Plain Rules for Railway Speculators. 307
THE STEAM ENGINE
EXPLAINED AND ILLUSTRATED.
CHAPTER I.
PRELIMINARY MATTER.
Motion the Agent in Manufactures. -- Animal Power. -- Power
depending on Physical Phenomena. -- Purpose of a Machine. --
Prime Mover. -- Mechanical qualities of the Atmosphere. -- Its
Weight. -- The Barometer. -- Fluid Pressure. -- Pressure of
Rarefied Air. -- Elasticity of Air. -- Bellows. -- Effects of
Heat. -- Thermometer. -- Method of making one. -- Freezing and
Boiling Points. -- Degrees. -- Dilatation of Bodies. --
Liquefaction and Solidification. -- Vaporisation and
Condensation. -- Latent heat of Steam. -- Expansion of Water in
Evaporating. -- Effects of Repulsion and Cohesion. -- Effect of
Pressure upon Boiling-Point. -- Formation of a Vacuum by
Condensation.
(1.) Of the various productions designed by nature to supply the wants
of man, there are few which are suited to his necessities in the state
in which the earth spontaneously offers them: if we except atmospheric
air, we shall scarcely find another instance: even water, in most
cases, requires to be transported from its streams or reservoirs; and
food itself, in almost every form, requires culture and preparation.
But if, from the mere necessities of physical existence in a primitive
state, we rise to the demands of civil and social life,--to say
nothing of luxuries and refinements,--we shall find that everything
which contributes to our convenience, or ministers to our pleasure,
requires a previous and extensive expenditure of labour. In most
cases, the objects of our enjoyment derive all their excellences, not
from any qualities originally inherent in the natural substances out
of which they are formed, but from those qualities which have been
bestowed upon them by the application of human labour and human skill.
In all those changes to which the raw productions of the earth are
submitted in order to adapt them to our wants, one of the principal
agents is _motion_. Thus, for example, in the preparation of clothing
for our bodies, the various processes necessary for the culture of the
cotton require the application of moving power, first to the soil, and
subsequently to the plant from which the raw material is obtained: the
wool must afterwards be picked and cleansed, twisted into threads, and
woven into cloth. In all these processes motion is the agent: to
cleanse the wool and arrange the fibres of the cotton, the wool must
be beaten, teased, carded, and submitted to other processes, by which
all the foreign and coarser matter may be separated, and the fibres or
threads arranged evenly, side by side. The threads must then receive a
rotatory motion, by which they may be twisted into the required form;
and finally peculiar motions must be given to them in order to produce
among them that arrangement which characterises the cloth which it is
our final purpose to produce.
In a rude state of society, the motions required in the infant
manufactures are communicated by the immediate application of the
hand. Observation and reflection, however, soon suggest more easy and
effectual means of attaining these ends: the strength of animals is
first resorted to for the relief of human labour. Further reflection
and inquiry suggest still better expedients. When we look around us in
the natural world, we perceive inanimate matter undergoing various
effects in which motion plays a conspicuous part: we see the falls of
cataracts, the currents of rivers, the elevation and depression of the
waters of the ocean, the currents of the atmosphere; and the question
instantly arises, whether, without sharing our own means of
subsistence with the animals whose force we use, we may not equally,
or more effectually, derive the powers required from these various
phenomena of nature? A difficulty, however, immediately presents
itself: we require motion of a particular kind; but wind will not
blow, nor water fall as we please, nor as suits our peculiar wants,
but according to the fixed laws of nature. We want an _upward_ motion;
water falls _downwards_: we want a _circular_ motion; wind blows in a
_straight_ line. The motions, therefore, which are in actual existence
must be modified to suit our purposes: the means whereby these
modifications are produced, are called _machines_. A machine,
therefore, is an instrument interposed between some natural force or
motion, and the object to which force or motion is desired to be
transmitted. The construction of the machine is such as to modify the
natural motion which is impressed upon it, so that it may transmit to
the object to be moved that peculiar species of motion which it is
required to have. To give a very obvious example, let us suppose that
a circular or rotatory motion is required to be produced, and that the
only natural source of motion at our command is a perpendicular fall
of water: a wheel is provided, placed upon the axle destined to
receive the rotatory motion; this wheel is furnished with cavities in
its rim; the water is conducted into the cavities near the top of the
wheel on one side; and being caught by these, its weight bears down
that side of the wheel, the cavities on the opposite side being empty
and in an inverted position. As the wheel turns, the cavities on the
descending side discharge their contents as they arrive near the
lowest point, and ascend empty on the other side. Thus a load of water
is continually pressing down one side of the wheel, from which the
other side is free, and a continued motion of rotation is produced.
In every machine, therefore, there are three objects demanding
attention:--first, The power which imparts motion to it, this is
called the _prime mover_; secondly, The nature of the _machine_
itself; and thirdly, The object to which the motion is to be conveyed.
In the steam engine the first mover arises from certain phenomena
which are exhibited when heat is applied to liquids; but in the
details of the machine and in its application there are several
physical effects brought into play, which it is necessary perfectly to
understand before the nature of the machine or its mode of operation
can be rendered intelligible. We propose therefore to devote the
present chapter to the explanation and illustration of these
phenomena.
(2.) The physical effects most intimately connected with the
operations of steam engines are some of the mechanical properties of
atmospheric air. The atmosphere is the thin transparent fluid in which
we live and move, and which, by respiration, supports animal life.
This fluid is apparently so light and attenuated, that it might be at
first doubted whether it be really a body at all. It may therefore
excite some surprise when we assert, not only that it is a body, but
also that it is one of considerable _weight_. We shall be able to
prove that it presses on every _square inch_[1] of surface with a
weight of about 15lb. avoirdupois.
[Footnote 1: As we shall have frequent occasion to mention this
magnitude, it would be well that the reader should be familiar with
it. It is a _square_, each side of which is an inch. Such as A B C D,
Fig. 1.]
