suppose that the water in B is raised to the temperature of 213°,
the stopcock C being open; the vessel A will then be filled with
steam of the same temperature, and having a pressure of 15 lbs.
per square inch. This will be common steam. If the stopcock be now
closed, and the whole apparatus be exposed to the temperature of
243°; the steam in A will preserve the same density, but its
pressure will be [Pg171] increased from 15 lbs. to a little more
than 16 lbs. per square inch. Let the stopcock C be then opened
and while the temperature of the steam in A shall continue to be
243°, the pressure will suddenly rise from 16 lbs. to about 26
lbs. per square inch. The weight of the steam in A will be at the
same time increased in the same proportion of 16 to 26 as its
pressure. The steam thus produced in A will then be common steam,
and any abstraction of heat from it would be attended with partial
condensation.
(96.) The law, according to which the pressure of elastic fluids
in general, whether gases or vapours, increases with their
temperature, was simultaneously discovered by Dalton and Gay
Lussac. If the pressure which the gas or vapour would have at the
temperature of melting ice, were expressed by 10,000, then the
increase of pressure which it would receive for every degree of
temperature by which it would be raised, its volume being supposed
to be preserved, would be expressed by 208-1/3. Thus, if the
pressure of gas, or vapour, on a surface of a certain magnitude at
the temperature of 32° were 10,000 ounces, then the same gas or
vapour would acquire an additional pressure of 208-1/3 ounces for
every degree of temperature which would be imparted to it above
32°. This law is common to all gases and vapours.
It may be objected that water cannot exist in the state of vapour
under the usual pressures at so low a temperature as melting ice.
This, however, does not hinder the application of the above law,
for that law will equally hold good by computing the pressure
which the vapour would have if it were a permanent gas, and if it
could therefore exist in the elastic form at that low temperature.
(97.) Another law, common to all elastic fluids, and of equal
importance with the former, was discovered by Mariotte. By this
law it appears that every gas or vapour, so long as its
temperature is unchanged, will have a pressure directly
proportional to its density. If therefore, while we compress steam
into half its volume, we could preserve its temperature unaltered,
we should increase its pressure in a two-fold proportion; but if
the process of compression should cause its temperature to
increase, [Pg172] then its increase of pressure will be greater
than its increase of density, since it will be due conjointly to
the increase of density and to the increase of temperature. In
this case the increased pressure may be deduced from the combined
application of the two laws just explained; that of Mariotte will
determine that increase of pressure which is due to the increase
of density, and that of Dalton and Gay Lussac will determine the
further increase of pressure which will be due to the increase of
temperature. The full investigation of these effects, and the
formulæ expressing them, will be found in the Appendix to this
volume.
(98.) The fixed relations which exist between the temperatures of
common steam and its pressure and density, have never been
discovered from any general physical principles. The pressures and
the densities however, which correspond to a great variety of
temperatures throughout the thermometric scale, have been
ascertained by extensive series of experiments instituted by
philosophers of this and other countries. From a comparison of the
temperatures and pressures thus found by experiment, empirical
formulæ have been constructed, which exhibit, with an approximation
sufficiently close for practice, this relation; and these formulæ
may accordingly be used for the computation of tables exhibiting the
pressures, temperatures, and densities of common steam; and such
tables will have sufficient numerical accuracy for all practical
purposes. These formulæ, and the tables resulting from them, will be
found in the Appendix to this volume.
(99.) It has been explained, that to effect the conversion of
water into steam, it is only necessary to impart to it as much
heat as, added to the temperature which it has, would, if it
continued in the liquid form, raise it to the temperature of
1212°. This condition is necessary, and sufficient to effect the
transition of water into vapour. If, for example, as much heat
were imparted to the water evaporated, as would maintain it in the
liquid state to 1300°, then the steam so produced would be
superheated steam, having 80° of heat more than is necessary to
maintain it in the vaporous form. From such steam, therefore, 80°
of heat may be abstracted without producing any condensation.
[Pg173]
(100.) Common steam being raised from water at any pressure and
temperature, and being afterwards separated from the water, if the
same steam be compressed into a small volume, or allowed to expand
into a greater volume, it will still maintain its quality of
common steam, and will have the same pressure and temperature,
whatever volume it may assume, as it would have if immediately
raised from water at that pressure. Thus if steam be raised from
water under a pressure of 30 lbs. per square inch, and, being
separated from the water, be allowed to dilate, until its pressure
is reduced to 15 lbs. per square inch, its temperature will then
be reduced to 213°, which is that temperature which it would have
if immediately raised from water under a pressure of 15 lbs. per
square inch; and if any heat be abstracted from such steam,
whether under its original pressure, or under the diminished
pressure of 15 lbs. per square inch, a condensation will be
produced, the amount of which will be the same, if the same
quantity of heat be abstracted from the steam. These are
consequences which immediately flow from the fact, that the sum of
the latent and sensible heats of steam is always the same.[20]
It appears, therefore, that supposing the steam used in an engine
to receive no additional heat after it leaves the boiler, however
it may be changed in its density by subsequent expansion, it will
still retain its character of common steam, and cannot lose any
portion of heat, however small, without suffering partial
condensation. The mechanical force also exerted by such steam,
after expansion, must be computed in the same manner as if it were
raised immediately.
