the sites of the two
by actual simultaneous observation.

At the east side of the rampart, multitudes of these comparatively
minute craters may also be detected, although not so closely crowded
together as those on the west side; but among those on the east may be
seen myriads of minute prominences roughening the surface; on close
scrutiny these are seen to be small mounds of extruded matter which, not
having been ejected with sufficient energy to cause the erupted material
to assume the crater form around the vent of ejection, have simply
assumed the mound form so well known to be the result of volcanic
ejection of moderate force.

Were we to select a comparatively limited portion of the lunar surface
abounding in the most unmistakable evidence of volcanic action in every
variety that can characterize its several phases, we could not choose
one yielding in all respects such instructive examples as Copernicus and
its immediate surroundings.

GASSENDI, 90—(59·7—123·3). Frontispiece.

An interesting crater about 54 miles diameter; the height of the most
elevated portion of the surrounding wall from the plateau being about
9600 feet. The centre is occupied by a group of conical mountains, three
of which are most conspicuous objects and rise to nearly 7000 feet above
the level of the plateau. As in other similar cases, these central
mountains are doubtless the result of the expiring effort of the
eruption which had formed the great circular wall of the crater. The
plateau is traversed by several deep cracks or chasms nearly one mile
wide.

Both the interior and exterior of the wall of the crater are terraced
with the usual segmental ridges or landslips. A remarkable detached
portion of the interior bank is to be seen on the east side, while on
the west exterior of the wall may be seen an equally remarkable example
of an outburst of lava subsequent to the formation of the wall or bank
of the crater; it is of conical form and cannot fail to secure the
attention of a careful observer.

Interpolated on the north wall of the crater may be seen a crater of
about 18 miles diameter which has burst its bank in towards the great
crater, upon whose plateau the lava appears to have discharged itself.

The neighbourhood of Gassendi is diversified by a vast number of mounds
and long ridges of exudated matter, and also traversed by enormous
chasms and cracks, several of which exceed one mile wide and are fully
100 miles in length, and, as is usual with such cracks, traverse plain
and mountain alike, disregarding all surface inequalities.

Numbers of small craters are scattered around; the whole forming an
interesting and instructive portion of the lunar surface.

EUDOXUS, 208 (29·7—88·0), and ARISTOTLE, 209 (30·0—84·6). Plate X.

Two gigantic craters, Eudoxus being nearly 35 miles in diameter and
upwards of 11,000 feet deep, while Aristotle is about 48 miles in
diameter, and about 10,000 feet deep (measuring from the summit of the
rampart to the plateau). These two magnificent craters present all the
true volcanic characteristics in a remarkable degree. The outsides as
well as the insides of their vast surrounding walls or banks display on
the grandest scale the landslip feature, the result of the over-piling
of the ejected material, and the consequent crushing down and crumbling
of the substructure. The true eruptive character of the action which
formed the craters is well evinced by the existence of the groups of
conical mountains which occupy the centres of their circular plateaux,
since these conical mountains, there can be little doubt, stand over
what were once the vents from whence the ejected matter of the craters
was discharged.

On the west side of these grand craters may be seen myriads of
comparatively minute ones (we use the expression “comparatively minute,”
although most of them are fully a mile in diameter). So thickly are
these small craters crowded together, that counting them is totally out
of the question; in our original notes we have termed them “Froth
craters” as the most characteristic description of their aspect.

The exterior banks of Aristotle are characterized by radial ridges or
spurs: these are most probably the result of the flowing down of great
currents of very fluid lava. To the east of the craters some very lofty
mountains of exudation may be seen, and immediately beyond them an
extensive district of smaller mountains of the same class, so thickly
crowded together as under favourable illumination to present a multitude
of brilliant points of light contrasted by intervening deep shade. On
the west bank of Aristotle a very perfect crater may be seen, 27 miles
in diameter, having all the usual characteristic features.

About 40 miles to the east of Eudoxus there is a fine example of a crack
or fissure extending fully 50 miles—30 miles through a plain, and the
remaining 20 miles cutting through a group of very lofty mountains. This
great crack is worthy of attention, as giving evidence of the
deep-seated nature of the force which occasioned it, inasmuch as it
disregards all surface impediments, traversing plain and group of
mountains alike.

There are several other features in and around these two magnificent
craters well worthy of careful observation and scrutiny, all of them
excellent types of their respective classes.

TRIESNEKER, 150 (35·5—109·8). Plate XI.

A fine example of a normal lunar volcanic crater, having all the usual
characteristic features in great perfection. Its diameter is about 20
miles, and it possesses a good example of the central cone and also of
interior terracing.

The most notable feature, however, in connection with this crater, and
on account of which we have chosen it as a subject for one of our
illustrations, is the very remarkable display of chasms or cracks which
may be seen to the west side of it. Several of these great cracks
obviously diverge from a small crater near the west external bank of the
great one, and they subdivide or branch out, as they extend from the
apparent point of divergence, while they are crossed or intersected by
others. These cracks or chasms (for their width merits the latter
appellation) are nearly one mile broad at the widest part, and after
extending to fully 100 miles, taper away till they become invisible.
Although they are not test objects of the highest order of difficulty,
yet to see them with perfect distinctness requires an instrument of some
perfection and all the conditions of good vision. When such are present,
a keen and practised eye will find many details to rivet its attention,
among which are certain portions of the edges of these cracks or chasms
which have fallen in and caused interruptions to their continuity.

THEOPHILUS, 97 (22·3—120·0). CYRILLUS, 96 (23·5—121·3). CATHARINA, 95
(24·7—124·0). Plate XII.

These three magnificent craters form a very conspicuous group near the
middle of the south-east quarter of the lunar disc.

Their respective diameters and depths are as follows:—

Theophilus, 64 miles diameter; depth of plateau from summit of crater
wall, 16,000 feet; central cone, 5200 feet high.

Cyrillus, 60 miles diameter; depth of plateau from summit of crater
wall, 15,000 feet; central cone, 5800 feet high.

Catharina, 65 miles diameter; depth of plateau from summit of crater
wall, 13,000 feet; centre of plateau occupied by a confused group of
minor craters and débris.

Each of these three grand craters is full of interesting details,
presenting in every variety the characteristic features which so
fascinate the attention of the careful observer of the moon’s wonderful
surface, and affording unmistakable evidence of the tremendous energy of
the volcanic forces which at some inconceivably remote period piled up
such gigantic formations.

Theophilus by its intrusion within the area of Cyrillus shows in a very
striking manner that it is of comparatively more recent formation than
the latter crater. There are many such examples in other parts of the
lunar disc, but few of so very distinct and marked a character.

The flanks or exterior banks of Theophilus, especially those on the west
side, are studded with apparently minute craters, all of which when
carefully scrutinized are found to be of the true volcanic type of
structure; and minute as they are, by comparison, they would to a
beholder close to them appear as very imposing objects; but so gigantic
are the more notable craters in the neighbourhood, that we are apt to
overlook what are in themselves really large objects. It is only by duly
training the mind, as we have previously urged, so as ever to keep
before us the vast scale on which the volcanic formations of the lunar
surface are displayed, that we can do them the justice which their
intrinsic grandeur demands. We trust that our illustrations may in some
measure tend to educate the mind’s eye, so as to derive to the full the
tranquil enjoyment which results from the study of the manifestation of
one of the Creator’s most potent agencies in dealing with the materials
of his worlds, namely, volcanic force. So rich in wonderful features and
characteristic details is this magnificent group and its neighbourhood,
that a volume might be filled in the attempt to do justice, by
description, to objects so full of suggestive subject for study.

THEBIT, 85—(40·8—126·8).

A crater about 32 miles in diameter and about 9700 feet deep, devoid of
a central cone. It appears on the upper part and near the middle of
Plate XIII. The plateau has five minute craters upon it. On the east
outside are two small craters, the lesser of which, about 2·75 miles
diameter, has a central cone. We specially note this fact, because it is
the smallest crater but one in which we have detected a central cone: no
doubt, however, many smaller craters possess this unmistakable stamp of
true volcanic origin, but so minute are the specks of light which the
central cones of such very small craters reflect, that they fail to be
visible to us.

East of Thebit is a very remarkable straight cliff 60 miles long by
about 1000 feet high, called by some observers the “Railway,” and
apparently the result either of an upheaval or of a down-sinking of the
surface of the circular area across whose diameter it stretches.

Under moderate magnifying power, this cliff appears straight, but with
higher power and under favourable conditions, its face is seen to be
serrated, and along the upper edge may be detected several very minute
craters. A more conspicuous small crater is seen at the north end of the
cliff. To the east of the cliff nearly opposite the centre are two
craters, from the east side of the larger of which proceeds a fine crack
parallel to the cliff and passing through a dome-shaped hill of low
eminence.

PLATO, 210 (41·8—81·8). Plate XIV.