(3.) Take a glass tube A B (fig. 2.) more than 32 inches long, open at
one end A, and closed at the other end B, and let it be filled with
mercury (quicksilver.) Let a glass vessel or cistern C, containing a
quantity of mercury, be also provided. Applying the finger at A so as
to prevent the mercury in the tube from falling out, let the tube be
inverted, and the end, stopped by the finger, plunged into the mercury
in _C_. When the end of the tube is below the surface of the mercury
in C (fig. 3.) let the finger be removed. It will be found that the
mercury in the tube will not, as might be expected, fall to the level
of the mercury in the cistern C, which it would do were the end B open
so as to admit the air into the upper part of the tube. On the other
hand, the level D of the mercury in the tube will be about 30 inches
above the level C of the mercury in the cistern.
(4.) The cause of this effect is, that the weight of the atmosphere
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 we suppose the section of the tube to be equal to the magnitude of
a square inch, the weight of the column of mercury in the tube above
the level C will be exactly equal to the weight of the atmosphere on
each square inch of the surface C. The height of the level D above C
being about 30 inches, and a column of mercury two inches in height,
and having a base of a square inch, weighing about one pound
avoirdupois, it follows that the weight with which the atmosphere
presses on each square inch of a level surface is about 15lb.
avoirdupois.
An apparatus thus constructed, and furnished with a scale to indicate
the height of the level D above the level C, is the _common
barometer_. The difference of these levels is subject to a small
variation, which indicates a corresponding change in the atmospheric
pressure. But we take 30 inches as a standard or average.
(5.) 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 since the downward pressure or
weight of the atmosphere is about 15lb. 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.
Let four glass tubes A, B, C, D, (fig. 4.) 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,[2] and that the difference of the levels in all will be the same.
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, and in D by its oblique pressure; and as the
difference of the levels is the same in all, these pressures are
exactly equal.
[Footnote 2: This experiment with the tube A requires to be very
carefully executed, and the tube should be one of small bore.]
(6.) In the experiment described in (3.) the space B D (fig. 3.) at
the top of the tube from which the mercury has fallen is perfectly
void and empty, containing neither air nor any other fluid: it is
called therefore a _vacuum_. If, however, a small quantity of air be
introduced into that space, it will immediately begin to exert a
pressure on D, which will cause the surface D to descend, and it will
continue to descend until the column of mercury C D is so far
diminished that the weight of the atmosphere is sufficient to sustain
it, as well as the pressure exerted upon it by the air in the space B
D.
The quantity of mercury which falls from the tube in this case is
necessarily an equivalent for the pressure of the air introduced, so
that the pressure of this air may be exactly ascertained by allowing
about one pound per square inch for every two inches of mercury which
has fallen from the tube. The pressure of the air or any other fluid
above the mercury in the tube, may at once be ascertained by comparing
the height of the mercury in the tube with the height of the
barometer; the difference of the heights will always determine the
pressure on the surface of the mercury in the tube. This principle
will be found of some importance in considering the action of the
modern steam engines.
The air which we have supposed to be introduced into the upper part of
the tube, presses on the surface of the mercury with a force much
greater than its weight. For example, if the space B D (fig. 3.) were
filled with atmospheric air in its ordinary state, it would exert a
pressure on the surface D equal to the whole pressure of the
atmosphere, although its weight might not amount to a single grain.
The property in virtue of which the air exerts this pressure is its
_elasticity_, and this force is diminished in precisely the proportion
in which the space which the air occupies is increased.
Thus it is known that atmospheric air in its ordinary state exerts a
pressure on the surface of any vessel in which it is confined,
amounting to about 15lb. on every square inch. If the capacity of the
vessel which contains it be doubled, it immediately expands and fills
the double space, but in doing so it loses half its elastic force, and
presses only with the force of 7-1/2lb. on every square inch. If the
capacity of the vessel had been enlarged five times, the air would
still have expanded so as to fill it, but would exert only a fifth
part of its first pressure, or 3lb. on every square inch.
This property of losing its elastic force as its volume or bulk is
increased, is not peculiar to air. It is common to all elastic fluids,
and we accordingly find it in steam; and it is absolutely necessary to
take account of it in estimating the effects of that agent.
(7.) There are numerous instances of the effects of these properties
of atmospheric air which continually fall under our observation. If
the nozzle and valve-hole of a pair of bellows be stopped, it will
require a very considerable force to separate the boards. This effect
is produced by the diminished elastic force of the air remaining
between the boards upon the least increase of the space within the
bellows, while the atmosphere presses, with undiminished force, on the
external surfaces of the boards. If the boards be separated so as to
double the space within, the elastic force of the included air will be
about 7-1/2lb. on every square inch, while the pressure on the
external surfaces will be 15lb. on every square inch; consequently, it
will require as great a force to sustain the boards in such a
position, as it would to separate them if each board were forced
against the other, with a pressure of 7-1/2lb. per square inch on
their external surfaces.
When boys apply a piece of moistened leather to a stone, so as to
exclude the air from between them, the stone, though it be of
considerable weight, may be lifted by a string attached to the
leather: the cause of which is the atmospheric pressure, which keeps
the leather and the stone in close contact.
(8.) The next class of physical effects which it is necessary to
explain, are those which are produced when heat is imparted or
abstracted from bodies.
In general, when heat is imparted to a body, an enlargement of bulk
will be the immediate consequence, and at the same time the body will
become warmer to the touch. These two effects of expansion and
increase of warmth going on always together, the one has been taken as
a measure of the other; and upon this principle the common thermometer
is constructed. That instrument consists of a tube of glass,
terminated in a bulb, the magnitude of which is considerable, compared
with the bore of the tube. The bulb and part of the tube are filled
with mercury, or some other liquid. When the bulb is exposed to any
source of heat, the mercury contained in it, being warmed or increased
in temperature, is at the same time increased in bulk, or expanded or
dilated, as it is called. The bulb not having sufficient capacity to
contain the increased bulk of mercury, the liquid is forced up in the
tube, and the quantity of expansion is determined by observing the
ascent of the column in the tube.
