rests on the surface C of the mercury in the cistern, and tends
thereby to press it up, or rather to resist its fall in the tube;
and as the fall is not assisted by the weight of the atmosphere on
the surface D (since B is closed), it follows, that as much
mercury remains suspended in the tube above the level C, as the
weight of the atmosphere is able to support.
If the section of the tube were equal to the magnitude of a square
inch, the weight of the column of mercury in the tube above the
level C would be exactly equal to the weight of the atmosphere on
each square inch of the surface C.
(22.) If such an apparatus be observed from time to time, it will
be found that the column of mercury sustained in the tube will be
subject to variation between certain limits, never falling below
twenty-eight inches, and never rising above thirty-one inches.
This variation of the mercurial column is produced by a
corresponding variation in the weight of the atmosphere.
If the apparatus be transported to any height above its ordinary
position, it will have a less quantity of atmosphere above it, and
therefore the surface of the mercury in the cistern will be
pressed by a less weight, and consequently the [Pg041] column of
mercury will fall proportionally. In virtue of this effect, such
an instrument has been rendered a means of measuring heights, such
as the heights of mountains, the ascents of balloons, &c. &c.
(23.) If a proper scale be attached to the tube containing the
mercurial column, showing the absolute height of the column
sustained at any time, and indicating its changes of height, the
instrument becomes a BAROMETER.
Two cubic inches of mercury weigh very nearly one pound
avoirdupois.[6] Hence, when the barometric column measures thirty
inches, the weight of the atmosphere resting on each square inch
of surface is about fifteen pounds.
(24.) It is an established property of fluids, that they press
equally in all directions; and air, like every other fluid,
participates in this quality. Hence, it follows, that when the
downward pressure or weight of the atmosphere is fifteen pounds on
the square inch, the lateral, upward, and oblique pressures are of
the same amount. But, independently of the general principle, it
may be satisfactory to give experimental proof of this.
[Illustration: _Fig._ 10.]
Let four glass tubes, A, B, C, D (_fig._ 10.), be constructed of
sufficient length, closed at one end, A, B, C, D, and open at the
other. Let the open ends of three of them be bent, as represented
in the tubes B, C, D. Being previously filled with mercury, let
them all be gently inverted, so as to have their closed ends up,
as here represented. It will be found that the mercury will be
sustained in all, and that the difference of the levels in all
will be the same.[7] Thus, the mercury is sustained in A by the
upward pressure of the atmosphere; in B, by its horizontal or
lateral pressure; in C, by its downward pressure; [Pg042] and in
D, by its oblique pressure: and, as the difference of the levels
is the same in all, these pressures are exactly equal.
(25.) The same arrangement by which the pressure of the atmosphere
is measured by a mercurial column of equivalent weight, also
supplies the means of measuring the pressure or elasticity of
atmospheric air, or any other gas or vapour, whether in a more or
less compressed or rarefied state; and as instruments constructed
on this principle are of considerable use in steam engines, we
shall take this occasion to explain their principle and
application.
In the experiments described in (21), the space D B in the top of
the barometer-tube, from which the mercury descended, is a vacuum.
If, however, it were occupied by a quantity of air in a rarefied
state, or any other gas or vapour, such gas or vapour would press
on the surface of the mercury at D, with a force determined by its
elasticity. In that case, the atmospheric pressure acting on the
surface of the mercury C in the cistern, would be balanced by the
combined forces of the weight of the mercurial column sustained in
the tube, and the elasticity of the gas or vapour in the upper
part of it. Now if we know the actual amount of the atmospheric
pressure,—that is to say, the height of the column of mercury
which it would be capable of sustaining,—we should then be able
to determine the pressure of the rarefied air in the space C D.
For example, let us suppose that the barometric column, when B D
(_fig._ 9.) is a vacuum, measures thirty inches: the atmospheric
pressure, therefore, would be equal to the weight of a column of
mercury of that height. Let us suppose that the elasticity of the
gas or vapour occupying the upper part of the tube D B causes the
column to fall to the height of twenty-six inches: it is evident,
then, that the pressure of the air in the top of the tube would be
equal to the weight of a column of mercury of four inches. In
fine, to determine the pressure of the rarefied gas or vapour in
the top of the tube, it is only necessary to observe the
difference between the height of the column of mercury actually
sustained in the tube, and the column sustained at the same time
and [Pg043] place in a common barometer: the difference of the
two will be the column of mercury whose weight will represent the
pressure of the vapour or gas in the top of the tube.