This crater, besides being a conspicuous object on account of its great
diameter, has many interesting details in and around it requiring a fine
instrument and favourable circumstances to render them distinctly
visible. The diameter of the crater is 70 miles; the surrounding wall or
rampart varies in height from 4000 to upwards of 8000 feet, and is
serrated with noble peaks which cast their black shadows across the
plateau in a most picturesque manner, like the towers and spires of a
great cathedral. Reference to our illustration will convey a very fair
idea of this interesting appearance. On the north-east inside of the
circular wall or rampart may be observed a fine example of landslip, or
sliding down of a considerable mass of the interior side of the crater’s
wall. The landslip nature of this remarkable detail is clearly
established by the fact of the bottom edge of the downslipped mass
projecting in towards the centre of the plateau to a considerable
extent. Other smaller landslip features may be seen, but none on so
grand and striking a scale as the one referred to. A number of
exceedingly minute craters may be detected on the surface of the
plateau. The plateau itself is remarkable for its low reflective power,
which causes it to look like a dingy spot when Plato is viewed with a
small magnifying power. The exterior of the crater wall is remarkable
for the rugged character of its formation, and forms a great contrast in
that respect to the comparatively smooth unbroken surface of the
plateau, which by the way is devoid of a central cone. The surrounding
features and objects indicated in our illustration are of the highest
interest, and a few of them demand special description.

THE VALLEY OF THE ALPS (37·0—86·0). Plate XIV.

This remarkable object lays somewhat diagonally to the west of Plato;
when seen with a low magnifying power (80 or 100), it appears as a rut
or groove tapering towards each extremity. It measures upwards of 75
miles long by about six miles wide at the broadest part. When examined
under favourable circumstances, with a magnifying power of from 200 to
300, it is seen to be a vast flat-bottomed valley bordered by gigantic
mountains, some of which attain heights upwards of 10,000 feet; towards
the south-east of this remarkable valley, and on both sides of it, are
groups of isolated mountains, several of which are fully 8000 feet high.
This flat-bottomed valley, which has retained the integrity of its form
amid such disturbing forces as its immediate surroundings indicate, is
one of the many structural enigmas with which the lunar surface abounds.
To the north-west of the valley a vast number of isolated mounds or
small mountains of exudation may be seen; so numerous are they as to
defy all attempts to count them with anything like exactness; and among
them, a power of 200 to 300 will enable an observer, under favourable
circumstances, to detect vast numbers of small but perfectly-formed
craters.

PICO, 211 (41·9—87·3). Plate XIV.

This is one of the most interesting examples of an isolated volcanic
“mountain of exudation,” and it forms a very striking object when seen
under favourable circumstances. Its height is upwards of 8000 feet, and
it is about three times as long at the base as it is broad. The summit
is cleft into three peaks, as may be ascertained by the three-peaked
shadow it casts on the plain. Five or six minute craters of very perfect
form may be detected close to the base of this magnificent mountain.
There are several other isolated peaks or mountains of the same class
within 30 or 40 miles of it which are well worthy of careful scrutiny,
but Pico is the master of the situation, and offers a glorious subject
for realizing a lunar day-dream in the mind’s eye, if we can only by an
effort of imagination conceive its aspect under the fiercely brilliant
sunshine by which it is illuminated, contrasted with the intensely black
lunar heavens studded with stars shining with a steady brightness of
which, by reason of _our_ atmosphere intervening, we can have no
adequate conception save by the aid of a well-directed imagination.

TYCHO, 30 (43·0—142·3). Plate XVI.

This magnificent crater, which occupies the centre of the crowded group
in our Plate, is 54 miles in diameter, and upwards of 16,000 feet deep,
from the highest ridge of the rampart to the surface of the plateau,
whence rises a grand central cone 5000 feet high. It is one of the most
conspicuous of all the lunar craters, not so much on account of its
dimensions as from its occupying the great focus of disruption from
whence diverge those remarkable bright streaks, many of which may be
traced over 1000 miles of the moon’s surface, disregarding in their
course all interposing obstacles. There is every reason to conclude that
Tycho is an instance of a vast disruptive action which rent the solid
crust of the moon into radiating fissures, which were subsequently
occupied by extruded molten matter, whose superior luminosity marks the
course of the cracks in all directions from the crater as their common
centre of divergence. So numerous are these bright streaks when examined
by the aid of the telescope, and they give to this region of the moon’s
surface such an extra degree of luminosity, that, when viewed as a
whole, their locality can be distinctly seen at full moon by the
unassisted eye as a bright patch of light on the southern portion of the
disc. (See Plate III.) The causative origin of the streaks is discussed
and illustrated in Chapter XI.

The interior of this fine crater presents striking examples of the
concentric terrace-like formations that we have elsewhere assigned to
vast landslip actions. Somewhat similar concentric terraces may be
observed in other lunar craters; some of these, however, appear to be
the results of some temporary modification of the ejective force, which
has caused the formation of more or less perfect inner ramparts: what we
conceive to be true landslip terraces are always distinguished from
these by their more or less fragmentary character.

On reference to Plate III., showing the full moon, a very remarkable and
special appearance will be observed in a dingy district or zone
immediately surrounding the exterior of the rampart of Tycho, and of
which we venture to hazard what appears to us a rational explanation:
namely, that as Tycho may be considered to have acted as a sort of
safety-valve to the rending and ejective force which caused, in the
first instance, the cracking of this vast portion of the moon’s
crust—the molten matter that appears to have been forced up through
these cracks, on finding a comparatively free exit by the vent of Tycho,
so relieved the district immediately around him as to have thereby
reduced, in amount, the exit of the molten matter, and so left a zone
comparatively free from the extruded lava which, according to our view
of the subject, came up simultaneously through the innumerable fissures,
and, spreading sideways along their courses, left everlasting records of
the original positions of the radiating cracks in the form of the bright
streaks which we now behold.

“WARGENTIN,” 26 (57·5—140·2). Plate XVII.

This object is quite unique of its kind—a crater about 53 miles across
that to all appearance has been filled to the brim with lava that has
been left to consolidate. There are evidences of the remains of a
rampart, especially on the south-west portion of the rim. The general
aspect of this extraordinary object has been not unaptly compared to a
“thin cheese.” The terraced and rutted exterior of the rampart has all
the usual characteristic details of the true crater. The surface of the
high plateau is marked by a few ridges branching from a point nearly in
its centre, together with some other slight elevations and depressions;
these, however, can only be detected when the sun’s rays fall nearly
parallel to the surface of the plateau.

To the north of this interesting object is the magnificent ring
formation Schickard, whose vast diameter of 123 miles contrasts
strikingly with that of the sixteen small craters within his rampart,
and equally so with a multitude of small craters scattered around. There
are many objects of interest on the portion of the lunar surface
included within our illustration, but as they are all of the usual type,
we shall not fatigue the attention of our readers by special
descriptions of them.

ARISTARCHUS, 176 (6·3—99·2), and HERODOTUS, 175 (63·2—99·6). Plate XVIII.

These two fine examples of lunar volcanic craters are conspicuously
situated in the north-east quarter of the moon’s disc. Aristarchus has a
circular rampart nearly 28 miles diameter, the summit of which is about
7500 feet above the surface of the plateau, while its height above the
general surface of the moon is 2600 feet. A central cone having several
subordinate peaks completes the true volcanic character of this crater:
its rampart banks, both outside and inside, have fine examples of the
segmental crescent-shaped ridges or landslips, which form so constant
and characteristic a feature in the structure of lunar craters. Several
very notable cracks or chasms may be seen to the north of these two
craters. They are contorted in a very unusual and remarkable manner, the
result probably of the force which formed them having to encounter very
varying resistance near the surface.

Some parts of these chasms gape to the width of two to three miles, and
when closely scrutinized are seen to be here and there partly filled by
masses which have fallen inward from their sides. Several smaller
craters are scattered around, which, together with the great chasms and
neighbouring ridges, give evidence of varied volcanic activity in this
locality. We must not omit to draw attention to the parallelism or
general similarity of “strike” in the ridges of extruded matter; this
appearance has special interest in the eyes of geologists, and is well
represented in our illustration.

Aristarchus is specially remarkable for the extraordinary capability
which the material forming its interior and rampart banks has of
reflecting light. Although there are many portions of the lunar surface
which possess the same property, yet few so remarkably as in the case of
Aristarchus, which shines with such brightness, as compared with its
immediate surroundings, as to attract the attention of the most
unpractized observer. Some have supposed this appearance to be due to
active volcanic discharge still lingering on the lunar surface, an idea
in which, for reasons to be duly adduced, we have no faith. Copernicus,
in the remarkable bright streaks which radiate from it, and Tycho also,
as well as several other spots, are apparently composed of material very
nearly as highly reflective as that of Aristarchus. But the comparative
isolation of Aristarchus, as well as the extraordinary light-reflecting
property of its material, renders it especially noticeable, so much so
as to make it quite a conspicuous object when illuminated only by
earth-light, when but a slender crescent of the lunar disc is
illuminated, or when, as during a lunar eclipse, the disc of the moon is
within the shadow of the earth, and is lighted only by the rays
refracted through the earth’s atmosphere.