An instrument of this kind, exposed to heat or cold, will fluctuate
accordingly, the mercury rising as the heat to which it is exposed is
increased, and falling by exposure to cold. In order, however, to
render it an accurate measure of temperature, it is necessary to
connect with it a scale by which the elevation or depression of the
mercury in the tube may be measured. Such a scale is constructed for
thermometers in this country in the following manner:--Let us suppose
the instrument immersed in a vessel of melting ice: the column of
mercury in the tube will be observed to fall to a certain point, and
there maintain its position unaltered: let that point be marked upon
the tube. Let the instrument be now transferred to a vessel of boiling
water at a time when the barometer stands at the altitude of 30
inches: the mercury in the tube will be observed to rise until it
attain a certain elevation, and will there maintain its position. It
will be found, that though the water continue to be exposed to the
action of the fire, and continue to boil, the mercury in the tube will
not continue to rise, but will maintain a fixed position: let the
point to which the mercury has risen, in this case, be likewise marked
upon the tube.
The two points, thus determined, are called the _freezing_ and the
_boiling_ points. If the distance upon the tube between these two
points be divided into 180 equal parts, each of these parts is called
a _degree_; and if this division be continued, by taking equal
divisions below the freezing point, until 32 divisions be taken, the
last division is called the _zero_, or _nought_ of the thermometer. It
is the point to which the mercury would fall, if the thermometer were
immersed in a certain mixture of snow and salt. When thermometers were
first invented, this point was taken as the zero point, from an
erroneous supposition that the temperature of such a mixture was the
lowest possible temperature.
The degrees upon the instrument thus divided are counted upwards from
the zero, and are expressed, like the degrees of a circle, by placing
a small ° over the number. Thus it will be perceived that the freezing
point is 32° of our thermometer, and the boiling-point will be found
by adding 180° to 32°; it is therefore 212°.
The temperature of a body is that elevation to which the thermometer
would rise when the mercury enclosed in it would acquire the same
temperature. Thus, if we should immerse the thermometer, and should
find that the mercury would rise to the division marked 100°, we
should then affirm that the temperature of the water was 100°.
(9.) The dilatation which attends an increase of temperature is one of
the most universal effects of heat. It varies, however, in different
bodies: it is least in solid bodies; greater in liquids; and greatest
of all in bodies in the aeriform state. Again, different solids are
differently susceptible of this expansion. Metals are the most
susceptible of it; but metals of different kinds are differently
expansible.
As an increase of temperature causes an increase of bulk, so a
diminution of temperature causes a corresponding diminution of bulk,
and the same body always has the same bulk at the same temperature.
A flaccid bladder, containing a small quantity of air, will, when
heated, become quite distended; but it will again resume its flaccid
appearance when cold. A corked bottle of fermented liquor, placed
before the fire, will burst by the effort of the air contained in it
to expand when heated.
Let the tube A B (fig. 5.) open at both ends, have one end inserted in
the neck of a vessel C D, containing a coloured liquid, with common
air above it; and let the tube be fixed so as to be air-tight in the
neck: upon heating the vessel, the warm air inclosed in the vessel C D
above the liquid will begin to expand, and will press upon the surface
of the liquid, so as to force it up in the tube A B.
In bridges and other structures, formed of iron, mechanical
provisions are introduced to prevent the fracture or strain which
would take place by the expansion and contraction which the metal must
undergo by the changes of temperature at different seasons of the
year, and even at different hours of the day.
Thus all nature, animate and inanimate, organized and unorganized, may
be considered to be incessantly breathing heat; at one moment drawing
in that principle through all its dimensions, and at another moment
dismissing it.
(10.) Change of bulk, however, is not the only nor the most striking
effect which attends the increase or diminution of the quantity of
heat in a body. In some cases, a total change of form and of
mechanical qualities is effected by it. If heat be imparted in
sufficient quantity to a solid body, that body, after a certain time,
will be converted into a liquid. And again, if heat be imparted in
sufficient quantity to this liquid, it will cease to exist in the
liquid state, and pass into the form of vapour.
By the abstraction of heat, a series of changes will be produced in
the opposite order. If from the vapour produced in this case, a
sufficient quantity of heat be taken, it will return to the liquid
state; and if again from this liquid heat be further abstracted, it
will at length resume its original solid state.
The transmission of a body from the solid to the liquid state, by the
application of heat, is called _fusion_ or _liquefaction_, and the
body is said to be fused, _liquefied_, or melted.
The reciprocal transmission from the liquid to the solid state, is
called _congelation_, or _solidification_; and the liquid is said to
be _congealed_ or _solidified_.
The transmission of a body from the liquid to the vaporous or aeriform
state, is called _vaporization_, and the liquid is said to be
_vaporized_ or _evaporated_.
The reciprocal transmission of vapour to the liquid state is called
_condensation_, and the vapour is said to be _condensed_.
We shall now examine more minutely the circumstances which attend
these remarkable and important changes in the state of body.
(11.) Let us suppose that a thermometer is imbedded in any solid body;
for example, in a mass of sulphur; and that it stands at the ordinary
temperature of 60 degrees: let the sulphur be placed in a vessel, and
exposed to the action of fire. The thermometer will now be observed
gradually to rise, and it will continue to rise until it exhibit the
temperature of 218°. Here, however, notwithstanding the continued
action of the fire upon the sulphur, the thermometer will become
stationary; proving, that notwithstanding the supply of heat received
from the fire, the sulphur has ceased to become hotter. At the moment
that the thermometer attains this stationary point, it will be
observed that the sulphur has commenced the process of fusion; and
this process will be continued, the thermometer being stationary,
until the whole mass has been liquefied. The moment the liquefaction
is complete, the thermometer will be observed again to rise, and it
will continue to rise until it attain the elevation of 570°. Here,
however, it will once more become stationary; and notwithstanding the
heat supplied to the sulphur by the fire, the liquid will cease to
become hotter: when this happens, the sulphur will boil; and if it
continue to be exposed to the fire a sufficient length of time, it
will be found that its quantity will gradually diminish, until at
length it will all disappear from the vessel which contained it. The
sulphur will, in fact, be converted into vapour.
