The first machine in which Watt realised the conceptions which
we mentioned in the last chapter, is that which was afterwards called
his _Single-acting Steam Engine_. We shall now describe the working
apparatus in this machine.
The cylinder is represented at C (fig. 12.)–in which the piston P
moves steam-tight. It is closed at the top, and the piston-rod being
very accurately turned, runs in a steam-tight collar B furnished with
a stuffing-box, and constantly supplied with melted tallow or wax.
Through a funnel in the top of the cylinder, melted grease flows upon
the piston so as to maintain it steam-tight. Two boxes A A, containing
the valves for admitting and withdrawing the steam, connected by a
tube of communication T, are attached to the cylinder; the action of
these valves will be presently described. Below the cylinder, placed
in a cistern of cold water, is a close cylindrical vessel D, called
the condenser, communicating with the cylinder by a tube T´, leading
to the lower valve-box A. In the side of this condenser is inserted a
tube, the inner end of which is pierced with holes like the rose of a
watering-pot; and a cock E in the cold cistern is placed on the
outside, through which, when open, the water passing, rises in a jet
on the inside.
The tube S, which conducts steam from the boiler, enters the top of
the upper valve-box at F. Immediately under it is placed a valve G,
which is opened and closed by a lever or rod G´. This valve, when
open, admits steam to the top of the piston, and also to the tube T,
which communicates between the two valve-boxes, and when closed
suspends the admission of steam. There are two valves in the lower
box, one H in the top worked by the lever H´, and one I in the bottom
worked by the lever I´. The valve H, when open, admits steam to pass
from the cylinder _above_ the piston, by the tube T, to the cylinder
_below_ the piston, the valve I being supposed in this case to be
closed. This valve I, when open, (the valve H being closed,) admits
steam to pass from below the cylinder through T´ to the condenser.
This steam, entering the condenser, meets the jet, admitted to play by
the valve E, and is condensed.
The valve G is called the _upper steam valve_; H, _lower steam valve_;
I, the _exhausting-valve_; and E, the _condensing valve_. Let us now
consider how these valves must be worked in order to produce the
alternate ascent and descent of the piston.
It is in the first place necessary that all the air which fills the
cylinder, tubes, and condenser should be expelled. To accomplish this
it is only necessary to open at once the valves G, H, and I. The steam
then rushing from F through the valve G will pass into the upper part
of the cylinder, and through the tube T and the valve H into the lower
part, and also through the valve I into the condenser. After the steam
ceases to be condensed by the cold of the apparatus, it will rush out
mixed with air through the valve M, which opens outward; and this will
continue until all the air has been expelled, and the apparatus filled
with pure steam. Then suppose all the valves again closed. The
cylinder both above and below the piston is filled with steam; and the
steam which filled the condenser being cooled by the cold surface, a
vacuum has been produced in that vessel.
The apparatus being in this state, let the upper steam valve G, the
exhausting-valve I, and the condensing valve E be opened. Steam will
thus be admitted through G to press on the top of the piston; and this
steam will be prevented from circulating to the lower part of the
cylinder by the lower steam-valve H being closed. Also the steam which
filled the cylinder below the piston rushes through the open
exhausting-valve I to the condenser, where it meets the jet allowed to
play by the open condensing valve E. It is thus instantly condensed,
and a vacuum is left in the cylinder below the piston. Into this
vacuum the piston is pressed without resistance by the steam which is
admitted through G. When the piston has thus been forced to the bottom
of the cylinder, let the three valves G, I, and E, which were before
opened, be closed, and let the lower steam-valve H be opened. The
effects of this change are easily perceived. By closing the upper
steam-valve G, the further admission of steam to the apparatus is
stopped. By closing the exhausting-valve I, all transmission of steam
from the cylinder to the condenser is stopped. Thus the steam which is
_in_ the cylinder, valve-boxes, and tubes is shut up in them, and no
more admitted, nor any allowed to escape. By closing the condensing
valve E, the play of the jet in the condenser is suspended.
Previously to opening the valve H, the steam contained in the
apparatus was confined to the part of the cylinder _above_ the piston
and the tube T and the valve-box A. But on opening this valve, the
steam is allowed to circulate above and below the piston; and in fact
through every part included between the upper steam valve G, and the
exhausting-valve I. The same steam circulating on both sides, the
piston is thus equally pressed upward and downward.
In this case there is no force tending to retain the piston at the
bottom of the cylinder except its own weight. Its ascent is produced
in the same manner as the ascent of the piston in the atmospheric
engine. The piston-rod is connected by chains G to the arch-head of
the beam, and the weight of the pump-rod R, or any other counterpoise
acting on the chains suspended from the other arch-head, draws the
piston to the top of the cylinder.