There are no features about Herodotus of any such speciality as to call
for remark, except it be the breach of the north side of its rampart by
the southern extremity of a very remarkable contorted crack or chasm,
which to all appearance owes its existence to some great disruptive
action subsequent to the formation of the crater.

WALTER, 48 (37·8—131·9), and adjacent Intrusive Craters. Plate XX.

This Plate represents a southern portion of the moon’s surface measuring
170 by 230 miles. It includes upwards of 200 craters of all dimensions,
from Walter, whose rampart measures nearly 70 miles across, down to
those of such small apparent diameter as to require a well practized eye
to detect them. In the interior of the great crater Walter a remarkable
group of small craters may be observed surrounding his central cone,
which in this instance is not so perfectly in the centre of the rampart
as is usually the case. The number of small craters which we have
observed within the rampart is 20, exclusive of those on the rampart
itself. The entire group represented in the Plate suggests in a striking
manner the wild scenery which must characterize many portions of the
lunar surface; the more so if we keep in mind the vast proportions of
the objects which they comprise, upon which point we may remark that the
smallest crater represented in this Plate is considerably larger than
that of Vesuvius.

ARCHIMEDES, 191 (40·3—95·8), AUTOLYCUS, 189 (36·8—95·5), ARISTILLUS,
190 (37·0—93·3), and the APENNINES. Plate IX.

This group of three magnificent craters, together with their remarkable
surroundings, especially including the noble range of mountains termed
the Apennines, forms on the whole one of the most striking and
interesting portions of the lunar surface. If the reader is not
acquainted with what the telescope can reveal as to the grandeur of the
effect of sunrise on this very remarkable portion of the moon’s surface,
he should carefully inspect and study our illustration of it; and if he
will pay due regard to our previously repeated suggestion concerning the
attached scale of miles, he will, should he have the good fortune to
study the actual objects by the aid of a telescope, be well prepared to
realize and duly appreciate the magnificence of the scene which will be
presented to his sight.

Were we to attempt an adequate detail description of all the interesting
features comprised within our illustration, it would, of itself, fill a
goodly volume; as there is included within the space represented every
variety of feature which so interestingly characterizes the lunar
surface. All the more prominent details are types of their class; and
are so favourably situated in respect to almost direct vision, as to
render their nature, forms, and altitudes above and depths below the
average surface of the moon most distinctly and impressively cognizable.

Archimedes is the largest crater in the group; it has a diameter of
upwards of 52 miles, measuring from summit to summit of its vast
circular rampart or crater wall, the average height of which, above the
plateau, is about 4300 feet; but some parts of it rise considerably
higher, and, in consequence, cast steeple-like shadows across the
plateau when the sun’s rays are intercepted by them at a low angle. The
plateau of this grand crater is devoid of the usual central cone. Two
comparatively minute but beautifully-formed craters may be detected
close to the north-east interior side of the surrounding wall of the
great crater. Both outside and inside of the crater wall may be seen
magnificent examples of the landslip subsidence of its overloaded banks;
these landslips form vast concentric segments of the outer and inner
circumference of the great circular rampart, and doubtless belong to its
era of formation. Two very fine examples of cracks, or chasms, may be
observed proceeding from the opposite external sides of the crater, and
extending upwards of 100 miles in each direction; these cracks, or
chasms, are fully a mile wide at their commencement next the crater, and
narrow away to invisibility at their further extremity. Their course is
considerably crooked, and in some parts they are partially filled by
masses of the material of their sides, which have fallen inward and
partially choked them. The depths of these enormous chasms must be very
great, as they probably owe their existence to some mighty upheaving
action, which there is every reason to suppose originated at a profound
depth, since the general surface on each side of the crater does not
appear to be disturbed as to altitude, which would have been the case
had the upheaving action been at a moderate depth beneath. We would
venture to ascribe a depth of not less than ten miles as the most
moderate estimate of the profundity of these terrible chasms. If the
reader would realize the scale of them, let him for a moment imagine
himself a traveller on the surface of the moon coming upon one of them,
and finding his onward progress arrested by the sudden appearance of its
vast black yawning depths; for by reason of the angle of his vision
being almost parallel to the surface, no appearance of so profound a
chasm would break upon his sight until he came comparatively close to
its fearful edge. Our imaginary lunar traveller would have to make a
very long détour, ere he circumvented this terrible interruption to his
progress. If the reader will only endeavour to realize in his mind’s eye
the terrific grandeur of a chasm a mile wide and of such dark profundity
as to be, to all appearance, fathomless—portions of its rugged sides
fallen in wild confusion into the jaws of the tortuous abyss, and
catching here and there a ray of the sun sufficient only to render the
darkness of the chasm more impressive as to its profundity—he will, by
so doing, learn to appreciate the romantic grandeur of this, one of the
many features which the study of the lunar surface presents to the
careful observer, and which exceed in sublimity the wildest efforts of
poetic and romantic imagination. The contemplation of these views of the
lunar world are, moreover, vastly enhanced by special circumstances
which add greatly to the impressiveness of lunar scenery, such as the
unchanging pitchy-black aspect of the heavens and the death-like silence
which reigns unbroken there.

These digressions are, in some respects, a forestallment of what we have
to say by-and-by, and so far they are out of place; but with the
illustration to which the above remarks refer placed before the reader,
they may, in some respects, enhance the interest of its examination.

The upper portion of our illustration is occupied by the magnificent
range of volcanic mountains named after our Apennines, extending to a
length of upwards of 450 miles. This mountain group rises gradually from
a comparatively level surface towards the south-west, in the form of
innumerable comparatively small mountains of exudation, which increase
in number and altitude towards the north-east, where they culminate and
suddenly terminate in a sublime range of peaks, whose altitude and
rugged aspect must form one of the most terribly grand and romantic
scenes which imagination can conceive. The north-east face of the range
terminates abruptly in an almost vertical precipitous face, and over the
plain beneath intense black steeple or spire-like shadows are cast, some
of which at sunrise extend fully 90 miles, till they lose themselves in
the general shading due to the curvature of the lunar surface. Nothing
can exceed the sublimity of such a range of mountains, many of which
rise to heights of 18,000 to 20,000 feet at one bound from the plane at
their north-east base. The most favourable time to examine the details
of this magnificent range is from about a day before first quarter to a
day after, as it is then that the general structure of the range as well
as the character of the contour of each member of the group can, from
the circumstances of illumination then obtaining, be most distinctly
inferred.

Several comparatively small perfectly-formed craters are seen
interspersed among the mountains, giving evidence of the truly volcanic
character of the surrounding region, which, as before said, comprises in
a comparatively limited space the most perfect and striking examples of
nearly every class of lunar volcanic phenomena.

We have endeavoured on Plate XXIII. to give some idea of a landscape
view of a small portion of this mountain range.

Continue Reading

COSMICAL HEAT

In this Chapter we propose to treat briefly of the probable formation of
the various members of the solar system from matter which previously
existed in space in a condition different from that in which we at
present find it—_i.e._, in the form of planets and satellites.

It is almost impossible to conceive that our world with its satellite,
and its fellow worlds with their satellites, and also the great centre
of them all, have always, from the commencement of time, possessed their
present form: all our experiences of the working of natural laws rebel
against such a supposition. In every phenomenon of nature upon this
earth—the great field from which we must glean our experiences and form
our analogies—we see a constant succession of changes going on, a
constant progression from one stage of development to another taking
place, a perpetual mutation of form and nature of the same material
substance occurring: we see the seed transformed into the plant, the
flower into the fruit, and the ovum into the animal. In the inorganic
world we witness the operation of the same principle; but, by reason of
their slower rate of progression, the changes there are manifested to us
rather by their resulting effects than by their visible course of
operation. And when we consider, as we are obliged to do, that the same
laws work in the greatest as well as the smallest processes of nature,
we are compelled to believe in an antecedent state of existence of the
matter that composes the host of heavenly bodies, and amongst them the
earth and its attendant moon.

In the pursuit of this course of argument we are led to inquire whether
there exists in the universe any matter from which planetary bodies
could be formed, and how far their formation from such matter can be
explained by the operation of known material laws.