From this process we infer, that all the heat supplied during the
processes of liquefaction and vaporization is consumed in effecting
these changes in the state of the body; and that under such
circumstances, it does not increase the temperature of the body on
which the change is produced.
These effects are general: all solid bodies would pass into the
liquid state by a sufficient application of heat; and all liquid
bodies would pass into the vaporous state by the same means. In all
cases the thermometer would be stationary during these changes, and
consequently the temperature of the body, in those periods, would be
maintained unaltered.
(12.) Solids differ from one another in the temperatures at which they
become liquid. These temperatures are called their _melting points_.
Thus the melting point of ice is 32°; that of lead 612°; that of gold
5237°.[3] The heat which is supplied to a body during the processes of
fusion or vaporization, and which does not affect the thermometer, or
increase the temperature of the body fused or vaporized, is said to
become _latent_. It can be proved to exist in the body fused or
vaporized, and may even be taken from that body. In parting with it
the body does not fall in temperature, and consequently the loss of
this heat is not indicated by the thermometer any more than its
reception. The term latent heat is merely intended to express this
fact, of the thermometer being insensible to the presence or absence
of this portion of heat, and is not intended to express any
theoretical notions concerning it.
[Footnote 3: Temperatures above 650° cannot be measured by the
mercurial thermometer. They can be inferred only with probability by
pyrometers.]
(13.) In explaining the construction and operation of the steam
engine, although it is necessary occasionally to refer to the effects
of heat upon bodies in general, yet the body, which is by far the most
important to be attended to, so far as the effects of heat upon it are
concerned, is water. This body is observed to exist in the three
different states, the solid, the liquid, and the vaporous, according
to the varying temperature to which it is exposed. All the
circumstances which have been explained in reference to metals, and
the substance sulphur in particular, will, _mutatis mutandis_, be
applicable to water. But in order perfectly to comprehend the
properties of the steam engine, it is necessary to render a more
rigorous and exact account of these phenomena, so far as they apply to
the changes produced upon water by the effects of heat.
Let us suppose a mass of ice immersed in the mixture of snow and salt
which determines the zero point of the thermometer: this mass, if
allowed to continue a sufficient length of time submerged in the
mixture, will necessarily acquire its temperature, and the thermometer
immersed in it will stand at zero. Let the ice be now withdrawn from
the mixture, still keeping the thermometer immersed in it, and let it
be exposed to the atmosphere at the ordinary temperature, say 60°. At
first the thermometer will be observed gradually and continuously to
rise until it attain the elevation of 32°; it will then become
stationary, and the ice will begin to melt: the thermometer will
continue standing at 32° until the ice shall be completely liquefied.
The liquid ice and the thermometer being contained in the same vessel,
it will be found, when the liquefaction is completed, that the
thermometer will again begin to rise, and will continue to rise until
it attain the temperature of the atmosphere, viz. 60°. Hitherto the
ice or water has received a supply of heat from the surrounding air;
but now an equilibrium of temperature having been established, no
further supply of heat can be received; and if we would investigate
the further effects of increased heat, it will be necessary to expose
the liquid to fire, or some other source of heat. But previous to
this, let us observe the time which the thermometer remains stationary
during the liquefaction of the ice: if noted by a chronometer, it
would be found to be a hundred and forty times the time during which
the water in the liquid state was elevated one degree; the inference
from which is, that in order to convert the solid ice into liquid
water, it was necessary to receive from the surrounding atmosphere one
hundred and forty times as much heat as would elevate the liquid water
one degree in temperature; or, in other words, that to liquefy a
given weight of ice requires as much heat as would raise the same
weight of water 140° in temperature: or from 32° to 172°.
The latent heat of water acquired in liquefaction is therefore 140°.
(14.) Let us now suppose that, a spirit lamp being applied to the
water already raised to 60°, the effects of a further supply of heat
be observed: the thermometer will continue to rise until it attain the
elevation of 212°, the barometer being supposed to stand at 30 inches.
The thermometer having attained this elevation will cease to rise; the
water will therefore cease to become hotter, and at the same time
bubbles of steam will be observed to be formed at the bottom of the
vessel containing the water, near the flame of the spirit lamp. These
bubbles will rise through the water, and escape at the surface,
exhibiting the phenomena of ebullition, and the water will undergo the
process of _boiling_.
During this process, the thermometer will constantly be maintained at
the same elevation of 212°; but if the time be noted, it will be found
that the water will be altogether evaporated, if the same source of
heat be continued to be applied to it six and a half times as long as
was necessary to raise it from the freezing to the boiling-point.
Thus, if the application of the lamp to water at 32°, be capable of
raising that water to 212° in one hour, the same lamp will require to
be applied to the boiling water for six hours and a half, in order to
convert the whole of it into steam. Now if the steam into which it is
thus converted were carefully preserved in a receiver, maintained at
the temperature of 212°, this steam would be found to have that
temperature, and not a greater one; but it would be found to fill a
space about 1700 times greater than the space it occupied in the
liquid state, and it would possess an elastic force equal to the
pressure of the atmosphere under which it was boiled; that is to say,
it would press the sides of the vessel which contained it with a
pressure equivalent to that of a column of mercury of 30 inches in
height; or what is the same thing, at the rate of about 15lb. on every
square inch of surface.
(15.) As the quantity of heat expended in raising the water from 32°
to 212°, is 180°; and as the quantity of heat necessary to convert the
same water into steam is six and a half times this quantity, it
follows that the quantity of heat requisite for converting a given
weight of water into steam, will be found by multiplying 180° by
5-1/2. The product of these numbers being 990°, it follows, that, to
convert a given weight of water at 212° into steam of the same
temperature, under the pressure of the atmosphere, when the barometer
stands at 30 inches, requires as much heat as would be necessary to
raise the same water 990° higher in temperature. The heat, not being
sensible to the thermometer, is latent heat; and accordingly it may be
stated, that the latent heat, necessary to convert water into steam
under this pressure is, in round numbers, 1000°.