(101.) If the law of Mariotte were strictly applicable to steam,
its mechanical effect would be the same as has been already
explained in all states of density; but since its temperature will
rise and fall as its density is increased or diminished, a
corresponding change will be produced in its [Pg174] mechanical
efficacy. It is therefore necessary in the calculation of the
mechanical effect of steam, whether it be used at a uniform
pressure without the principle of expansion, or with the
application of that principle to any given extent, to take into
account the combined operation of the laws of Mariotte and Dalton.
Formulæ exhibiting the relation between the temperatures,
pressures, volumes, and densities of steam, and the mechanical
effect produced by the evaporation of water, whether acting with
or without expansion, together with the tables necessary for the
practical application of these, will be found in the Appendix.
(102.) One of the methods of equalising the varying force of
expanding steam, would be to work it at the same time in two
cylinders connected with the same beam; so that while its force in
one would be augmented, its force in the other would be
diminished, the combination of the two producing a uniform effect.
Soon after the expansive principle was promulged by Mr. Watt, this
expedient was accordingly resorted to by an engineer named
Hornblower.
[Illustration: _Fig._ 30.]
In the year 1781, Hornblower conceived the notion of working an
engine with two cylinders of different sizes, by allowing the
steam to flow freely from the boiler until it fills the smaller
cylinder, and then permitting it to expand into the greater one,
employing it thus to press down two pistons in the following
manner.
Let C, _fig._ 30., be the centre of the great working-beam,
carrying two arch heads, on which the chains of the piston rods
play. The distances of these arch heads from the centre C must be
in the same proportion as the length of the cylinders, in order
that the same play of the beam may correspond to [Pg175] the
plays of both pistons. Let F be the steam-pipe from the boiler,
and G a valve to admit the steam above the lesser piston. H is a
tube by which a communication may be opened by the valve I,
between the top and bottom of the lesser cylinder B. K is a tube
communicating by the valve L, between the bottom of the lesser
cylinder B and the top of the greater cylinder A. M is a tube
communicating, by the valve N, between the top and bottom of the
greater cylinder A; and P a tube leading to the condenser by the
exhausting valve O.
At the commencement of the operation, suppose all the valves
opened, and steam allowed to flow through the engine until the air
be completely expelled, and then let all the valves be closed. To
start the engine, let the exhausting valve O and the steam valves
G and L be opened, as in _fig._ 30. The steam will flow freely
from the boiler, and press upon the lesser piston, and at the same
time the steam below the greater piston will flow into the
condenser, leaving a vacuum in the greater cylinder. The valve L
being opened, the steam which is under the piston in the lesser
cylinder will flow through K, and press on the greater piston,
which, having a vacuum beneath it, will consequently descend. At
the commencement of the motion, the lesser piston is as much
resisted by the steam below it, as it is urged by the steam above
it; but after a part of the descent has been effected, the steam
below the piston, in the lesser cylinder, passing into the
greater, expands into an increased space, and therefore loses part
of its elastic force. The steam above the lesser piston retaining
its full force by having a free communication with the boiler by
the valve G, the lesser piston will be urged by a force equal to
the excess of the pressure of this steam above the diminished
pressure of the expanded steam below it. As the pistons descend,
the steam which is between them is continually increasing in its
bulk, and therefore decreasing in its pressure, from whence it
follows, that the force which resists the lesser piston is
continually decreasing, while that which presses it down remains
the same, and therefore the effective force which impels it must
be continually increasing. [Pg176]
On the other hand, the force which urges the greater piston is
continually decreasing, since there is a vacuum below it, and the
steam which presses it is continually expanding into an increased
bulk.
[Illustration: _Fig._ 31.]
Impelled in this way, let us suppose the pistons to have arrived
at the bottoms of the cylinders, and let the valves G, L, and O,
be closed, and the valves I and N opened. No steam is allowed to
flow from the boiler, G being closed, nor any allowed to pass into
the condenser, since O is closed, and all communication between
the cylinders is stopped by closing L. By opening the valve I, a
free communication is made between the top and bottom of the
lesser piston through the tube H, so that the steam which presses
above the lesser piston will exert the same pressure below it, and
the piston is in a state of indifference. In the same manner the
valve N being open, a free communication is made between the top
and bottom of the greater piston, and the steam circulates above
and below the piston, and leaves it free to rise. A counterpoise
attached to the pump-rods, in this case, draws up the piston, as
in Watt's single engine; and when they arrive at the top, the
valves I and N are closed, and G, L, and O, opened, and the next
descent of the pistons is produced in the manner already
described, and so the process is continued.