Before the telescope revealed the hidden wonders of the skies, and
brought its rich fruits into our garner of knowledge concerning the
nature of the universe, the philosophic minds of some early astronomers,
Kepler and Tycho Brahe to wit, entertained the idea that the sun and the
stars—the suns of distant systems—were formed by the condensation of
celestial vapours into spherical bodies; Kepler basing his opinion on
the phenomena of the sudden shining forth of new stars on the margin of
the Milky Way. But it was when the telescope pierced into the depths of
celestial space, and brought to light the host of those marvellous
objects, the nebulæ, that the strongest evidence was afforded of the
probable validity of these suppositions. The mention of “nebulous stars”
made by the earlier astronomers refers only to clusters of telescopic
stars which the naked eye perceives as small patches of nebulous light;
and it does not appear that even the nebula in Andromeda, although so
plainly discernible as to be often now-a-days mistaken by the
uninitiated for a comet, was known, until it was discovered by means of
a telescope, in 1612, by Simon Marius, who described it as resembling a
candle shining through semi-transparent horn, as in a lantern, and
without any appearance of stars. Forty years after this date Huygens
discovered the splendid nebula in the sword handle of Orion, and in 1665
another was detected by Hevelius. In 1667 Halley (afterwards Astronomer
Royal) discovered a fourth; a fifth was found by Kirsch in 1681, and a
sixth by Halley again in 1714. Half a century after this the labours of
Messier expanded the list of known nebulæ and clusters to 103, a
catalogue of which appeared in the “Connaissance du Temps” (the French
“Nautical Almanac”) for the years 1783-1784. But this branch of
celestial discovery achieved its most brilliant results when the rare
penetration, the indomitable perseverance, and the powerful instruments
of the elder Herschel were brought to bear upon it. In the year 1779
this great astronomer began to search after nebulæ with a seven-inch
reflector, which he subsequently superseded by the great one of forty
feet focus and four feet aperture. In 1786 he published his first
catalogue of 1000 nebulæ; three years later he astonished the learned
world by a second catalogue containing 1000 more, and in 1802 a third
came forth comprising other 500, making 2500 in all! This number has
been so far increased by the labours of more recent astronomers that the
last complete catalogue, that of Sir John Herschel, published a few
years ago, contains the places of 5063 nebulæ and clusters.

At the earlier periods of Herschel’s observations, that illustrious
observer appears to have inclined to the belief that all nebulæ were but
remote clusters of stars, so distant, so faint, and so thickly
agglomerated as to affect the eye only by their combined luminosity, and
at this period of the nebular history it was supposed that increased
telescopic power would resolve them into their component stars. But the
familiarity which Herschel gained with the phases of the multitudinous
nebulæ that passed in review before his eyes, led him ultimately to
adopt the opinion, advanced by previous philosophers, that they were
composed of some vapoury or elementary matter out of which, by the
process of condensation, the heavenly bodies were formed; and this led
him to attempt a classification of the known nebulæ into a cosmical
arrangement, in which, regarding a chaotic mass of vapoury matter as the
primordial state of existence, he arranged them into a series of stages
of progressive development, the individuals of one class being so nearly
allied to those in the next that, to use his own expression, not so much
difference existed between them “as there would be in an annual
description of the human figure were it given from the birth of a child
till he comes to be a man in his prime.” (_Philosophical Transactions,
Vol. CI., pp. 271_, _et seq._)

His category comprises upwards of thirty classes or stages of
progression, the titles of a few of which we insert here to illustrate
the completeness of his scheme.

Class 1. Of extensive diffused nebulosity. (A table of 52
patches of such nebulosity actually observed is given,
some of which extend over an area of five or six square
degrees, and one of which occupies nine square degrees.)
” 6. Of milky nebulosity with condensation.
” 15. Of nebulæ that are of an irregular figure.
” 17. Of round nebulæ.
” 20. Of nebulæ that are gradually brighter in the middle.
” 25. Of nebulæ that have a nucleus.
” 29. Of nebulæ that draw progressively towards a period of
final condensation.
” 30. Of planetary nebulæ.
” 33. Of stellar nebulæ nearly approaching the appearance of
stars.

In a walk through a forest we see trees in every stage of growth, from
the tiny sapling to the giant of the woods, and no doubt can exist in
our minds that the latter has sprung from the former. We cannot at a
passing glance discern the process of development actually going on; to
satisfy ourselves of this, we must record the appearance of some single
tree from time to time through a long series of years. And what a walk
through a forest is to an observer of the growth of a tree, a lifetime
is to the observer of changes in such objects as the nebulæ. The
transition from one state to another of the nebulous development is so
slow that a lifetime is hardly sufficient to detect it. Nor can any
precise evidence of change be obtained by the comparison of drawings or
descriptions of nebulæ at various epochs, with whatever care or skill
such drawings be made, for it will be admitted that no two draughtsmen
will produce each a drawing of the most simple object from the same
point of view, in which every detail in the one will coincide exactly
with every detail in the other. There is abundant evidence of this in
the existing representations of the great nebula in Orion; a comparison
of the drawings that have been lately made of this object, with the most
perfect instruments and by the most skilful of astronomical draughtsmen,
reveals varieties of detail and even of general appearance such as could
hardly be imagined to occur in similar delineations of one and the same
subject; and any one who himself makes a perfectly unbiassed drawing at
the telescope will find upon comparison of it with others that it will
offer many points of difference. The fact is that the drawing of a man,
like his penmanship, is a personal characteristic, peculiar to himself,
and the drawings of two persons cannot be expected to coincide any more
than their handwritings. The appearance of a nebula varies also to a
great extent with the power of the telescope used to observe it and the
conditions under which it is observed; the drawings of nebulæ made with
the inferior telescopes of a century or two centuries ago, the only ones
that, by comparison with those made in modern times, could give
satisfactory evidence of changes of form or detail, are so rude and
imperfect as to be useless for the purpose, and it is reasonable to
suppose that those made in the present day will be similarly useless a
century or two hence. Since then we can obtain no evidence of the
changes we must assume these mysterious objects to be undergoing, _ipso
facto_, by observation of _one nebula_ at _various periods_, we must for
the present accept the _primâ facie_ evidence offered (as in the case of
the trees in a forest) by the observation of _various nebulæ_ at _one
period_.

“The total dissimilitude,” says Herschel at the close of the
observations we have alluded to, “between the appearance of a diffusion
of the nebulous matter and of a star, is so striking, that an idea of
the conversion of the one into the other can hardly occur to any one who
has not before him the result of the critical examination of the
nebulous system which has been displayed in this [his] paper. The end I
have had in view, by arranging my observations in the order in which
they have been placed, has been to show that the above mentioned
extremes may be connected by such nearly allied intermediate steps, as
will make it highly probable that every succeeding state of the nebulous
matter is the result of the action of gravitation upon it while in a
foregoing one, and by such steps the successive condensation of it has
been brought up to the planetary condition. From this the transit to the
stellar form, it has been shown, requires but a very small additional
compression of the nebulous matter.”

Where the researches of Herschel terminated those of Laplace commenced.
Herschel showed how a mass of nebulous matter so diffused as to be
scarcely discernible might be and probably was, by the mere action of
gravitation, condensed into a mass of comparatively small dimensions
when viewed in relation to the immensity of its primordial condition.
Laplace demonstrated how the known laws of gravitation could and
probably did from such a partially condensed mass of matter produce an
entire planetary system with all its subordinate satellites.

The first physicist who ventured to account for the formation of the
various bodies of our solar system was Buffon, the celebrated French
naturalist. His theory, which is fully detailed in his renowned work on
natural history, supposed that at some period of remote antiquity the
sun existed without any attendant planets, and that a comet having
dashed obliquely against it, ploughed up and drove off a portion of its
body sufficient in bulk to form the various planets of our system. He
suggests that the matter thus carried off “at first formed a torrent the
grosser and less dense parts of which were driven the farthest, and the
densest parts, having received only the like impulsion, were not so
remotely removed, the force of the sun’s attraction having retained
them:” that “the earth and planets therefore at the time of their
quitting the sun were burning and in a state of liquefaction;” that “by
degrees they cooled, and in this state of fluidity they took their
form.” He goes on to say that the obliquity of the stroke of the comet
might have been such as to separate from the bodies of the principal
planets small portions of matter, which would preserve the same
direction of motion as the principal planets, and thus would form their
attendant satellites.

The hypothesis of Buffon, however, is not sufficient to explain all the
phenomena of the planetary system; and it is imperfect, inasmuch as it
begins by assuming the sun to be already existing, whereas any theory
accounting for the primary formation of the solar system ought
necessarily to include the origination of the most important body
thereof, the sun itself. Nevertheless, it is but due to Buffon to
mention his ideas, for the errors of one philosophy serve a most useful
end by opening out fields of inquiry for subsequent and more fortunate
speculators.

Laplace, dissatisfied with Buffon’s theory, sought one more probable,
and thus was led to the proposition of the celebrated _nebular
hypothesis_ which bears his name, and which, in spite of its
disbelievers, has never been overthrown, but remains the only probable,
and, with our present knowledge, the only possible explanation of the
cosmical origin of the planets of our system. Although Laplace puts
forth his conjectures, to use his own words, “with the deference which
ought to inspire everything that is not a result of observation and
calculation,” yet the striking coincidence of all the planetary
phenomena with the conditions of his system gives to those conjectures,
again to use his modest language, “a probability strongly approaching
certitude.”

