close the communication between the tube T and the boiler, and to
stop the further supply of steam to the vessel V; and at the same
time let the condensing pipe G be brought over the vessel V, and
the cock opened so as to let a stream of cold water flow upon it.
This will cool the vessel V, and the steam with which it is filled
will be condensed and fall in a few drops of water, leaving the
interior of the vessel a vacuum. The valve B will be kept closed
by the atmospheric pressure. But the elastic force of the air
between the valve A and the surface of the water in the well, or
reservoir, will open A, so that a part of this air will rush in,
and occupy the vessel V. The air in the suction pipe S, being thus
allowed an increased space, will be proportionally diminished in
its elastic force, and its pressure will no longer balance that of
the atmosphere acting on the external surface of the water in the
reservoir. This pressure will, therefore, force water up in the
tube S until its weight, together with the elastic force of the
air above it, balances the atmospheric pressure. When this has
taken place, the water will cease to ascend.
Let us now suppose that, by shifting the regulator, the
communication is opened between T and the boiler, so that steam
flows again into V. The condensing cock G being removed, the
vessel will be again heated as before, the air expelled, and its
place filled by the steam. The condensing pipe being again allowed
to play upon the vessel V, and the further supply of steam being
stopped, a vacuum will be produced in V, and the atmospheric
pressure will force the water through the valve A into the vessel
V, which it will nearly fill, a small quantity of air, however,
remaining above it.
Thus far the mechanical agency employed in elevating the water is
the atmospheric pressure; and the power of steam is no further
employed than in the production of a vacuum. [Pg054] But, in
order to continue the elevation of the water through the force
pipe F, above the level of the steam vessel, it will be necessary
to use the elastic pressure of the steam. The vessel V is now
nearly filled by the water which has been forced into it by the
atmosphere. Let us suppose that, the regulator being shifted
again, the communication between the tube T and the boiler is
opened, the condensing cock removed, and that steam flows into V.
At first, coming in contact with the cold surface of the water and
that of the vessel, it is condensed; but the vessel is soon
heated, and the water formed by the condensed steam collects in a
sheet or film upon the surface of the water in V, so as to form a
surface as hot as boiling water.[10] The steam then being no
longer condensed, presses on the surface of the water with its
elastic force; and when that pressure becomes greater than the
atmospheric pressure, the valve B is forced open, and the water
issuing through it, passes through E into the force-pipe F; and
this is continued until the steam has forced all the water from V,
and occupies its place.
The further admission of steam through T is once more stopped by
moving the regulator; and the condensing pipe being again allowed
to play on V, so as to condense the steam which fills it, produces
a vacuum. Into this vacuum, as before, the atmospheric pressure
will force the water, and fill the vessel V. The condensing pipe
being then closed, and steam admitted through T, the water in V
will be forced by its pressure through the valve B and tube E into
F, and so the process is continued.
We have not yet noticed the other steam vessel V′, which, as far
as we have described, would have remained filled with common
atmospheric air, the pressure of which on the valve A′ would have
prevented the water raised in the suction pipe S from passing
through it. However, this is not the case; for, during the entire
process which has been described in V, similar effects have been
produced in V′, which we have only omitted to notice to avoid the
confusion which the two processes might produce. It will be
remembered, that after the steam, in the first instance, having
flowed from the boiler [Pg055] through T, has blown the air out
of V through B, the communication between T and the boiler is
closed. Now the same motion of the regulator which closes this,
opens the communication between T′ and the boiler; for the sliding
plate R (_fig._ 11.) is moved from the one tube to the other, and
at the same time, as we have already stated, the condensing pipe
is brought to play on V. While, therefore, a vacuum is being
formed in V by condensation, the steam, flowing through T′, blows
out the air through B′, as already described in the other vessel
V; and while the air in S is rushing up through A into V, followed
by the water raised in S by the atmospheric pressure, the vessel
V′ is being filled with steam, and the air is completely expelled
from it.