(26.) Whenever the air contained in any vessel or other enclosed
space has by any means had its pressure reduced so as to be
rendered less than that of the external air, the external air will
have a tendency to rush into such vessel or enclosed space with a
force proportionate to the excess of the pressure of such external
air over that of the air within; and if any communication be
opened between the interior of such vessel or enclosed space, and
the external air, the latter will rush in until an equilibrium be
established between the pressures within and without. It is
evident that the force thus obtained by diminishing the pressure
of air within a vessel may be applied to any mechanical purpose.
It is by such means that water is raised in an ordinary pump. A
portion of the air contained between the piston of the pump and
the surface of the water below, is withdrawn by the action of the
piston, and the pressure of the air remaining under the piston is
thereby diminished. The superior pressure of the atmosphere upon
the external surface of the water in the well then forces up a
column of water in the pump-barrel, and this is continued as the
air is more and more rarefied by the action of the piston. By
whatever means, therefore, the air can be wholly or partially
withdrawn from any space, a mechanical power will be thereby
developed, proportional in its amount and efficacy to the quantity
of air so withdrawn. If, however, such air be withdrawn by any
mechanical process, such as by a syringe, by a common pump, or by
an air-pump, the quantity of force expended in withdrawing it is
always equivalent to the amount of mechanical power obtained by
the vacuum or partial vacuum so produced. Indeed the power
expended is greater than the power so obtained, inasmuch as the
friction, leakage, &c. of the exhausting apparatus must be allowed
for.
(27.) There are, however, various other means by which air may be
partially expelled from a vessel besides the direct application of
mechanical force. Thus if heat be applied to [Pg044] the vessel,
the air, as has been already explained, will acquire increased
elasticity, and will rush from the vessel with a force proportionate
to the excess of its elasticity above that of the external air, and
this process may be continued by increasing the heat to which the
vessel is exposed, until a very considerable portion of the air has
been expelled. If the orifice by which the air has escaped be then
closed, and the vessel be allowed to cool, the air within, by having
its temperature reduced to that of the external air, will lose all
the elasticity which it had gained from the heat, and will be in the
same condition as if an equivalent quantity of air had been
withdrawn by any mechanical agent. The external air, therefore, will
have a tendency to rush in with a force corresponding to the
difference of pressures.
The process of filling thermometers with mercury shows one use of
producing a high degree of rarefaction by heat. To construct the
instrument it is necessary to fill the bulb and a part of the tube
with mercury; but the bore of the tube is so small that the
mercury cannot be introduced by any ordinary means. It is
therefore held over flame until heated to a high temperature. The
air within it gradually increasing in pressure as its temperature
is raised, is forced through the small bore of the tube, until the
pressure of the air within becomes no more than equal to the
pressure of the external atmosphere; this air being so rarefied
that quantity in the bulb bears a very small proportion to its
contents at common temperatures. The mouth of the tube is then
plunged into mercury, and as the bulb cools, the air within it
loses its elasticity, and the superior pressure upon the external
surface forces the mercury into the tube. This continues until the
air remaining within the bulb has been so contracted, that its
pressure combined with the weight of the mercury, shall balance
the atmospheric pressure. The tube is then reversed, and the air
which remained rises in a bubble to the surface, and escapes.
(28.) Let us now return to the proceedings of Papin. How great a
power would result from such a machine as he conceived, will be
apparent, if it be considered that the unresisted atmosphere
exercises a pressure of about fifteen pounds on [Pg045] each
square inch of surface exposed to it, and that if the piston in
the cylinder imagined by Papin, had a diameter of only one foot,
its superficial magnitude would be about 114 square inches. The
pressure of the atmosphere upon it, therefore, would be 114 times
fifteen pounds, or 1710 pounds. Papin first proposed to produce
the vacuum under the piston by means of common air pumps, worked
by a water-wheel; and by such means he conceived that the power of
a river, stream, or waterfall might be conveyed by pipes to a
distance. While he was in England, in 1687, he laid his
contrivance before the Royal Society of London, but was met by
objections and difficulties, the nature of which he does not
explain.