Laplace conceived the sun to have been at one period the nucleus of a
vast nebula, the attenuated surrounding matter of which extended beyond
what is now the orbit of the remotest planet of the system. He supposed
that this mass of matter in process of condensation possessed a rotatory
motion round its centre of gravity, and that the parts of it that were
situated at the limits where centrifugal force exactly counterbalanced
the attractive force of the nucleus were abandoned by the contracting
mass, and thus were formed successively a number of rings of matter
concentric with and circulating around the central nucleus. As it would
be improbable that all the conditions necessary to preserve the
stability of such rings of matter in their annular form could in all
cases exist, they would break up into masses which would be endued with
a motion of rotation, and would in consequence assume a spheroidal form.
These masses, which hence constituted the various planets, in their turn
condensing, after the manner of the parent mass, and abandoning their
outlying matter, would become surrounded by similarly concentric rings,
which would break up and form the satellites surrounding the various
planetary masses; and, as a remarkable exception to the rule of the
instability of the rings and their consequent breakage, Laplace cited
the case of Saturn surrounded by his rings as the only instances of
unbroken rings that the whole system offers us; unless indeed we include
the zodiacal light, that cone of hazy luminosity that is frequently seen
streaming from our luminary shortly before and after sunset, and which
Laplace supposed to be formed of molecules of matter, too volatile to
unite either with themselves or with the planets, and which must hence
circulate about the sun in the form of a nebulous ring, and with such an
appearance as the zodiacal actually presents.

This hypothesis, although it could not well be refuted, has been by many
hesitatingly received, and for a reason which was at one time cogent. In
the earlier stages of nebular research it was clearly seen, as we have
previously remarked, that many of the so-called nebulæ, which appeared
at first to consist of masses of vapoury matter, became, when
scrutinised with telescopes of higher power, resolved into clusters
containing countless numbers of stars, so small and so closely
agglomerated, that their united lustre only impressed the more feeble
eye as a faint nebulosity; and as it was found that each accession of
telescopic power increased the numbers of nebulæ that were thus
resolved, it was thought that every nebula would at some period succumb
to the greater penetration of more powerful instruments; and if this
were the case, and if no real nebulæ were hence found to exist, how, it
was argued, could the nebular hypothesis be maintained? One of the most
important nebulæ bearing upon this question was the great one in the
sword handle of Orion, one of the grandest and most conspicuous in the
whole heavens. On account of the brightness of some portions of this
object, it seemed as though it ought to be readily resolvable, supposing
all nebulæ to consist of stars, but all attempts to resolve it were in
vain, even with the powerful telescopes of Sir John Herschel and the
clear zenithal sky of the Cape of Good Hope. At length the question was
thought to be settled, for upon the completion of Lord Rosse’s giant
reflector, and upon examination of the nebula with it, his lordship
stated that there could be little, if any, doubt as to its
resolvability, and then it was maintained, by the disbelievers in the
nebular theory, that the last stronghold of that theory had been broken
down.

But the truths of nature are for ever playing at hide and seek with
those who follow them:—the dogmas of one era are the exploded errors of
the next. Within the past few years a new science has arisen that
furnishes us with fresh powers of penetration into the vast and secret
laboratories of the universe; a new eye, so to speak, has been given us
by which we may discern, by the mere light that emanates from a
celestial body, something of the chemical elements of which it is
composed. When Newton two hundred years ago toyed with the prism he
bought at Stourbridge fair, and projected its pretty rainbow tints upon
the wall, his great mind little suspected that that phantom riband of
gorgeous colours would one day be called upon to give evidence upon the
probable cosmical origin of worlds. Yet such in truth has been the case.
Every substance when rendered luminous gives off light of some colour or
degree of refrangibility peculiar to itself, and although the eye cannot
detect any difference between one character of light and another, the
prism gives the means of ascertaining the quality and degree of
refrangibility of the light emanating from any source however distant,
and hence of gaining some knowledge of the nature of that source. If,
for instance, a ray of light from a solid body in combustion is passed
through a prism, a spectrum is produced which exhibits light of all
colours or all degrees of refrangibility; if the light from such a body,
before passing through the prism, be made to pass through gases or
certain metallic vapours, the resulting spectrum is found to be crossed
transversely by numbers of fine dark lines, apparently separating the
various colours, or cutting the spectrum into bands. The solar spectrum
is of this class; the once mysterious lines first observed by Wollaston,
and subsequently by Fraunhoffer, and known as “Fraunhoffer’s lines,”
have now been interpreted, chiefly by the sagacious German chemist
Kirchoff, and identified as the effects of absorption of certain of the
sun’s rays by chemical vapours contained in his atmosphere. The fixed
stars yield spectra of the same character, but varying considerably in
feature, the lines crossing the stellar spectra differing in position
and number from those of the sun, and one star from another, proving the
stars to possess varied chemical constitutions. But there is another
class of spectra, exhibited when light from other sources is passed
through the prism. These consist, not of a luminous riband of light like
the solar spectrum, but of bright isolated lines of coloured light with
comparatively wide dark spaces separating them. Such spectra are yielded
only by the light emitted from luminous gases and metals or chemical
elements in the condition of incandescent vapour. Every gas or element
in the state of luminous vapour yields a spectrum peculiar to itself,
and no two elements when vapourized before the prism show the same
combinations of luminous lines.

Now in the course of some observations upon the spectra of the fixed
stars by Dr. Huggins, it occurred to that gentleman to turn his
telescope, armed with a spectroscope, upon some of the brighter of the
nebulæ, and great was his surprise to find that instead of yielding
continuous spectra, as they must have done had their light been made up
of that of a multitude of stars, they gave spectra containing only two
or three isolated bright lines; such a spectrum could only be produced
by some luminous gas or vapour, and of this form of matter we are now
justified in declaring, upon the strength of numerous modern
observations, these remarkable bodies are composed; and it is a curious
and interesting fact that some of the nebulæ styled resolvable, from the
fact of their exhibiting points of light like stars, yield these gaseous
spectra, whence Dr. Huggins concludes that the brighter points taken for
stars are in reality nuclei of greater condensation of the nebular
matter: and so the fact of the apparent resolvability of a nebula
affords no positive proof of its non-nebulous character.

These observations—which have been fully confirmed by Father Secchi of
the Roman College—by destroying the evidence in favour of nebulæ being
remote clusters, add another attestation to the probability of the truth
of the nebular hypothesis, and we have now the confutation of the
luminologist to add to that of the astronomers who, in the person of the
illustrious Arago, asserted that the ideas of the great author of the
“Mécanique Céleste” “were those only which by their grandeur, their
coherence, and their mathematical character could be truly considered as
forming a physical cosmogony.”

Confining, then, our attention to the single object of the universe it
is our task to treat of—the Moon—and without asserting as an
indisputable fact that which we can never hope to know otherwise than by
inference and analogy, we may assume that that body once existed in the
form of a vast mass of diffused or attenuated matter, and that, by the
action of gravitation upon the particles of that matter, it was
condensed into a comparatively small and compact planetary body.

But while the process of condensation or compaction was going on,
another important law of nature—but recently unfolded to our
knowledge—was in powerful operation, the discussion of which law we
reserve for a separate Chapter.

In the preceding Chapter we endeavoured to show how the action of
gravitation upon the particles of diffused primordial matter would
result in the formation, by condensation and aggregation, of a spherical
planetary body. We have now to consider another result of the
gravitating action, and for this we must call to our aid a branch of
scientific enquiry and investigation unrecognized as such at the period
of Laplace’s speculations, and which has been developed almost entirely
within the past quarter of a century.

The “great philosophical doctrine of the present era of science,” as the
subject about to engage our attention has been justly termed, bears the
title of the “Conservation of Force,” or—as some ambiguity is likely to
attend the definition of the term “Force”—the “Conservation of Energy.”
The basis of the doctrine is the broad and comprehensive natural law
which teaches us that the quantity of force comprised by the universe,
like the quantity of matter contained in it, is a fixed and invariable
amount, which can be neither added to nor taken from, but which is for
ever undergoing change and transformation from one form to another. That
we cannot create force ought to be as obvious a fact as that we cannot
create matter; and what we cannot create we cannot destroy. As in the
universe we see no new matter created, but the same matter constantly
disappearing from one form and reappearing in another, so we can find no
new force ever coming into action—no description of force that is not to
be referred to some previous manner of existence.

Without entering upon a metaphysical discussion of the term “force,” it
will be sufficient for our purpose to consider it as something which
produces or resists motion, and hence we may argue that the ultimate
effect of force is motion. The force of gravity on the earth results in
the motion or tendency of all bodies towards its centre, and, similarly,
the action of gravitation upon the atoms or particles of a primeval
planet resulted in the motion of those particles towards each other. We
cannot conceive force otherwise than by its effects, or the motion it
produces.