The communication between T and the boiler is now again opened,
and the communication between T′ and the boiler closed by moving
the regulator R (_fig._ 11.) from the tube T to T′; at the same
time the condensing pipe is removed from over V, and brought to
play upon V′. While the steam once more expels the air from V
through B, a vacuum is formed by condensation in V′, into which
the water in S rushes through the valve A′. In the mean time V is
again filled with steam. The communication between T and the
boiler is now closed, and that between T′ and the boiler is
opened, and the condensing pipe removed from V′, and brought to
play on V. While the steam from the boiler forces the water in V′
through B′ into the force-pipe F, a vacuum is being produced in V,
into which water is raised by the atmospheric pressure.
Thus each of the vessels V V′ is alternately filled from S, and
the water thence forced into F. The same steam which forces the
water from the vessels into F, having done its duty, is condensed,
and brings up the water from S, by giving effect to the
atmospheric pressure.
During this process, two alternate motions or adjustments must be
constantly made; the communication between T and the boiler must
be opened, and that between T′ and the boiler closed, which is
done by one motion of the regulator. The condensing pipe at the
same time must be brought from V to play on V′, which is done by
the lever placed upon it. Again [Pg056] the communication between
T′ and the boiler is to be opened, and that between T and the
boiler closed; this is done by moving back the regulator. The
condensing pipe is brought from V′ to V by moving back the other
lever, and so on alternately.
For the clearness and convenience of description, some slight and
otherwise unimportant changes have been made in the position of
the parts. A perspective view of this engine is represented at the
head of this chapter. The different parts already described will
easily be recognised.
The engine of Savery was very clearly described in a small work
published in London in 1702, entitled, _The Miner's Friend, or an
Engine to raise Water by Fire described, and the Manner of Fixing
it in Mines; with an Account of the several Uses it is applicable
unto, and an Answer to the Objection made against it; by Thomas
Savery, Gentleman_. This volume was dedicated to William III. (to
whom the engine had been exhibited at Hampton Court palace), to
the Royal Society, and to the mining adventurers of England. The
following are the uses to which Savery proposed the engine should
be applied: _First_, to raise water for turning all sorts of
mills; _second_, supplying palaces and houses with water, and
supplying means of extinguishing fire therein by the water so
raised; _third_, the supplying cities and towns with water;
_fourth_, draining fens or marshes; _fifth_, for ships; _sixth_,
the drainage of mines.
Dr. Harris, in his _Lexicon Technicum, or Dictionary of Arts and
Sciences_, mentions a machine of Savery's for propelling a vessel
in a calm, by paddle-wheels placed at the side; but it does not
appear that Savery contemplated the application of a steam engine
to work these wheels.
It is only from scattered passages in publications of the day that
it can be ascertained to what extent the engines of Savery were
practically applied. In his address to the Royal Society, he
speaks of the "difficulties and expense which he encountered in
instructing artisans to make engines according to his wish; but
that after much experience the workmen had become such masters of
the thing, that they bound themselves to deliver the engines
'exactly tight and fit for [Pg057] service, and such as he
(Savery) dare warrant them to every one that has occasion for
them.'"
In his address to the miners of England he also says, "that the
frequent disorders and cumbersomeness of water engines then in use
encouraged him to invent engines to work by this new force; that
though they were obliged to encounter the oddest and almost
insuperable difficulties, yet he spared neither time, pains, nor
money, till he had conquered them."
In Bradley's _Improvements of Planting and Gardening_, 1718, the
author thus speaks of an engine erected by Savery:—
"Supposing the situation of a house or garden to be a considerable
height above any pond, river, or spring, and that it has at
present no other conveniency of water than what is brought
continually by men or horses to it. In this case, the wonderful
invention of the late Mr. Savery, F.R.S., for raising water by
fire, will not only supply the defect, by flinging up as much
water as may be desired, but may be maintained with very little
trouble and very small expense.