(16.) All the effects of heat which we have just described may be
satisfactorily accounted for, by supposing that the principle of heat
imparts to the constituent atoms of bodies a force, by virtue of which
they acquire a tendency to repel each other. But in conjunction with
this, it is necessary to notice another force, which is known to exist
in nature: there is observable among the corpuscles of bodies a force,
in virtue of which they have a tendency to cohere, and collect
themselves together in solid concrete masses: this force is called the
attraction of cohesion. These two forces--the natural cohesion of the
particles, and the repulsive energy introduced by heat--are directly
opposed to one another, and the state of the body will be decided by
the predominance of the one or the other, or their mutual equilibrium.
If the natural cohesion of the constituent particles of the body
considerably predominate over the repulsive energy introduced by the
heat, then the cohesion will take effect; the particles of the body
will coalesce, the mass will become rigid and solid, and the
particles will hold together in one invariable mass, so that they
cannot drop asunder by the mere effect of their weight. In such cases,
a more or less considerable force must be applied, in order to break
the body, or to tear its parts asunder. Such is the quality which
characterises the state, which in mechanics is called the state of
solidity.
If the repulsive energy introduced by the application of heat be
equal, or nearly equal, to the natural cohesion with which the
particles of the body are endued, then the predominance of the
cohesive force may be insufficient to resist the tendency which the
particles may have to drop asunder by their weight. In such a case,
the constituent particles of the body cannot cohere in a solid mass,
but will separate by their weight, fall asunder, and drop into the
various corners, and adapt themselves to the shape of any vessel in
which the body may be contained. In fact, the body will take the
liquid form. In this state, however, it does not follow that the
cohesive principle will be altogether inoperative: it may, and does,
in some cases, exist in a perceptible degree, though insufficient to
resist the separate gravitation of the particles. The tendency which
particles of liquids have, in some cases, to collect in globules,
plainly indicates the predominance of the cohesive principle: drops of
water collected upon the window pane; drops of rain condensed in the
atmosphere; the tear which trickles on the cheek; drops of mercury,
which glide over any flat surface, and which it is difficult to
subdivide or scatter into smaller parts; are all obvious indications
of the predominance of the cohesive principle in liquids.
By the due application of heat, even this small degree of cohesion may
be conquered, and a preponderance of the opposite principle of
repulsion may be created. But another physical influence here
interposes its aid, and conspires with cohesion in resisting the
transmission of the body from the liquid to the vaporous state: this
force is no other than the pressure of the atmosphere, already
explained. This pressure has an obvious tendency to restrain the
particles of the liquid, to press them together, and to resist their
separation. The repulsive principle of the heat introduced must
therefore not only neutralize the cohesion, but must also impart to
the atoms of the liquid a sufficient elasticity or repulsive energy to
enable them to fly asunder, and assume the vaporous form in spite of
this atmospheric resistance.
Now it is clear, that if this atmospheric resistance be subject to any
variation in its intensity, from causes whether natural or artificial,
the repulsive energy necessary to be introduced by the heat, will vary
proportionally: if the atmospheric pressure be diminished, then less
heat will be necessary to vaporize the liquid. If, on the other hand,
this pressure be increased, a greater quantity of heat will be
required to impart the necessary elasticity.
(17.) From this reasoning we must expect that any cause, whether
natural or artificial, which diminishes the atmospheric pressure upon
the surface of a liquid, will cause that liquid to boil at a lower
temperature: and on the other hand, any cause which may increase the
atmospheric pressure upon the liquid, will render it necessary to
raise it to a higher temperature before it can boil.
These inferences we accordingly find supported by experience. Under a
pressure of 15lb. on the square inch, _i. e._ when the barometer is at
30 inches, water boils at the temperature of 212° of the common
thermometer. But if water at a lower temperature, suppose 180°, be
placed under the receiver of an air-pump, and, by the process of
exhaustion the atmospheric pressure be removed, or very much
diminished, the water will boil, although its temperature still remain
at 180°, as may be indicated by a thermometer placed in it.
On the other hand, if a thermometer be inserted air-tight in the lid
of a close digester containing water with common atmospheric air above
it, when the vessel is heated the air acquires an increased
elasticity; and being confined by the cover, presses, with increased
force, on the surface of the water. By observing the thermometer while
the vessel is exposed to the action of heat, it will be seen to rise
considerably above 212°, suppose to 230°, and would continue so to
rise until the strength of the vessel could no longer resist the
pressure within it.
The temperature at which water boils is commonly said to be 212°,
which is called _the boiling-point_ of the thermometer; but, strictly
speaking, this is only true when the barometer stands at 30 inches. If
it be lower, water will boil at a lower temperature, because the
atmospheric pressure is less; and if it be higher, as at 31, water
will not boil until it receives a higher temperature, the pressure
being greater.
According as the cohesive forces of the particles of liquids are more
or less active, they boil at greater or less temperatures. In general
the lighter liquids, such as _alcohol_ and _ether_, boil at lower
temperatures. These fluids, in fact, would boil by merely removing the
atmospheric pressure, as may be proved by placing them under the
receiver of an air-pump, and withdrawing the air. From this we may
conclude that these and similar substances would never exist in the
liquid state at all, but for the atmospheric pressure.
(18.) The elasticity of vapour raised from a boiling liquid, is equal
to the pressure under which it is produced. Thus, steam raised from
water at 212°, and, therefore, under a pressure of 15lb. on the square
inch, is endued with an elastic force which would exert a pressure on
the sides of any vessel which confines it, also equal to 15lb. on the
square inch. Since an increased pressure infers an increased
temperature in boiling, it follows, that where steam of a higher
pressure than the atmosphere is required, it is necessary that the
water should be boiled at a higher temperature.