And force we are taught is indestructible; therefore motion must be
indestructible also. But when a falling body strikes the earth, or a
gunshot strikes its target, or a hammer delivers a blow upon an anvil,
or a brake is pressed against a rotating wheel, motion is arrested, and
it would seem natural to infer that it is destroyed. But if we say it is
indestructible, what becomes of it? The philosophical answer to the
question is this—that the motion of the mass becomes transferred to the
particles or molecules composing it, and transformed to molecular
motion, and this molecular motion manifests itself to us as heat. The
particles or atoms of matter are held together by cohesion, or, in other
words, by the action of molecular attraction. When heat is applied to
these particles, motion is set up among them, they are set in vibration,
and thus, requiring and making wider room, they urge each other apart,
and the well-known _expansion by heat_ is the result. If the heat be
further continued a more violent molecular motion ensues, every increase
of heat tending to urge the atoms further apart, till at length they
overcome their cohesive attraction and move about each other, and a
_liquid or molten condition_ results. If the heat be still further
increased, the atoms break away from their cohesive fetters altogether
and leap off the mass in the form of vapour, and the matter thus assumes
the _gaseous or vaporous form_. Thus we see that the phenomena of heat
are phenomena of motion, and of motion only.

This mutual relation between heat and work presented itself as an embryo
idea to the minds of several of the earlier philosophers, by whom it was
maintained in opposition to the _material theory_ which held heat to be
a kind of matter or subtle fluid stored up in the inter-atomic spaces of
all bodies, capable of being separated and procured from them by rubbing
them together, but not generated thereby. Bacon, in his “Novum Organum,”
says that “heat itself, its essence and quiddity, is motion and nothing
else.” Locke defines heat as “a very brisk agitation of the insensible
parts of an object, which produces in us that sensation from whence we
denominate the object hot; so what in our sensation is _heat_, in the
object is nothing but _motion_.” Descartes and his followers upheld a
similar opinion. Richard Boyle, two hundred years ago, actually wrote a
treatise entitled “The Mechanical Theory of Heat and Cold,” and the
ingenious Count Rumford made some highly interesting and significant
experiments on the subject, which are described in a paper read before
the Royal Society in 1798, entitled “An Inquiry concerning the Source of
Heat excited by Friction.” But the conceptions of these authors remained
isolated and unfruitful for more than a century, and might have passed,
meantime, into the oblivion of barren speculation, but for the impulse
which this branch of inquiry has lately received. Now, however, they
stand forth as notable instances of truth trying to force itself into
recognition while yet men’s minds were unprepared or disinclined to
receive it. The key to the beautiful mechanical theory of heat was found
by these searching minds, but the unclasping of the lock that should
disclose its beauty and value was reserved for the philosophers of the
present age.

Simultaneously and independently, and without even the knowledge of each
other, three men, far removed from probable intercourse, conceived the
same ideas and worked out nearly similar results concerning the
mechanical theory of heat. Seeing that motion was convertible into heat,
and heat into motion, it became of the utmost importance to determine
the exact relation that existed between the two elements. The first who
raised the idea to philosophic clearness was Dr. Julius Robert Mayer, a
physician of Heilbronn in Germany. In certain observations connected
with his medical practice it occurred to him that there must be a
necessary equivalent between work and heat, a necessary numerical
relation between them. “The variations of the difference of colour of
arterial and venous blood directed his attention to the theory of
respiration. He soon saw in the respiration of animals the origin of
their motive powers, and the comparison of animals to thermic machines
afterwards suggested to him the important principle with which his name
will remain for ever connected.”

Next in order of publication of his results stands the name of Colding,
a Danish engineer, who about the year 1843 presented a series of memoirs
on the steam engine to the Royal Society of Copenhagen, in which he put
forth views almost identical with those of Mayer.

Last in publication order, but foremost in the importance of his
experimental treatment of the subject, was our own countryman, Dr. Joule
of Manchester. “Entirely independent of Mayer, with his mind firmly
fixed upon a principle, and undismayed by the coolness with which his
first labours appear to have been received, he persisted for years in
his attempts to prove the invariability of the relation which subsists
between heat and ordinary mechanical power.” (We are quoting from
Professor Tyndall’s valuable work on “Heat considered as a Mode of
Motion.”) “He placed water in a suitable vessel, agitated the water by
paddles, and determined both the amount of heat developed by the
stirring of the liquid and the amount of labour expended in its
production. He did the same with mercury and sperm oil. He also caused
discs of cast iron to rub against each other, and measured the heat
produced by their friction, and the force expended in overcoming it. He
urged water through capillary tubes, and determined the amount of heat
generated by the friction of the liquid against the sides of the tubes.
And the results of his experiments leave no shadow of doubt upon the
mind that, under all circumstances, the quantity of heat generated by
the same amount of force is fixed and invariable. A given amount of
force, in causing the iron discs to rotate against each other, produced
precisely the same amount of heat as when it was applied to agitate
water, mercury, or sperm oil. * * * * _The absolute amount of heat_
generated by the same expenditure of power, was in all cases the same.”

“In this way it was found that the quantity of heat which would raise
one pound of water one degree Fahrenheit in temperature, is exactly
equal to what would be generated if a pound weight, after having fallen
through a height of 772 feet, had its moving force destroyed by
collision with the earth. Conversely, the amount of heat necessary to
raise a pound of water one degree in temperature, would, if all applied
mechanically, be competent to raise a pound weight 772 feet high, or it
would raise 772 pounds one foot high. The term ‘foot pounds’ has been
introduced to express in a convenient way the lifting of one pound to
the height of a foot. Thus the quantity of heat necessary to raise the
temperature of a pound of water one degree Fahrenheit being taken as a
standard, 772 foot-pounds constitute what is called the _mechanical
equivalent_ of heat.”

By a process entirely different, and by an independent course of
reasoning, Mayer had, a few months previous to Joule, determined this
equivalent to be 771·4 foot-pounds. Such a remarkable coincidence
arrived at by pursuing different routes gives this value a strong claim
to accuracy, and raises the Mechanical Theory of Heat to the dignity of
an exact science, and its enunciators to the foremost place in the ranks
of physical philosophers.

In linking together the labours of the two remarkable men above alluded
to, Prof. Tyndall remarks, that “Mayer’s labours have in some measure
the stamp of profound intuition, which rose however to the energy of
undoubting conviction in the author’s mind. Joule’s labours, on the
contrary, are an experimental demonstration. Mayer _thought_ his theory
out, and rose to its grandest applications. Joule _worked_ his theory
out, and gave it the solidity of natural truth. True to the speculative
instinct of his country, Mayer drew large and mighty conclusions from
slender premises; while the Englishman aimed above all things at the
firm establishment of facts…. To each belongs a reputation which will
not quickly fade, for the share he has had, not only in establishing the
dynamical theory of heat, but also in leading the way towards a right
appreciation of the general energies of the universe.”

But from these generalities we must pass to the application of the
mechanical theory of heat to our special subject. We have learnt that
every form of motion is convertible into heat. We know that the falling
meteor or shooting star, whose motion is impeded by friction against the
earth’s atmosphere, is heated thereby to a temperature of incandescence.
Let us then suppose that myriads of such cosmical particles came into
collision from the effect of their mutual attraction, or that the
component atoms of a vast nebulous mass violently converged under the
like influence. What would follow? Obviously the generation of an
intense heat by the arrest of converging motion, such a heat as would
result in the fusion of the whole into one mass. Mayer, in one of his
most remarkable papers (“Celestial Dynamics”) remarks that the
“Newtonian theory of gravitation, whilst it enables us to determine,
from its present form, the earth’s state of aggregation in ages past, at
the same time points out to us a source of heat powerful enough to
produce such a state of aggregation—powerful enough to melt worlds: it
teaches us to consider the molten state of a planet as the result of the
mechanical union of cosmical masses, and to derive the radiation of the
sun and the heat in the bowels of the earth from a common origin.”

And the same laws that governed the formation of the earth, governed
also the formation of the moon: the variations of Nature’s operations
are _quantitative_ only and not _qualitative_. The Divine Will that made
the earth made the moon also, and the means and mode of working were the
same for both. The geological phenomena of the earth afford
unmistakeable evidence of its original fluid or molten condition, and
the appearance of the moon is as unmistakeably that of a body once in an
igneous or molten state. The enigma of the earth’s primary formation is
solved by the application of the dynamical theory of heat. By this
theory the generation of cosmical heat is removed from the quicksands of
conjecture and established upon the firm ground of direct calculation:
for the absolute amount of heat generated by the collision of a given
amount of matter is (of course, with some little uncertainty) deducible
from a mathematical formula. Mayer has computed the amount of heat that
the matter of the earth would have generated, if it had been formed
originally of only two parts drawn into collision by their mutual
attraction, and has found that it would be from 0 to 32,000 or 47,000[1]
Centigrade degrees, according as one part was infinitely small as
compared with the other, or as the two parts were of equal size.
Professor Helmholtz, another labourer in the same field of science, has
computed the amount of heat generated by the condensation of the whole
of the matter composing the solar system: this he finds would be
equivalent to the heat that would be required to raise the temperature
of a mass of water equal to the sum of the masses of all the bodies of
the system to 28,000,000 (twenty-eight million) degrees of the
Centigrade scale.