It is, however, apparent, from what has been already explained,
that such a method of proceeding would amount to a mere transfer
of power, and would not, properly speaking, be itself a moving
force: the moving power would, in reality, be the force of the
water by which the water-wheel would be driven; and the air-pumps,
tubes, together with the piston and cylinder, would be merely
means of conveying the power of the water-wheel to the objects to
be moved, or the machinery to be driven. Papin states, that, long
before this, he had attempted to expel the air from his cylinder
by means of gunpowder; but, notwithstanding all the precautions
which he could take, there always remained a considerable
quantity; so much, indeed, as to deprive the vacuum of more than
half its proper force. At length he adopted an expedient for the
production of a vacuum which forms a most important step in the
progressive invention of the steam engine, and which gives to
Papin's name a high place in the history of that machine. This
method is explained in the following paragraph of a work published
by Papin in 1695, at Cassel, entitled "_Recueil de diverses Pièces
touchant quelques nouvelles Machines_", p. 53.
"I have endeavoured," says he, "to attain this end (viz. the
production of a vacuum in the cylinder) in another way. As water
has the property of elasticity, when converted into steam by heat,
and afterwards of being so completely recondensed by cold, that
there does not remain the least [Pg046] appearance of this
elasticity, I have thought that it would not be difficult to work
machines in which, by means of a moderate heat and at a small
cost, water might produce that perfect vacuum which has vainly
been sought by means of gunpowder."
This remarkable passage is given in the work just cited, as an
extract from the "Leipsic Acts," of August, 1690.
Let us pause here to explain more fully this important discovery.
(29.) We have explained that, in its conversion into vapour, by
the application of heat, water, besides acquiring the property of
elasticity, undergoes a vast enlargement of bulk, filling, under
ordinary circumstances, about 1700 times more space than it
occupied in the liquid form. This fact was known generally, though
not with numerical accuracy, by Papin, having been the foundation
of the machines previously invented and published by De Caus and
Lord Worcester; the happy idea of reversing the process occurred
to him. If water in its conversion into steam swelled into many
hundred times its original bulk, it would necessarily follow, that
steam, being reconverted into water, would shrink into its
primitive dimensions. Papin therefore saw, that if he could by any
means expel the air from his cylinder under the piston, and
replace it by the pure vapour of water, he could cause that vapour
to be reconverted into a comparatively minute quantity of water by
depriving it of the heat which sustained it in the state of steam,
and that by accomplishing this, the space in the cylinder under
the piston would become a vacuum; that by such means, the pressure
of the atmosphere above the piston would take full effect, and
would urge the piston down; that by introducing more steam under
the piston, it might be again raised by the elastic force of the
steam, the destruction of which by cold water would again produce
the descent of the piston with the same mechanical force; and that
in this way the alternate ascent and descent of the piston might
be continued indefinitely.
In accordance with these ideas, Papin constructed a model
consisting of a small cylinder, in which was placed a solid
piston; [Pg047] and in the bottom of the cylinder under the
piston was contained in a small quantity of water. The piston
being in immediate contact with this water, so as to exclude the
atmospheric air, on applying fire to the bottom of the cylinder,
steam was produced, the elastic force of which raised the piston
to the top of the cylinder; the fire being then removed, and the
cylinder being cooled by the surrounding air, the steam was
condensed and reconverted into water, leaving a vacuum in the
cylinder into which the piston was pressed by the force of the
atmosphere. The fire being applied and subsequently removed,
another ascent and descent were accomplished; and in the same
manner the alternate motion of the piston might be continued.
Papin described no other form of machine by which this property
could be rendered available in practice; but he states generally,
that the same end may be attained by various forms of machines
easy to be imagined.[8]
THOMAS SAVERY, 1698.
(30.) The discovery of the method of making a vacuum by the
condensation of steam was reproduced, before 1698, by Captain
Thomas Savery, to whom a patent was granted in that year for a
steam engine to be applied to the raising of water, &c. Savery
proposed to combine the machine described by the Marquis of
Worcester with an apparatus for raising water by suction into a
vacuum produced by the condensation of steam.