When the piston has arrived at the top of the cylinder, suppose the
three valves G, I, and E to be again opened, and H closed. Steam
passes from the steam-pipe F through the upper steam-valve G to the
top of the piston, and at the same time the steam which filled the
cylinder below the piston is drawn off through the open
exhausting-valve I into the condenser, where it is condensed by the
jet allowed to play by the open condensing valve E. The pressure of
the steam above the piston then forces it without resistance into the
vacuum below it, and so the process is continued.
It should be remembered, that of the four valves necessary to work the
piston, three are to be opened the moment the piston reaches the top
of the cylinder, and the fourth is to be closed; and on the piston
arriving at the bottom of the cylinder, these three are to be closed
and the fourth opened. The three valves which are thus opened and
closed together are the upper steam-valve, the exhausting-valve, and
the condensing valve. The lower steam-valve is to be opened at the
same instant that these are closed, and _vice versâ_. The manner of
working these valves we shall describe hereafter.
The process which has just been described, if continued for any
considerable number of reciprocations of the piston, would be attended
with two very obvious effects which would obstruct and finally destroy
the action of the machine. First, the condensing water and condensed
steam would collect in the condenser D, and fill it; and secondly, the
water in the cistern in which the condenser is placed would gradually
become heated, until at last it would not be cold enough to condense
the steam when introduced in the jet. Besides this, it will be
recollected that water boils in a vacuum at a very low temperature
(17); and, therefore, the hot water collected in the bottom of the
condenser would produce steam which, rising into the cylinder through
the exhausting-valve, would resist the descent of the piston, and
counteract the effects of the steam above it. A further disadvantage
arises from the air or other permanently elastic fluid which enters in
combination with the water, both in the boiler and condensing jet, and
which is disengaged by its own elasticity.
To remove these difficulties, a pump is placed near the condenser
communicating with it by a valve M, which opens from the condenser
into the pump. In this pump is placed a piston which moves air-tight,
and in which there is a valve N, which opens upwards. Now suppose the
piston at the bottom of the pump. As it rises, since the valve in it
opens _upwards_, no air can pass _down_ through it, and consequently
it leaves a vacuum _below_ it. The water and any air which may be
collected in the condenser open the valve M, and pass into the lower
part of the pump from which they cannot return in consequence of the
valve M opening _outwards_. On the descent of the pump-piston, the
fluids which occupy the lower part of the pump, force open the
piston-valve N; and passing through it, get _above_ the piston, from
which their return is prevented by the valve N. In the next ascent,
the piston lifts these fluids to the top of the pump, whence they are
discharged through a conduit into a small cistern B by a valve K which
opens outwards. The water which is thus collected in B is heated by
the condensed steam, and is reserved in B, which is called the hot
well for feeding the boiler, which is effected by means which we shall
presently explain. The pump which draws off the hot water and air from
the condenser is called the _air-pump_.
(50.) We have not yet explained the manner in which the valves and the
air-pump piston are worked. The rod Q of the latter is connected with
the working beam, and the pump is therefore wrought by the engine
itself. It is not very material to which arm of the beam it is
attached. If it be on the same side of the centre of the beam with the
cylinder, it rises and falls with the steam-piston; but if it be on
the opposite side, the pump-piston rises when the steam-piston falls,
and _vice versâ_. In the single-engine there are some advantages in
the latter arrangement. As the steam-piston _descends_, the steam
rushes into the condenser, and the jet is playing; and this,
therefore, is the most favourable time for drawing out the water and
condensed steam from the condenser by the ascent of the pump-piston,
since by this means the descent of the steam-piston is assisted; an
effect which would not be produced if the steam-piston and pump-piston
descended together.
With respect to the method of opening and closing the valves, it is
evident that the three valves which are simultaneously opened and
closed may be so connected as to be worked by the same lever. This
lever may be struck by a pin fixed upon the rod Q of the air-pump, so
that when the pistons have arrived at the top of the cylinders the pin
strikes the lever and opens the three valves. A catch or detent is
provided for keeping them open during the descent of the piston, from
which they are disengaged in a similar manner on the arrival of the
piston at the bottom of the cylinder, and they close by their own
weight.
In exactly the same way the lower steam-valve is opened on the arrival
of the piston at the bottom of the cylinder, and closed on its arrival
at the top by the action of a pin placed on the piston-rod of the
air-pump.