The valves are worked by the engine itself, by means similar to
some of those already described. By computation, we find the power
of this engine to be nearly the same as a similar engine on Watt's
expansive principle. It does not, however, appear, that any
adequate advantage was gained by this modification of the
principle, since no engines of this construction are now made.
(103.) The use of two cylinders was revived by Arthur Woolf in 1804,
who, in this and the succeeding year, obtained patents for the
application of steam raised under a high pressure to double-cylinder
engines. The specification of his patent states, that he has proved
by experiment that steam raised [Pg177] under a safety-valve loaded
with any given number of pounds upon the square inch will, if
allowed to expand into as many times its bulk as there are pounds of
pressure on the square inch, have a pressure equal to that of the
atmosphere. Thus, if the safety-valve be loaded with four pounds on
the square inch, the steam, after expanding into four times its
bulk, will have the atmospheric pressure; if it be loaded with 5, 6,
or 10 lbs. on the square inch, it will have the atmospheric pressure
when it has expanded into 5, 6, or 10 times its bulk, and so on. It
was, however, understood in this case, that the vessel into which it
was allowed to expand should have the same temperature as the steam
before it expands.
It is very unaccountable how a person of Mr. Woolf's experience in
the practical application of steam could be led into errors so
gross as those involved in the averments of this patent; and it is
still more unaccountable how the experiments could have been
conducted which led him to conclusions not only incompatible with
all the established properties of elastic fluids, but even
involving in themselves palpable contradiction and absurdity. If
it were admitted that every additional pound avoirdupois which
should be placed upon the safety-valve would enable steam, by its
expansion into a proportionally enlarged space, to attain a
pressure equal to the atmosphere, the obvious consequence would
be, that a physical relation would subsist between the atmospheric
pressure and the pound avoirdupois! It is wonderful that it did
not occur to Mr. Woolf, that, granting his principle to be true at
any given place, it would necessarily be false at another place,
where the barometer would stand at a different height! Thus, if
the principle were true at the foot of a mountain, it would be
false at the top of it; and if it were true in fair weather, it
would be false in foul weather, since these circumstances would be
attended by a change in the atmospheric pressure, without making
any change in the pound avoirdupois.[21]
[Pg178]
(104.) For several years after the extension of Watt's first
patent had been obtained from parliament, he was altogether
engrossed by the labour of bringing to perfection the application
of the steam-engine to the drainage of mines, and in surmounting
the numerous difficulties which presented themselves to its
general adoption, even after its manifold advantages were
established and admitted. When, however, these obstacles had been
overcome, and the works for the manufacture of engines for pumping
water, at Soho, had been organised and brought into active
operation, he was relieved from the pressure of these anxieties,
and was enabled to turn his attention to the far more extensive
and important uses of which he had long been impressed with the
conviction that the engine was capable. His sagacious mind enabled
him to perceive that the machine he had created was an infant
force, which by the fostering influence of his own genius would
one day extend its vast power over the arts and manufactures, the
commerce and the civilisation of the world. Filled with such
aspirations, he addressed his attention about the year 1779, to
the adaptation of the steam-engine to move machinery, and thereby
to supersede animal power, and the natural agents, wind and water.
The idea that steam was capable of being applied extensively as a
prime mover, had prevailed from a very early period; and now that
we have seen its powers so extensively brought to bear, it will
not be uninteresting to revert to the faint traces by which its
agency was sketched in the crude speculations of the early
mechanical inventors.
(105.) Papin, to whom the credit of discovering the method of
producing a vacuum by the condensation of steam is due, was the
earliest and most remarkable of those projectors. With very limited
powers of practical application, he was, nevertheless, peculiarly
happy in his mechanical conceptions; and had his experience and
opportunities been proportionate to the clearsighted character of
his mind, he would doubtless have anticipated some of the most
memorable of his successors in the progressive improvement of the
steam engine.
In his work already cited, after describing his method of
imparting an alternate motion to a piston by the atmospheric
[Pg179] pressure acting against a vacuum produced by the
condensation of steam, he stated that his invention, besides being
applicable to pumping water, could be available for rowing vessels
against wind and tide, which he proposed to accomplish in the
following manner.