Savery appears to have been ignorant of the publication of Papin,
and stated that his discovery of the condensing principle arose
from the following circumstance:—
Having drunk a flask of Florence at a tavern, and flung the empty
flask on the fire, he called for a basin of water to wash his
hands. A small quantity which remained in the flask began to boil,
and steam issued from its mouth. It occurred to him to try what
effect would be produced by inverting the flask and plunging its
mouth in the cold water. Putting on a thick glove to defend his
hand from the heat, he seized the [Pg048] flask, and the moment
he plunged its mouth in the water the liquid immediately rushed up
into the flask and filled it.
Savery stated that this circumstance immediately suggested to him
the possibility of giving effect to the atmospheric pressure by
creating a vacuum in this manner. He thought that if, instead of
exhausting the barrel of a pump by the usual laborious method of a
piston and sucker, it was exhausted by first filling it with
steam, and then condensing the same steam, the atmospheric
pressure would force the water from the well into the pump-barrel,
and into any vessel connected with it, provided that vessel were
not more than about thirty-four feet above the elevation of the
water in the well. He perceived also, that, having lifted the
water to this height, he might use the elastic force of steam in
the manner described by the Marquis of Worcester to raise the same
water to a still greater elevation, and that the same steam which
accomplished this mechanical effect would serve, by its subsequent
condensation, to reproduce the vacuum, and draw up more water. It
was on this principle that Savery constructed the first engine in
which steam was ever brought into practical operation.
[Illustration: BRANCA'S ENGINE.]
FOOTNOTES:
[1] Arago, Eloge historique de James Watt; p. 22.
[2] Ibid., p. 21. note.
[3] Farey, Treatise on the Steam Engine, p. 93.
[4] Arago, sur les Machines à Vapeur, Annuaire, 1829, p. 165
[5] SPIRITUS, _breath_ or _air_.
[6] Exactly 15·68 oz. = 0·98 lb.
[7] This experiment with the tube A requires to be very
carefully executed, and the tube should be one of small bore.
[8] Recueil de diverses Pièces touchant quelques nouvelles
Machines, p. 38.
[Pg049]
[Illustration: SAVERY'S ENGINE.]
CHAP. II.
ENGINES OF SAVERY AND NEWCOMEN.
SAVERY'S ENGINE. — BOILERS AND THEIR APPENDAGES. — WORKING
APPARATUS. — MODE OF OPERATION. — DEFECTS OF THE ENGINE. —
NEWCOMEN AND CAWLEY. — ATMOSPHERIC ENGINE. — ACCIDENTAL
DISCOVERY OF CONDENSATION BY INJECTION. — HUMPHREY POTTER
MAKES THE ENGINE WORK ITSELF. — ADVANTAGES OF THE ATMOSPHERIC
ENGINE OVER THAT OF SAVERY. — IT CONTAINED NO NEW PRINCIPLE. —
ITS PRACTICAL SUPERIORITY.
(31.) The steam engine contrived by Savery, like every other which
has since been constructed, consists of two parts, essentially
distinct. The first is that which is employed to [Pg050] generate
the steam, which is called the boiler; and the second, that in
which the steam is applied as a moving power.
[Illustration: _Fig._ 11.]
The former apparatus in Savery's engine consists of two strong
boilers, sections of which are represented at D and E in _fig._
11.; D the greater boiler, and E the less. The tubes T and T′
communicate with the working apparatus, which we shall presently
describe. A thin plate of metal R, is applied closely to the top
of the great boiler D, turning on a centre C, so that by moving a
lever applied to the axis C on the outside of the top, the sliding
plate R can be brought from the mouth of the one tube to the mouth
of the other alternately. This sliding valve is called the
_regulator_, since it is by it that the communications between the
boiler and two steam vessels (hereafter described) are alternately
opened and closed, the lever which effects this being moved at
intervals by the hand of the attendant.