"It is now about six years since Mr. Savery set up one of them for
that curious gentleman Mr. Balle, at Cambden House, Kensington,
near London, which has succeeded so well that there has not been
any want of water since it has been built; and, with the
improvements since made to it, I am apt to believe will be less
subject to be out of order than any engine whatever."
It is remarkable that, notwithstanding the high pressure steam
necessary for the operation of Savery's engine, he does not appear
to have adopted the obvious expedient of a safety valve. The
safety valve had been previously known, having been invented about
the year 1681, by Papin, for his digester, which was a close
boiler, contrived by him for stewing meat and digesting bones, by
submitting them to a higher temperature than that of water boiling
in an open vessel.
The safety valve which has ever since been used for steam boilers
of every kind is a valve which opens outwards, and is fitted to an
aperture in the boiler, so as to be steam tight. It is pressed
down by a weight, the amount of which is regulated by the maximum
pressure to which it is intended the steam [Pg058] shall be
limited. Thus, if the magnitude of the valve be a square inch, and
the pressure of the steam be limited to 10 lbs. per square inch
above the pressure of the atmosphere, then the valve would be
loaded with a weight of 10 lbs.; but as it was found necessary to
vary from time to time the limiting pressure of the steam, or the
load of the safety valve, these valves were usually constructed so
as to be held down by the pressure of a lever having a sliding
weight upon it. By moving the weight on the arm of the lever, the
pressure on the valve could be increased or diminished at the
discretion of the engineer. This contrivance was first applied to
Savery's engines, by Desaguliers, about the year 1717, before
which year Savery died.
It is justly observed by Mr. Farey, in his treatise on the steam
engine, that, "when a comparison is made between Captain[11]
Savery's engine and those of his predecessors, the result will be
in every respect favourable to his character as an inventor, and
as a practical engineer; all the details of his invention are made
out in a masterly style, and accidents and contingencies are
provided for, so as to render it a real working engine; whereas De
Caus, the Marquis of Worcester, Sir Samuel Morland, and Papin,
though ingenious philosophers, only produced mere outlines, which
required great labour and skill of subsequent inventors to fill
up, and make them sufficiently complete to be put in execution."
About the year 1718 further improvements were made in the
construction of Savery's engine, by Dr. Desaguliers; but it is
probable that some of these were suggested by the proceedings of
the inventors of the atmospheric engine, which shall presently
describe.
(33.) In order duly to appreciate the value of improvements, it is
necessary first to perceive the defects which these improvements
are designed to remove. Savery's steam engine, considering how
little was known of the value and properties of steam, and how low
the general standard of mechanical knowledge was in his day, is
certainly highly [Pg059] creditable to his genius. Nevertheless
it had very considerable defects, and was finally found to be
inefficient for the most important purposes to which he proposed
applying it.
At the time of this invention, the mines in England had greatly
increased in depth, and the process of draining them had become
both expensive and difficult; so much so, that it was found in
many instances that their produce did not cover the cost of
working them. The drainage of these mines was the most important
purpose to which Savery proposed to apply his steam engine.
It has been already stated that the pressure of the atmosphere
amounts to about fifteen pounds on every square inch. Now, a
column of water, whose base is one square inch, and whose height
is thirty-four feet, weighs about fifteen pounds. If we suppose
that a perfect vacuum were produced in the steam vessels V V′
(_fig._ 12.) by condensation, the atmospheric pressure would fail
to force up the water, if the height of the top of these vessels
above the water to be raised exceeded thirty-four feet. It is
plain, therefore, that the engine cannot be more than thirty-four
feet above the water which it is intended to elevate. But in fact
it cannot be so much; for the vacuum produced in the steam vessels
V V′ is never perfect. Water, when not submitted to the pressure
of the atmosphere, will vaporise at a very low temperature, as we
shall hereafter explain; and it was found that a vapour possessing
a considerable elasticity would, notwithstanding the condensation,
remain in the vessels V V′ and the pipe S, and would oppose the
ascent of the water. In consequence of this, the engine could
never be placed with practical advantage at a greater height than
twenty-six feet above the level of the water to be raised.