Paddle-wheels, such as have since been brought into general use,
were to be placed at the sides, and attached to a shaft extending
across the vessel. Within the vessel, and under this shaft, he
proposed to place several cylinders supplied with pistons, to be
worked by the atmospheric pressure. On the piston-rods were to be
constructed racks furnished with teeth: these teeth were to work in
the teeth of wheels or pinions, placed on the shaft of the
paddle-wheels. These pinions were not to be fixed on the shaft, but
to be connected with it by a ratchet; so that when they turned in
one direction, they would revolve without causing the shaft to
revolve; but when driven in the other direction, the catch of the
ratchet-wheel would act upon the shaft so as to compel the shaft and
paddle-wheels to revolve with the motion of the pinion or wheel upon
it. By this arrangement, whenever the piston of any cylinder was
forced down by the atmospheric pressure, the rack descending would
cause the corresponding pinion of the paddle-shaft to revolve; and
the catch of the ratchet wheel, being thus in operation, would cause
the paddle-shaft and paddle-wheels also to revolve; but whenever the
piston would rise, the rack driving the pinion in the opposite
direction, the catch of the ratchet wheel would merely fall from
tooth to tooth, without driving the paddle-shaft.
It is evident that by such an arrangement a single cylinder and
piston would give an intermitting motion to the paddle-shaft, the
motion of the wheel being continued only during the descent of the
piston; but if several cylinders were provided, then their motion
might be so managed, that when one would be performing its
ascending stroke, and therefore giving no motion to the
paddle-shaft, another should be performing its descending stroke,
and therefore driving the paddle-shaft. As the interval between
the arrival of the piston at the bottom of the cylinder and the
commencement [Pg180] of its next descent would have been, in the
imperfect machine conceived by Papin, much longer than the time of
the descent, it was evident that more than two cylinders would be
necessary to insure a constantly acting force on the paddle-shaft,
and, accordingly, Papin proposed to use several cylinders.
In addition to this, Papin proposed to construct a boiler having a
fireplace surrounded on every side by water, so that the heat
might be imparted to the water with such increased rapidity as to
enable the piston to make four strokes per minute. These projects
were promulged in 1690, but it does not appear that they were ever
reduced to experiment.
(106.) Savery proposed, in his original patent, in 1698, to apply
his steam engine as a general prime mover for all sorts of
machinery, by causing it to raise water to make an artificial
fall, by which overshot water-wheels might be driven. This
proposal was not acted on during the lifetime of Savery, but it
was at a subsequent period partially carried into effect. Mr.
Joshua Rigley erected several steam engines on this principle at
Manchester, and other parts of Lancashire, to impel the machinery
of some of the earliest manufactories and cotton mills in that
district. The engines usually raised the water from sixteen to
twenty feet high, from whence it was conveyed to an overshot
wheel, to which it gave motion. The same water was repeatedly
elevated by the engine, so that no other supply was necessary,
save what was sufficient to make good the waste. These engines
continued in use for some years, until superseded by improved
machines.[22]
(107.) In 1736, Jonathan Hulls obtained a patent for a method of
towing ships into or out of harbour against wind and tide. This
method was little more than a revival of that proposed by Papin in
1690. The motion, however, was to be communicated to the
paddle-shaft by a rope passing over a pulley fixed on an axis, and
was to be maintained during the returning stroke of the piston by
the descent of a weight which was elevated during the descending
stroke. There is no record, however, of this plan, any more than
that of Papin, ever having been reduced to experiment.
(108.) During the early part of the last century the [Pg181]
manufactures of this country had not attained to such an extent as
to render the moving power supplied by water insufficient or
uncertain to any inconvenient degree; and accordingly mills, and
other works in which machinery required to be driven by a moving
power, were usually built along the streams of rivers. About the
year 1750 the general extension of manufactures, and their
establishment in localities where water power was not accessible,
called the steam engine into more extensive operation. In the year
1752, Mr. Champion, of Bristol, applied the atmospheric engine to
raise water, by which a number of overshot wheels were driven.
These were applied to move extensive brass-works in that
neighbourhood, and this application was continued for about twenty
years, but ultimately given up on account of the expense of fuel
and the improved applications of the steam engine.
About this time Smeaton applied himself with great activity and
success to the improvement of wind and water mills, and succeeded
in augmenting their useful effect in a twofold proportion with the
same supply of water. From the year 1750 until the year 1780 he
was engaged in the construction of his improved water mills, which
he erected in various parts of the country, and which were
imitated so extensively that the improvement of such mills became
general. In cases where a summer drought suspended the supply of
water, horse machinery was provided, either to work the mill or to
throw back the water. These improvements necessarily obstructed
for a time the extension of steam power to mill work; but the
increase of manufactures soon created a demand for power greatly
exceeding what could be supplied by such limited means.