These examples afford abundant evidence of sufficient heat having been
generated by the aggregation of the matter of the moon to reduce it to a
state of fusion, and so to produce, from a nebulous chaos of diffused
cosmical matter, a molten body of definite outline and size.

It is requisite here to remark that fusion does not necessarily imply
combustion. It has been frequently asked, How can a volcanic theory of
the lunar phenomena be upheld consistently with the condition that it
possesses no atmosphere to support Fire? To this we would reply that to
produce a state of incandescence or a molten condition it is _not_
necessary that the body be surrounded by an atmosphere. The intensely
rapid motion of the particles of matter of bodies, which the dynamical
theory shows to be the origin of the molten state, exists quite
independently of such external matter as an atmosphere. The complex
mixture of gases and vapours which we term “air,” has nothing whatever
to do with the fusion of substances, whatever it may have to do with
their combustion. Combustion is a chemical phenomenon, due to the
combination of the oxygen of that air with the heated particles of the
combustible matter: oxygen is the sole supporter of combustion, and
hence combustion is to be regarded rather as a phenomenon of oxygen than
as a phenomenon of the matter with which that oxygen combines. The
greatest intensity of heat may exist without oxygen, and consequently
without combustion. In support of this argument it will be sufficient to
adduce, upon the authority of Dr. Tyndall, the fact that a platinum wire
can be raised to a luminous temperature and actually _fused_ in a
perfect vacuum.

But while the mass of condensing cosmical matter was thus accumulating
and forming the globe of the moon, the heat consequent upon the
aggregation of its particles was suffering some diminution from the
effect of radiation. So long as the radiated heat lost fell short of the
dynamical heat generated, no effect of cooling would be manifest; but
when the _vis viva_ of the condensing matter was all converted into its
equivalent of heat, or when the accession of heat fell short of that
radiated, a necessary cooling must ensue, and this cooling would be
accompanied by a solidification of that part of the mass which was most
free to radiate its heat into surrounding space: that part would
obviously be the outer surface.

With the solidification of this external crust began the “year one” of
selenological history.

The phenomena attendant upon the cooling of the mass we will consider in
the next Chapter.

In the foregoing Chapters we have endeavoured to show, by the light of
modern science, first, how diffused cosmical matter was probably
condensed into a planetary mass by the mutual gravitation of its
particles, and secondly, how, the after destruction of the gravitative
force, by the collision of the converging particles of matter, resulted
in the generation of such sufficient heat as to reduce the whole mass to
a molten condition. Our present task is to consider the subsequent
cooling of the mass, and the phenomena attendant upon or resulting
therefrom. This brief Chapter is important to our subject, as we shall
have frequent occasion to refer to the leading principle we shall
endeavour to illustrate in it, in subsequently treating of the causes to
which the special selenological features are to be attributed.

First, then, as regards the cooling of the igneous mass that constituted
the moon at the inconceivably remote period when possibly that body was
really a “lesser light” shining with a luminosity of its own, due to its
then incandescent state, and not simply a reflector, as it is now, of
light which it receives from the sun. If we could conceive it possible
that the igneous mass in the act of cooling parted with its heat from
the central part first and so began to solidify from its centre, or if
it had been possible for the mass to have cooled uniformly and
simultaneously throughout its whole depth, or that each substratum had
cooled before its superstratum, we should have had a moon whose surface
would have been smooth and without any such remarkable asperities and
excresences as are now presented to our view. But these suppositions are
inadmissible: on the contrary we are compelled to consider that the
portion of the igneous or molten body that first cooled was its exterior
surface, which, radiating its heat into surrounding space, became solid
and comparatively cool while the interior retained its hot and molten
condition. So that at this early stage of the moon’s history it existed
in the form of a solid shell inclosing a molten interior.

Now at this period of its formation, the moon’s mass, partly cooled and
solidified and partly molten, would be subject to the influence of two
powerful molecular forces: the first of these would consist in the
diminution of bulk or contraction of volume which accompanies the
cooling of solidified masses of previously molten substances; the second
would arise from a phenomenon which we may here observe is by no means
so generally known as from its importance it deserves to be: and as we
shall have frequent occasion to refer to it as one of the chief agencies
in producing the peculiar structural characteristics of the moon’s
surface, it may be well here to give a few examples of its action, that
our reference to it hereafter may be more clearly understood.

The broad general principle of the phenomenon here referred to is
this:—that fusible substances are (with a few exceptions) specifically
heavier while in their molten condition than in the solidified state, or
in other words, that molten matter occupies less space, weight for
weight, than the same matter after it has passed from the melted to the
solid condition. It follows as an obvious corollary that such substances
contract in bulk in fusing or melting, and expand in becoming solid. It
is this expansion upon solidification that now concerns us.

Water, as is well known, increases in density as it cools, till it
reaches the temperature of 39° Fahrenheit, after which, upon a further
decrease of temperature, its density begins to decrease, or in other
words its bulk expands, and hence the well-known fact of ice floating in
water, and the inconvenient fact of water-pipes bursting in a frost.
This action in water is of the utmost importance in the grand economy of
nature, and it has been accepted as a marvellous exception to the
general law of substances increasing in density (or shrinking) as they
decrease in temperature. Water is, however, by no means the exceptional
substance that it has been so generally considered. It is a fact
perfectly familiar to iron-founders, that when a mass of solid cast-iron
is dropped into a pot of molten iron of identical quality, the solid is
found to float persistently upon the molten metal—so persistently that
when it is intentionally thrust to the bottom of the pot, it rises again
the moment the submerging agency is withdrawn. As regards the amount of
buoyancy we believe it may be stated in round numbers to be at least two
or three per cent. It has been suggested by some who are familiar with
this phenomenon that the solid mass may be kept up by a spurious
buoyancy imparted to it by a film of adhering air, or that surface
impurities upon the solid metal may tend to reduce the specific gravity
of the mass and thereby prevent it sinking, and that the fact of
floatation is not absolutely a proof of greater specific lightness. But
in controversion of these suggestions, we can state as the result of
experiment that pieces of cast-iron which have had their surface
roughness entirely removed, leaving the bright metal exposed, still
float on the molten metal, and further that when, under the influence of
the great heat of the molten mass, the solid is gradually melted away,
and consequently any possible surface impurities or adhering air must
necessarily have been removed, the remaining portion continues to float
to the last. The inevitable inference from this is that in the case of
cast-iron the solid is specifically lighter than the molten, and,
therefore, that in passing from the molten to the solid condition this
substance undergoes expansion in bulk.

We are able to offer a confirmation of this inference in the case of
cast-iron by a remarkable phenomenon well known to iron-founders, but of
which we have never met with special notice. When a ladle or pot of
molten iron is drawn from the melting furnace and allowed to stand at
rest, the surface presents a most remarkable and suggestive appearance.
Instead of remaining calm and smooth it is the scene of a lively
commotion: the thin coat of scoria or molten oxide which forms on the
otherwise bright surface of the metal is seen, as fast as it forms at
the circumference of the pot, to be swept by active convergent currents
towards the centre, where it accumulates in a patch. While this action
is proceeding, the entire upper surface of the metal appears as if it
were covered with animated vermicules of scoria, springing into
existence at the circumference of the pot, and from thence rapidly
streaming and wriggling themselves towards the centre.

[Illustration: Fig. 1.]

Our illustration (Fig. 1) is intended, so far as such means can do so,
to convey some idea of this remarkable appearance at one instant of its
continued occurrence. To interpret our illustration rightly it is
necessary to imagine this vermicular freckling to be constantly and
rapidly streaming from all points of the periphery of the pot towards
the centre, where, as we have said, it accumulates in the form of a
floating island. We may observe that the motion is most rapid when the
hot metal is first put into the cool ladle: as the fluid metal parts
with some of its heat and the ladle gets hot by absorbing it, this
remarkable surface disturbance becomes less energetic.