(19.) We have already stated that there is a certain point at which
the temperature of a liquid will cease to rise, and that all the heat
communicated to it beyond this is consumed in the formation of vapour.
It has been ascertained, that when water boils at 212°, under a
pressure equal to 30 inches of mercury, a cubic inch of water forms a
cubic foot[4] of steam, the elastic force of which is equal to the
atmospheric pressure, and the temperature of which is 212°. Since
there are 1728 cubic inches in a cubic foot, it follows, that when
water at this temperature passes from the liquid to the vaporous
state, it is dilated into 1728 times its bulk.
[Footnote 4: The terms _cubic inch_ and _cubic foot_ are easily
explained. A common die, used in games of chance, has the figure which
is called a cube. It is a solid having twelve straight edges equal to
one another. It has six sides, each of which is square, and which are
also equal to one another. If its edges be each one inch in length, it
is called a _cubic inch_, if one foot, a _cubic foot_, if one yard, a
_cubic yard_, &c. This figure is represented in perspective, in fig.
6.]
(20.) We have seen that about 1000 degrees of heat must be
communicated to any given quantity of water at 212°, in order to
convert it into steam of the same temperature, and possessing a
pressure amounting to about 15 pounds on the square inch, and that
such steam will occupy above 1700 times the bulk of the water from
which it was raised. Now we might anticipate, that by abstracting the
heat thus employed in converting the liquid into vapour, a series of
changes would be produced exactly the reverse of those already
described; and such is found to be actually the case. Let us suppose a
vessel, the capacity of which is 1728 cubic inches, to be filled with
steam, of the temperature of 212°, and exerting a pressure of 15
pounds on the square inch; let 5-1/2 cubic inches of water, at the
temperature of 32°, be injected into this vessel, immediately the
steam will impart the heat, which it has absorbed in the process of
vaporisation to the water thus injected, and will itself resume the
liquid form. It will shrink into its primitive dimensions of one cubic
inch, and the heat which it will dismiss will be sufficient to raise
the 5-1/2 cubic inches of injected water to the temperature of 212°.
The contents of the vessel will thus be 6-1/2 cubic inches of water at
the temperature of 212°. One of these cubic inches is in fact the
steam which previously filled the vessel reconverted into water, the
other 5-1/2 are the injected water which has been raised from the
temperature of 32° to 212° by the heat which has been dismissed by the
steam in resuming the liquid state. It will be observed that in this
transmission no temperature is lost, since the cubic inch of water
into which the steam is converted has the same temperature as the
steam had before the cold water was injected.
These consequences are in perfect accordance with the results already
obtained from observing the time necessary to convert a given quantity
of water into steam by the application of heat. From the present
result it follows, that in the reduction of a given quantity of steam
to water it parts with as much heat as is sufficient to raise 5-1/2
cubic inches from 32° to 212°, that is, 5-1/2 times 180° or 990°.
(21.) There is an effect produced in this process to which it is
material that we should attend. The steam which filled the space of
1728 cubic inches shrinks when reconverted into water into the
dimensions of 1 cubic inch. It therefore leaves 1727 cubic inches of
the vessel it contains unoccupied. By this property steam is rendered
instrumental in the formation of a vacuum.
By allowing steam to circulate through a vessel, the air may be
expelled from it, and its place filled by steam. If the vessel be then
closed and cooled the steam will be reduced to water, and, falling in
drops on the bottom and sides of the vessel, the space which it filled
will become a _vacuum_. This may be easily established by experiment.
Let a long glass tube be provided with a hollow ball at one end, and
having the other end open.[5] Let a small quantity of spirits be
poured in at the open end, and placing the glass ball over the flame
of a lamp, let the spirits be boiled. After some time the steam will
be observed to issue copiously from the open end of the tube which is
presented upwards. When this takes place, let the tube be inverted,
and its open end plunged in a basin of cold water. The heat being thus
removed, the cool air will reconvert the steam in the tube into
liquid, and a vacuum will be produced, into which the pressure of the
atmosphere on the surface of the water in the basin will force the
water through the tube, and it will rush up with considerable force,
and fill the glass ball.
[Footnote 5: A common glass flask with a long neck will answer the
purpose.]
In this experiment it is better to use spirits than water, because
they boil at a lower heat, and expose the glass to less liability to
break, and also the tube may more easily be handled.
CHAPTER II.
FIRST STEPS IN THE INVENTION.
Futility of early claims. -- Watt, the real Inventor. -- Hero of
Alexandria. -- Blasco Garay. -- Solomon de Caus. -- Giovanni
Branca. -- Marquis of Worcester. -- Sir Samuel Morland. -- Denis
Papin. -- Thomas Savery.
(22.) In the history of the progress of the useful arts and
manufactures, there is perhaps no example of any invention the credit
of which has been so keenly contested as that of the steam engine.
Claims to it have been advanced by different nations, and by different
individuals of the same nation. The partisans of the competitors for
this honour have argued their pretensions, and pressed their claims,
with a zeal which has occasionally outstripped the bounds of
discretion; and the contest has not unfrequently been tinged with
prejudices, both national and personal, and marked with a degree of
asperity quite unworthy of so noble a cause, and altogether beneath
the dignity of science.
The efficacy of the steam engine considered as a mechanical agent
depends, first, on the several physical properties from which it
derives its operation, and, secondly, on the various pieces of
mechanism and details of mechanical arrangement by which these
properties are rendered practically available. If the merit of the
invention must be ascribed to the discoverer and contriver of these,
then the contest will be easily decided, because it will be obvious
that the prize is not due to any one individual, but must be
distributed in different proportions among several. If, however, he is
best entitled to the credit of the invention, who has by the powers of
his mechanical genius imparted to the machine that form and those
qualities from which it has received its present extensive utility,
and by which it has become an agent of transcendent power, which has
spread its beneficial effects throughout every part of the civilized
globe, then the universal consent of mankind will, as it were by
acclamation, award the prize to one individual, whose pre-eminent
genius places him far above all other competitors, and from the
application of whose mental energies to this machine may be dated
those grand effects which have rendered it a topic of interest to
every individual for whom the progress of human civilization has any
attractions. Before the era marked by the discoveries of JAMES WATT,
the steam engine, which has since become an object of such universal
interest, was a machine of extremely limited power, greatly inferior
in importance to most other mechanical contrivances used as prime
movers. But from that time it is scarcely necessary here to state that
it became a subject not of British interest only, but one with which
the progress of the human race became intimately mixed up.