Two _gauge cocks_ are represented at G, G′, the use of which is to
determine the depth of water in the boiler. One, G, has its lower
aperture a little above the proper depth; and the other, G′, a
little below it. Cocks are attached to the upper ends G, G′, which
can be opened or closed at pleasure. The steam collected in the
top of the boiler pressing on the surface of the water, forces it
up in the tubes G, G′, if their lower ends be immersed. Upon
opening the cocks G, G′, if water be forced from both, there is
too much water in the boiler, since the mouth of G is _below_ its
level. If steam issue from both, there is too little water in the
boiler, since the mouth of G′ is _above_ its level. But if steam
issue from G, and water from G′, the water in the boiler is at its
proper level. This ingenious contrivance for determining the level
of the water in the boiler is the invention of Savery, and is used
in many instances at the present day.
The mouth of the pipe G should be at a level of a little less
[Pg051] than one third of the whole depth, and the mouth of G′ at
a level little lower than one third; for it is requisite that
about two thirds of the boiler should be kept filled with water.
The tube I forms a communication between the greater boiler D and
the lesser or feeding boiler E, descending nearly to the bottom of
it. This communication can be opened and closed at pleasure by the
cock K. A gauge pipe is inserted similar to G, G′, but extending
nearly to the bottom. From this boiler a tube F extends, which is
continued to a cistern C (_fig._ 12.), and a cock is placed at M,
which, when opened, allows the water from the cistern to flow into
the feeding boiler E, and which is closed when that boiler is
filled. The manner in which this cistern is supplied will be
described hereafter.
Let us now suppose that the principal boiler is filled to the
level between the gauge pipes, and that the subsidiary boiler is
nearly full of water, the cock K and the gauge cocks G G′ being
all closed. The fire being lighted beneath D, and the water
boiled, steam is produced, and is transmitted through one or other
of the tubes T, T′, to the working apparatus. When evaporation has
reduced the water in D below the level of G′, it will be necessary
to replenish the boiler D. This is effected thus:—A fire being
lighted beneath the feeding boiler E, steam is produced in it
above the surface of the water, which, having no escape, presses
on the surface so as to force it up in the pipe I. The cock K
being then opened, the boiling water is forced into the principal
boiler D, into which it is allowed to flow until water issues from
the gauge cock G′. When this takes place, the cock K is closed,
and the fire removed from E until the great boiler again wants
replenishing. When the feeding boiler E has been exhausted, it is
replenished from the cistern C (_fig._ 12.), through the pipe F,
by opening the cock M.
(32.) We shall now describe the working apparatus in which the
steam is used as a moving power.
Let V V′ (_fig._ 12.) be two steam vessels communicating by the
tubes T T′ (marked by the same letters in _fig._ 11.) with the
greater boiler D.
[Illustration: _Fig._ 12.]
Let S be a pipe, called the _suction pipe_, descending into
[Pg052] the well or reservoir from which the water is to be
raised, and communicating with each of the steam vessels through
tubes D D′, by valves A A′, which open upwards. Let F be a pipe
continued from the level of the engine to whatever higher level it
is intended to elevate the water. The steam vessels V V′
communicate with the _force-pipe_ F by valves B B′, which open
upwards, through the tubes E E′. Over the steam vessels and on the
force-pipe is placed a small cistern C, already mentioned, which
is kept filled with cold water from the force-pipe, and from the
bottom of which proceeds a pipe terminated with a cock G. This is
called the _condensing pipe_, and can be brought alternately over
each steam vessel. From this cistern another pipe communicates
with the feeding boiler (_fig._ 11.), by the cock M.[9]
The communication of the pipes T T′ with the boiler can be opened
and closed alternately, by the regulator R (_fig._ 11.), already
described.
Now suppose the steam vessels and tubes to be all filled with
common atmospheric air, and that the regulator be placed so that
the communication between the tube T and the boiler be opened, the
communication between the other tube T′ and the boiler being
closed, steam will flow into V through T. At first, while the
vessel V is cold, the steam will be condensed, and will fall in
drops of water on the bottom and sides of the vessel. The
continued supply of steam from the boiler will at length impart
such a degree of heat to the vessel V, that it will cease to
condense it. Mixed with the heated air [Pg053] contained in the
vessel V, it will have an elastic force greater than the
atmospheric pressure, and will therefore force open the valve B,
through which a mixture of air and steam will be driven until all
the air in the vessel V will have passed out, and it will contain
nothing but the pure vapour of water.