(34.) When the water is elevated to the engine, and the steam
vessels filled, if steam be introduced above the water in V, it
must first balance the atmospheric pressure, before it can force
the water through the valve B. Here, then, is a mechanical
pressure of fifteen pounds per square inch expended, without any
water being raised by it. If steam of twice that elastic force be
used, it will elevate a column in F of thirty-four feet in height;
and if steam of triple the force be used, it will raise a column
of sixty-eight feet high, [Pg060] which, added to twenty-six feet
raised by the atmosphere, gives a total lift of ninety-four feet.
In effecting this, steam of a pressure equal to three times that
of the atmosphere acts on the inner surface of the vessels V V′.
One third of this bursting pressure is balanced by the pressure of
the atmosphere on the external surface of the vessels; but an
effective pressure of thirty pounds per square inch still remains,
tending to burst the vessels. It was found that the apparatus
could not be constructed to bear more than this with safety; and,
therefore, in practice, the lift of such an engine was limited to
about ninety perpendicular feet. In order to raise the water from
the bottom of the mine by these engines, therefore, it was
necessary to place one at every ninety feet of the depth; so that
the water raised by one through the first ninety feet should be
received in a reservoir, from which it was to be elevated the next
ninety feet by another, and so on.
Besides this, it was found that sufficient strength could not be
given to those engines, if constructed upon a large scale.
They were, therefore, necessarily very limited in their
dimensions, and were incapable of raising the water with
sufficient speed. Hence arose a necessity for several engines at
each level, which greatly increased the expense.
(35.) These, however, were not the only defects of Savery's
engines. The consumption of fuel was enormous, the proportion of
heat wasted being much more than what was used in either forcing
up the water, or producing a _vacuum_. This will be very easily
understood by attending to the process of working the engine
already described.
When the steam is first introduced from the boiler into the steam
vessels V V′, preparatory to the formation of a vacuum, it is
necessary that it should heat these vessels up to the temperature
of the steam itself; for until then the steam will be condensed
the moment it enters the vessel by the cold surface. All this
heat, therefore, spent in raising the temperature of the steam
vessels is wasted. Again, when the water has ascended and filled
the vessels V V′, and steam is introduced to force this water
through B B′ into F, it is immediately condensed by the cold
surface in V V′, and does not [Pg061] begin to act until a
quantity of hot water, formed by condensed steam, is collected on
the surface of the cold water which fills these vessels. Hence
another source of the waste of heat arises.
When the steam begins to act upon the surface of the water in V
V′, and to force it down, the cold surface of the vessels is
gradually exposed to the steam, and must be heated while the steam
continues its action; and when the water has been forced out of
the vessel, the vessel itself has been heated to the temperature
of the steam which fills it, all which heat is dissipated by the
subsequent process of condensation. It must thus be evident that
the steam used in forcing up the the water in F, and in producing
a vacuum, bears a very small proportion indeed to what is consumed
in heating the apparatus after condensation.
(36.) There is also another circumstance which increases the
consumption of fuel. The water must be forced through B, not only
against the atmospheric pressure, but also against a column of
sixty-eight feet of water. Steam is therefore required of a
pressure of forty-five pounds on the square inch. Consequently the
water in the boiler must be boiled under this pressure. That this
should take place, it is necessary that the water should be raised
to a temperature considerably above 212°, even so high as 275°;
and thus an increased heat must be given to the boiler.
Independently of the other defects, this intense heat weakened and
gradually destroyed the apparatus.
Savery was the first who suggested the method of expressing the
power of an engine with reference to that of horses. In this
comparison, however, he supposed each horse to work but eight
hours a day, while the engine works for twenty-four hours. This
method of expressing the power of steam engines will be explained
hereafter.