Since, however, the question of the progressive developement of those
physical principles on which the steam engine depends, and of their
mechanical application, has of late years received some importance, as
well from the interest which the public manifest towards them as from
the rank of the writers who have investigated them, we have thought it
expedient to state briefly, but we trust with candour and fairness,
the successive steps which appear to have led to this invention.
The engine as it exists at present is not, strictly speaking, the
exclusive invention of any one individual: it is the result of a
series of discoveries and inventions which have for the last two
centuries been accumulating. When we attempt to trace back its
history, and to determine its first inventor, we experience the same
difficulty as is felt in tracing the head of a great river: as we
ascend its course, we are embarrassed by the variety of its tributary
streams, and find it impossible to decide which of those channels into
which it ramifies ought to be regarded as the principal stream; and it
terminates at length in a number of threads of water, each in itself
so insignificant as to be unworthy of being regarded as the source of
the majestic object which has excited the inquiry.
From a very early period the effects of heat upon liquids, and more
especially the production of steam or vapour, was regarded as a
probable source of mechanical power, and numerous speculators directed
their attention to it, and exerted their inventive faculties to derive
from it an effective mover. It was not, however, until the
commencement of the eighteenth century that any invention was produced
which was practically applied, even unsuccessfully. All the attempts
previous to that time were either suggestions which were limited to
paper or experiments confined to models; or, if they exceeded this,
they never outlived a single trial on a larger scale. Nevertheless
many of these suggestions and experiments being recorded and
accessible to future inquirers doubtless offered useful hints and some
practical aid to those more successful investigators who subsequently
contrived engines in such forms as to be practically available on a
large scale for mechanical purposes. It is right and just,
therefore--mere suggestions and abortive experiments though they may
have been--to record them, that each inventor and discoverer may
receive the just credit due to his share in this splendid mechanical
invention. We shall then in the present chapter briefly enumerate, in
chronological order, the successive steps so far as they have come to
our knowledge.
HERO OF ALEXANDRIA, 120 B. C.
[Illustration: Fig. 1.]
(23.) In a work entitled _Spiritalia seu Pneumatica_, one of the
numerous works of this philosopher which has remained to us, is
contained a description of a machine moved by vapour of water. A
hollow sphere, of which A B represents a section, is supported on two
pivots at A and B, which are the extremities of tubes A C D and B E F,
which pass into a boiler where steam is generated. This steam flows
through small apertures at the extremities A and B, and fills the
hollow sphere. One or more horizontal arms K G, I H, project from this
sphere, and are likewise filled with steam, but are closed at their
extremities. Conceive a small hole made near the extremity G, but at
one side of one of the tubes; the steam confined in the tube and globe
would immediately rush from the hole with a force proportionate to
its pressure within the globe. On the common principle of mechanics a
re-action would be produced, and the tube would recoil in the same
manner as a gun when discharged. The tubular arm K G being thus
pressed in a direction opposed to that in which the steam issues, the
sphere would revolve accordingly, and would continue to revolve so
long as the steam would continue to flow from the aperture. The force
of recoil would be increased by making a similar aperture in two or
more arms, care being taken that all the apertures should be placed so
as to cause the sphere to revolve in the same direction.
This motion being once produced might be transmitted by ordinary
mechanical contrivance to any machinery which its power might be
adequate to move.
This method of using steam is not adopted in any part or any form of
the modern steam engine.
BLASCO DE GARAY, A. D. 1543.
(24.) In the year 1826 there appeared in Zach's Correspondence a
communication from Thomas Gonsalez, Director of the Royal Archives of
Simancas, giving an account of an experiment reported to have been
made in the year 1543 by order of Charles V. in the port of Barcelona.
Blasco de Garay, a sea captain, had contrived a machine by which he
proposed to propel vessels without oars or sails. Garay concealed
altogether the nature of the machine which he used: all that was seen
during the experiment was that it consisted of a great boiler for
water, and that wheels were kept in revolution at each side of the
vessel. The experiment was made upon a vessel called the Trinity, of
200 tons burden, and was witnessed by several official personages,
whose presence on the occasion was commanded by the king. One of the
witnesses reported that it was capable of moving the vessel at the
rate of two leagues in three hours, that the machine was too
complicated and expensive, and was exposed to the danger of explosion.
The other witnesses, however, reported more favourably. The result of
the experiment was thought to be favourable: the inventor was
promoted, and received a pecuniary reward, besides having all his
expenses defrayed.
From the circumstance of the nature of the impelling power having been
concealed by the inventor it is impossible to say in what this machine
consisted, or even whether steam exerted any agency whatever in it,
or, if it did, whether it might not have been, as was most probably
the case, a reproduction of Hero's contrivance. It is rather
unfavourable to the claims advanced by the advocates of the Spaniard,
that although it is admitted that he was rewarded and promoted in
consequence of the experiment, yet it does not appear that it was
again tried, much less brought into practical use.
SOLOMON DE CAUS, 1615.
[Illustration: Fig. 2.]
(25.) A work entitled "Les Raisons des Forces Mouvantes, avec diverses
Machines tant utiles que plaisantes," published at Frankfort in 1615,
by Solomon de Caus, a native of France, contains the following
theorem:--
"_Water will mount by the help of fire higher than its level_," which
is explained and proved in the following terms:--
"The third method of raising water is by the aid of fire. On this
principle may be constructed various machines: I shall here describe
one. Let a ball of copper marked A, well soldered in every part, to
which is attached a tube and stopcock marked D, by which water may be
introduced; and also another tube marked B C, which will be soldered
into the top of the ball, and the lower end C of which shall descend
nearly to the bottom of the ball without touching it. Let the said
ball be filled with water through the tube D, then shutting the
stopcock D, and opening the stopcock in the vertical tube B C, let the
ball be placed upon a fire, the heat acting upon the said ball will
cause the water to rise in the tube B C."