Now if we carefully consider this peculiar action and seek a cause for
the phenomenon, we shall be led to the conclusion that it arises from
the expansion of that portion of the molten mass which is in contact
with or close proximity to the comparatively cool sides of the ladle,
which sides act as the chief agent in dispersing the heat of the melted
metal. The motion of the scoria betrays that of the fluid metal beneath,
and careful observation will show that the motion in question is the
result of an upward current of the metal around the circumference of the
ladle, as indicated by the arrows A, B, C in the accompanying sectional
drawing of the ladle (Fig. 2). The upward current of the metal can
actually be seen when specially looked for, at the rim of the pot, where
it is deflected into the convergent horizontal direction and where it
presents an elevatory appearance as shown in the figure. It is difficult
to assign to this effect any other cause than that of an expansion and
consequent reduction of the specific gravity of the fluid metal in
contact with or in close proximity to the cooler sides of the pot, as,
according to the generally entertained idea that contraction universally
accompanies cooling, it would be impossible for the cooler to float on
the hotter metal, and the curious surface-currents above referred to
would be in contrary direction to that which they invariably take,
_i.e._, they would diverge from the centre instead of converging to it.
The external arrows in the figure represent the radiation of the heat
from the outer sides of the pot, which is the chief cause of cooling.

[Illustration: Fig. 2.]

Turning from cast-iron to other metals we find further manifestations of
this expansive solidification. Bismuth is a notable example. In his
lectures on Heat, Dr. Tyndall exhibited an experiment in which a stout
iron bottle was filled with molten bismuth, and the stopper tightly
closed. The whole was set aside to cool, and as the metal within
approached consolidation the bottle was rent open by its expansion, just
as would have been the case had the bottle been filled with water and
exposed to freezing temperature. Mercury affords another example.
Thermometers which have to be exposed to Arctic temperatures are
generally filled with spirit instead of quicksilver, because the latter
has been found to burst the bulbs when the cold reached the congealing
point of the metal, the bursting being a consequence of the expansion
which accompanies the act of congelation. Silver also expands in passing
from the fluid to the solid state, for we are informed by a practical
refiner that solid floats on molten silver as ice floats on water; it
also, as likewise do gold and copper, exhibits surface converging
currents in the melting-pot like those depicted above for molten iron.

It may, however, be objected that metals are too distantly related to
volcanic substances to justify inferences being drawn from their
behaviour in explanation of volcanic phenomena. With a view therefore of
testing the question at issue with a substance admitted as closely
allied to volcanic material, we appealed to the furnace slag of
iron-works. The following are extracts from the letters of an iron
manufacturer of great experience[2] to whom we referred the question:—

“I beg to inform you that cold slag floats in molten slag in the same
way cold iron floats in molten iron.

“I filled a box with hot molten slag run quickly from a blast furnace;
the box was about 5½ feet square by 2 feet deep, and I dropped into the
slag a piece of cold slag weighing 16 lbs., when it came to the top in a
second. I pushed it down to the bottom several times and it always made
its appearance at the top: indeed a small portion of it remained above
the molten slag.”

[Illustration: Fig. 3.]

Here then we have a substance closely allied to volcanic material which
manifests the expansile principle in question; but we may go still
further and give evidence from the very fountain-head by instancing what
appears to be a most cogent example of its operation which we observed
on the occasion of a visit to the crater of Vesuvius in 1865 while a
modified eruption was in progress. On this occasion we observed
white-hot lava streaming down from apertures in the sides of a central
cone within the crater and forming a lake of molten lava on the plateau
or bottom of the crater; on the surface of this molten lake vast cakes
of the same lava which had become solidified were floating, exactly in
the same manner as ice floats in water. The solidified lava had cracked,
and divided into cakes, in consequence of its contraction and also of
the uprising of the accumulating fluid lava on which it floated, more
and more space being thus afforded for it to separate, on account of the
crater widening upwards, while through the joints or fissures the fluid
lava could be seen beneath. But for the decrease in density and
consequent expansion in volume which accompanied solidification, this
floating of the solidified lava on the molten could not have occurred.
Reference to Fig. 3, which represents a section of the crater of
Vesuvius on the occasion above referred to, will perhaps assist the
reader to a more clear idea of what we have endeavoured to describe. A A
are the streams of white-hot lava issuing from openings in the sides of
the central cone, and accumulating beneath the solidified crust B B in a
lake of molten lava at C C; the solidified crust B B as it was floated
upwards dividing into separate cakes as represented in Fig. 4. (See also
Plate I.)

[Illustration: Fig. 4.]

[Illustration: PLATE I.
CRATER OF VESUVIUS.
1865.]

Let us now consider what would be the effect produced upon a spherical
mass of molten matter in progress of cooling, first under the action of
the above described expansion which precedes solidification, and then by
the contraction which accompanies the cooling of a solidified body. The
first portion of such a mass to part with its heat being its external
surface, this portion would expand, but there being no obstacle to
resist the expansion there would be no other result than a temporary
slight enlargement of the sphere. This external portion would on cooling
form a solid shell encompassing a more or less fluid molten nucleus, but
as this interior has in its turn, on approaching the point of
solidification, to expand also, and there being, so to speak, no room
for its expansion, by reason of its confinement within its solid casing,
what would be the consequence?—the shell would be rent or burst open,
and a portion of the molten interior ejected with more or less violence
according to circumstances, and many of the characteristic features of
volcanic action would be thus produced: the thickness of the outer
shell, the size of the vent made by the expanding matter for its escape,
and other conditions conspiring to modify the nature and extent of the
eruption. Thus there would result vast floodings of the exterior surface
of the shell by the so extruded molten matter, volcanoes, extruded
mountains, and other manifestations of eruptive phenomena. The sectional
diagram (Fig. 5) will help to convey a clear idea of this action. Basing
our reasoning on the principle we have thus enunciated, namely, that
molten telluric matter expands on nearing the point of solidification,
and which we have endeavoured to illustrate by reference to actual
examples of its operation, we consider we are justified in assuming that
such a course of volcanic phenomena has very probably occurred again and
again upon the moon; that this expansion of volume which accompanies the
solidification of molten matter furnishes a key to the solution of the
enigma of volcanic action; and that such theories as depend upon the
agency of gases, vapour, or water are at all events untenable with
regard to the moon, where no gases, vapour, or water, appear to exist.

[Illustration: Fig. 5. A A. The solidified crust cooling,
contracting, and cracking; the cracking action enhanced by the
expansion of the substratum of molten matter, B B B, which,
expanding as it approaches the point of solidification, injects
portions of the molten matter up through the contractile cracks, and
results in producing craters, mountains of exudation, and districts
flooded with extruded lava, C C C. The nucleus of intensely hot
molten matter.]

That an upheaving and ejective force has been in action with varying
intensity beneath the whole of the lunar surface is manifest from the
aspect of its structural details, and we are impressed with the
conviction that the principle we have set forth, namely the paroxysms of
expansion which successively occurred as portions of its molten interior
approached solidification, supply us with a rational cause to which such
vast ejective and upheaving phenomena may be assigned. Many features of
terrestrial geology likewise require such an expansive force whereby to
explain them; we therefore venture to recommend this source and cause of
ejective action to the careful consideration of geologists.

[Illustration: Fig. 6.]

[Illustration: Fig. 7.]

When the molten substratum had burst its confines, ejected its
superfluous matter, and produced the resulting volcanic features, it
would, after final solidification, resume the normal process of
contraction upon cooling, and so retreat or shrink away from the
external shell. Let us now consider what would be the result of this.
Evidently the external shell or crust would become relatively too large
to remain at all points in close contact with the subjacent matter. The
consequence of too large a solid shell having to accommodate itself to a
shrunken body underneath, is that the skin, so to term the outer stratum
of solid matter, becomes shrivelled up into alternate ridges and
depressions, or wrinkles. In its attempt to crush down and follow the
contracting substratum, it would have to displace the superabundant or
superfluous material of its former larger surface by thrusting it (by
the action of tangential force) into undulating ridges as in Fig. 6, or
broken elevated ridges as in Fig. 7, or overlappings of the outer crust
as in Fig. 8, or ridges capped by more or less fluid molten matter
extruded from beneath, as indicated in Fig. 9, a class of action which
might occur contemporaneously with the elevation of the ridge or
subsequently to its formation.

[Illustration: Fig. 8.]

[Illustration: Fig. 9.]

A long-kept shrivelled apple affords an apt illustration of this wrinkle
theory; another example may be observed in the human face and hand, when
age has caused the flesh to shrink and so leave the comparatively
unshrinking skin relatively too large as a covering for it. We
illustrate both of these examples by actual photographs of the
respective objects, which are reproduced on Plate II. Whenever an outer
covering has to accommodate and apply itself to an interior body that
has become too small for it, wrinkles are inevitably produced. The same
action that shrivels the human skin into creases and wrinkles, has also
shrivelled certain regions of the igneous crust of the earth. A map of a
mountainous part of our globe affords abundant evidence of such a cause
having been in action; such maps are pictures of wrinkles. Several parts
of the lunar surface, as we shall by-and-by see, present us with the
same appearances in a modified degree.

To the few primary causes we have set forth in this chapter—to the
alternate expansion and contraction of successive strata of the lunar
sphere, when in a state of transition from an igneous and molten to a
cooled and solidified condition, we believe we shall be able to refer
well nigh all the remarkable and characteristic features of the lunar
surface which will come under our notice in the course of our survey.

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