(37.) The failure of the engines proposed by Captain Savery in the
work of drainage, from the causes which have been just mentioned,
and the increasing necessity for effecting this object, arising
from the large property in mines which became every year
unproductive by being flooded, stimulated the ingenuity [Pg062]
of mechanics to contrive some means of rendering those powers of
steam exhibited in Savery's engine available.
Thomas Newcomen, the reputed inventor of the atmospheric engine, was
an ironmonger, or, according to some, a blacksmith, in the town of
Dartmouth in Devonshire. From his personal acquaintance and
intercourse with Dr. Hooke, the celebrated natural philosopher, it
is probable that he was a person of some education, and therefore
likely to be above the position of a blacksmith. Being in the habit
of visiting the tin mines in Cornwall, Newcomen became acquainted
with the engine invented by Savery, and with the causes which led to
its inefficiency for the purposes of drainage.
It has been stated that Papin, about the year 1690, proposed the
construction of an engine working by the atmospheric pressure
acting on one side of a piston against a vacuum produced by the
condensation of steam on the other side. Papin was not conscious
of the importance of this principle; for, so far from ever having
attempted to apply it to practical purposes, he probably never
constructed, even on a small scale, any machine illustrating it.
On the contrary, he abandoned the project the moment he was
informed of the principle and structure of the steam engine of
Savery; and he then proposed an engine for raising water, acting
by the expansive force of steam similar to Savery's, but
abandoning the method of working by a vacuum.
This engine is described by Papin in a work published in 1707.
[Illustration: _Fig._ 13.]
A (_fig._ 13.) is an oval boiler, having a safety-valve B, which
limits the pressure of the steam. It is connected with a cylinder
C, by a curved pipe having a stop-cock at D. A pipe with a
stop-cock G opens from the top of the cylinder into the
atmosphere, and a safety-valve F is placed upon the cylinder. A
hollow copper piston H moves freely in the cylinder, and floats
upon the water. O is a funnel with a valve L in the bottom,
opening downwards, through which the cylinder C may be filled with
water to the level of the top of the funnel. A close air-vessel
communicates with the cylinder C by the curved tube, and has a
valve K opening upwards. The force-pipe through which the water is
raised communicates [Pg063] with the air-vessel I. If the cock D
be shut, and the cock G opened, water poured into the funnel O
will rise into the cylinder C, the air which fills the cylinder
escaping through the open pipe G. When the cylinder is thus filled
with water, let the cock G be closed, and the cock D opened. The
steam from the boiler, after heating the metal of the cylinder,
will force the piston downwards, and drive the water through the
curved tube into the vessel I, from which its return is prevented
by the valve K, which is closed by its weight. The air which
filled the vessel I will then be compressed, and by its elasticity
will drive a column of water up the pipe N. After the contents of
the cylinder have been thus discharged it may be refilled in the
same manner, and the process repeated.
It will be perceived that this project is nothing more than a
reproduction of the engine of the Marquis of Worcester. In the
preface to the work containing this description, Papin gives an
extract from a letter addressed by him to Leibnitz in 1698, from
which it appears that he had abandoned his idea of working the
piston by the atmospheric pressure acting against a vacuum,
considering it to be a contrivance inferior [Pg064] to the engine
now described. "We now raise water," he says, "by the force of
fire, _in a more advantageous manner than that which I had
published some years before_; for besides the suction, we now also
use the pressure which the water exerts upon other bodies in
dilating itself by heat; instead of which I before employed the
suction only, the effects of which are more limited."
From documents which have been preserved in the Royal Society, it
appears that Newcomen was acquainted with Papin's writings, and
therefore probably first derived from them the suggestion which he
subsequently realised in the atmospheric engine. Among some papers
of Dr. Hooke's have been found notes for the use of Newcomen, on
Papin's method of transmitting the force of a stream or fall of
water to a distance by pipes. Hooke dissuaded Newcomen from
attempting any machine on this principle, which, as first proposed
by Papin, was impracticable. He exposed the fallacy of Papin's
first project in several discourses before the Royal Society, and
considered his improved edition of it, though free from fallacy,
as impracticable.