(51.) Soon after the invention of these engines, Watt found that in
some instances inconvenience arose from the too rapid motion of the
steam-piston at the end of its stroke, owing to its being moved with
an _accelerated motion_. This was owing to the uniform action of the
steam-pressure upon it: for upon first putting it in motion at the top
of the cylinder, the motion was comparatively slow; but from the
continuance of the same pressure the velocity with which the piston
descended was continually increasing, until it reached the bottom of
the cylinder, where it acquired its greatest velocity. To prevent
this, and to render the descent as nearly as possible uniform, it was
proposed to cut off the steam before the descent was completed, so
that the remainder might be effected merely by the expansion of the
steam which was admitted to the cylinder. To accomplish this, he
contrived, by means of a pin on the rod of the air-pump, to close the
upper steam-valve when the steam-piston had completed one-third of its
entire descent, and to keep it closed during the remainder of the
descent, and until the piston again reached the top of the cylinder.
By this arrangement the steam pressed the piston with its full force
through one-third of the descent, and thus put it into motion; during
the other two thirds the steam thus admitted acted merely by its
expansive force, which became less in exactly the same proportion as
the space given to it by the descent of the piston increased. Thus,
during the last two thirds of the descent the piston is urged by a
gradually decreasing force, which in practice was found just
sufficient to sustain in the piston a uniform velocity.
(52.) We have already mentioned the difficulty arising from the water
in the cistern, in which the condenser and air-pump are placed,
becoming heated, and the condensation therefore being imperfect. To
prevent this, a waste-pipe is placed in this cistern, from which the
water is continually discharged, and a pump L (called the
_cold-water-pump_) is worked by the engine itself, which raises a
supply of cold water and sends it through a pipe in a constant stream
into the cold cistern. The waste-pipe, through which the water flows
from the cistern, is placed near the top of it, since the heated
water, being lighter than the cold, remains on the top. Thus the
heated water is continually flowing off, and a constant stream of cold
water supplied. The piston-rod of the cold-water-pump is attached to
the beam (by which it is worked), usually on the opposite side from
the cylinder.
Another pump O (called the _hot-water-pump_) enters the _hot well_ B;
and raising the water from it, forces it through a tube to the boiler
for the purpose of feeding it. The manner in which this is effected
will be more particularly described hereafter. A part of the heat
which would otherwise be lost, is thus restored to the boiler to
assist in the production of fresh steam. We may consider a portion of
the heat to be in this manner _circulating_ continually through the
machine. It proceeds from the boiler in steam, works the piston,
passes into the condenser, and is reconverted into hot water; thence
it is passed to the hot well, from whence it is pumped back into the
boiler, and is again converted into steam, and so proceeds in constant
circulation.
From what has been described, it appears that there are four pistons
attached to the great beam and worked by the piston of the
steam-cylinder. On the same side of the centre with the cylinder is
the piston-rod of the air-pump, and on the opposite side are the
piston-rods of the hot-water pump and the cold-water-pump; and lastly,
at the extremity of the beam opposite to that at which the
steam-piston works, is the piston of the pump to be wrought by the
engine.
(53.) The position of these piston-rods with respect to the centre of
the beam depends on the play necessary to be given to the piston. If
the play of the piston be short, its rod will be attached to the beam
near the centre; and if longer, more remote from the centre. The
cylinder of the air-pump is commonly half the length of the
steam-cylinder, and its piston-rod is attached to the beam at the
point exactly in the middle between the end of the beam and the
centre. The hot-water pump not being required to raise a considerable
quantity of water, its piston requires but little play, and is
therefore placed near the centre of the beam, the piston-rod of the
cold-water pump being farther from the centre.
(54.) It appears to have been about the year 1763, that Watt made
these improvements in the steam engine, and constructed a model which
fully realized his expectations. Either from want of influence or the
fear of prejudice and opposition, he did not make known his discovery
or attempt to secure it by a patent at that time. Having adopted the
profession of a land surveyor, his business brought him into
communication with Dr. Roebuck, at that time extensively engaged in
mining speculations, who possessed some command of capital, and was of
a very enterprising disposition. By Roebuck’s assistance and
countenance, Watt erected an engine of the new construction at a coal
mine on the estate of the Duke of Hamilton, at Kinneil near
Burrowstoness. This engine being a kind of experimental one, was
improved from time to time as circumstances suggested, until it
reached considerable perfection. While it was being erected, Watt in
conjunction with Roebuck applied for and obtained a patent to secure
the property in the invention. This patent was enrolled in 1769, six
years after Watt invented the improved engine.
Watt was now preparing to manufacture the new engines on an extensive
scale, when his partner Roebuck suffered a considerable loss by the
failure of a mining speculation in which he had engaged, and became
involved in embarrassments, so as to be unable to make the pecuniary
advances necessary to carry Watt’s designs into execution. Again
disappointed, and harassed by the difficulties which he had to
encounter, Watt was about to relinquish the further prosecution of his
plans, when Mr. Matthew Bolton, a gentleman who had established a
factory at Birmingham a short time before, made proposals to purchase
Dr. Roebuck’s share in the patent, in which he succeeded; and in 1773,
Watt entered into partnership with Bolton.
His situation was now completely changed. Bolton was not only a man of
extensive capital, but also of considerable personal influence, and
had a disposition which led him, from taste, to undertakings which
were great and difficult, and which he prosecuted with the most
unremitting ardency and spirit. “Mr. Watt,” says Playfair, “was
studious and reserved, keeping aloof from the world; while Mr. Bolton
was a man of address, delighting in society, active, and mixing with
people of all ranks with great freedom, and without ceremony. Had Mr.
Watt searched all Europe, he probably would not have found another
person so fitted to bring his invention before the public, in a manner
worthy of its merit and importance; and although of most opposite
habits, it fortunately so happened that no two men ever more cordially
agreed in their intercourse with each other.”
The delay in the progress of the manufacture of engines occasioned by
the failure of Dr. Roebuck was such, that Watt found that the duration
of his patent would probably expire before he would even be reimbursed
the necessary expenses attending the various arrangements for the
manufacture of the engines. He therefore, with the advice and
influence of Bolton, Roebuck, and other friends, in 1775, applied to
parliament for an extension of the terms of his patent, which was
granted for 25 years from the date of his application, so that his
exclusive privilege should expire in 1800.
An engine was now erected at Soho (the name of Bolton’s factory) as a
specimen for the examination of mining speculators, and the engines
were beginning to come into demand. The manner in which Watt chose to
receive remuneration from those who used his engines was as remarkable
for its ingenuity as for its fairness and liberality. He required that
one-third of the saving of coals effected by his engines, compared
with the atmospheric engines hitherto used, should be paid to him,
leaving the benefit of the other two-thirds to the public. Accurate
experiments were made to ascertain the saving of coals; and as the
amount of this saving in each engine depended on the length of time it
was worked, or rather on the number of descents of the piston, Watt
invented a very ingenious method of determining this. The vibrations
of the great working beam were made to communicate with a train of
wheelwork, in the same way as those of a pendulum communicate with the
work of a clock. Each vibration of the beam moved one tooth of a small
wheel, and the motion was communicated to a hand or index, which moved
on a kind of graduated plate like the dial plate of a clock. The
position of this hand marked the number of vibrations of the beam.
This apparatus, which was called the _counter_, was locked up and
secured by two different keys, one of which was kept by the
proprietor, and the other by Bolton and Watt, whose agents went round
periodically to examine the engines, when the counters were opened by
both parties and examined, and the number of vibrations of the beam
determined, and the value of the patent third found.[16]
[Footnote 16: The extent of the saving in fuel may be judged from
this: that for three engines erected at Chacewater mine in Cornwall,
it was agreed by the proprietors that they would compound for the
patent third at 2400_l._ per annum; so that the whole saving must have
exceeded 7200_l._ per annum.]
Notwithstanding the manifest superiority of these engines over the old
atmospheric engines; yet such were the influence of prejudice and the
dislike of what is new, that Watt found great difficulties in getting
them into general use. The comparative first cost also probably
operated against them; for it was necessary that all the parts should
be executed with great accuracy, which entailed proportionally
increased expense. In many instances they felt themselves obliged to
induce the proprietors of the old atmospheric engines to replace them
by the new ones, by allowing them in exchange an exorbitant price for
the old engines; and in some cases they were induced to erect engines
at their own expense, upon an agreement that they should only be paid
if the engines were found to fulfil the expectations, and brought the
advantages which they promised. It appeared since, that Bolton and
Watt had actually expended a sum of nearly 50,000_l._ on these engines
before they began to receive any return. When we contemplate the
immense advantages which the commercial interests of the country have
gained by the improvements in the steam engine, we cannot but look
back with disgust at the influence of that fatal prejudice which
opposes the progress of improvement under the pretence of resisting
innovation. It would be a problem of curious calculation to determine
what would have been lost to the resources of this country, if chance
had not united the genius of such a man as Watt with the spirit,
enterprise, and capital, of such a man as Bolton! The result would
reflect little credit on those who think novelty alone a sufficient
reason for opposition.