Such is the description of the apparatus of De Caus as given by
himself; and on this has been founded a claim to the invention of the
steam engine. It will be observed, that neither in the original
theorem nor in the description of the machine which accompanies it, is
the word _steam_ anywhere used. Now it was well known, by all
conversant in physics, long before the date of the publication
containing this description, that atmospheric air when heated acquires
an increased elastic force. As the experiment is described, the other
part of the ball A is filled with atmospheric air; the heat of the
fire acting upon the air through the external surface of the ball, and
likewise transmitted through the water, would of course raise the
temperature of the air contained in the vessel, would thereby increase
its elasticity, and would cause the water to rise in the tube B C,
upon a physical principle altogether independent of the qualities of
steam. The effect produced, therefore, is just what might have been
expected by any one acquainted with the common properties of air,
though entirely ignorant of those of steam; and, in point of fact, the
pressure of the air is as much concerned in this case in raising the
water as the pressure of the steam.
This objection, however, is combated by another theorem contained in
the same work, in which De Caus speaks of "the strength of the vapour
produced by the action of the fire, which causes water to mount; which
vapour will issue from the stopcock with great violence after the
water has been expelled."
If De Caus be admitted to have understood the elastic property of the
vapour of water, and to have attributed the ascent of the water in the
tube C B to the pressure of that vapour upon the surface of the water
confined in the copper ball, it must be admitted that he suggested
one of the ways of using the power of steam as a mechanical agent. In
the modern steam engine this pressure is not now used against a liquid
surface, but against the solid surface of a piston. This, however,
should not take from De Caus whatever credit be due to the suggestion
of the physical property in question.
GIOVANNI BRANCA, 1629.
(26.) In a work published at Rome in 1629, entitled "Le Machine del G.
Branca," is contained a description of a machine for propelling a
wheel by a blast of steam. This contrivance consists of a wheel
furnished with flat vanes upon its rim, like the boards of a
paddle-wheel. The steam is produced in a close vessel, and made to
issue with violence from the extremity of a pipe. Being directed
against the vanes, it causes the wheel to revolve, and this motion may
be imparted by the usual mechanical contrivances to any machinery
which it was intended to move.
This contrivance has no analogy whatever to any part of the modern
steam engines in any of their various forms.
EDWARD SOMERSET, MARQUIS OF WORCESTER, 1663.
(27.) Of all the individuals to whom the invention of the steam engine
has been ascribed the most celebrated was the Marquis of Worcester,
the author of a work entitled "The Scantling of One Hundred
Inventions," but which is more commonly known by the title "A Century
of Inventions." It is to him that by far the greater number of writers
and inquirers on this subject ascribe the merit of the discovery of
the invention. This contrivance is described in the following terms in
the sixty-eighth invention in the work above named:--
"I have invented an admirable and forcible way to drive up water by
fire; not by drawing or sucking it upwards, for that must be, as the
philosopher terms it, _infra spæram activitatis_, which is but at such
a distance. But this way hath no bounder if the vessels be strong
enough. For I have taken a piece of whole cannon whereof the end was
burst, and filled it three quarters full of water, stopping and
screwing up the broken end, as also the touch-hole and making a
constant fire under it; within twenty-four hours, it burst and made a
great crack. So that, having a way to make my vessels so that they are
strengthened by the force within them, and the one to fill after the
other, I have seen the water run like a constant fountain stream forty
feet high. One vessel of water rarefied by fire driveth up forty of
cold water, and a man that tends the work has but to turn two cocks;
that one vessel of water being consumed, another begins to force and
refill with cold water, and so successively; the fire being tended and
kept constant, which the self-same person may likewise abundantly
perform in the interim between the necessity of turning the said
cocks."
These experiments must have been made before the year 1663, in which
the "Century of Inventions" was published. The description of the
machine here given, like other descriptions in the same work, was only
intended to express the effects produced, and the physical principle
on which their production depends. It is, however, sufficiently
explicit to enable any one conversant with the subsequent contrivance
of Savery, to perceive that Lord Worcester must have contrived a
machine containing all that part of Savery's engine in which the
direct force of steam is employed. As in the above description, the
separate boiler or generator of steam is distinctly mentioned; that
the steam from this is conducted into another vessel containing the
cold water to be raised; that this water is raised by the pressure of
steam acting upon its surface; that when one vessel of water has thus
been discharged, the steam acts upon the water contained in another
vessel, while the first is being replenished; and that a continued
upward current of water is maintained by causing the steam to act
alternately upon two vessels, employing the interval to fill one while
the water is discharged from the other.
On comparing this with the contrivance previously suggested by De
Caus, it will be observed, that even if De Caus knew the physical
agent by which the water was driven upwards in the apparatus contrived
by him, still it was only a means of causing a vessel of boiling water
to empty itself; and before a repetition of the process could be
obtained, the vessel should be refilled, and again boiled. In the
contrivance of Lord Worcester, on the other hand, the agency of the
steam was employed in the same manner as it is in the steam engines of
the present day, being generated in one vessel, and used for
mechanical purposes in another. Nor must this distinction be regarded
as trifling and insignificant, because on it depends the whole
practicability of using steam as a mechanical agent. Had its action
been confined to the vessel in which it was produced, it never could
have been employed for any useful purpose.
SIR SAMUEL MORLAND, 1683.
(28.) It appears, by a MS. in the Harleian Collection in the British
Museum, that a mode of applying steam to raise water was proposed to
Louis XIV. by Sir Samuel Morland. It contains, however, nothing more
than might have been collected from Lord Worcester's description, and
is only curious, because of the knowledge the writer appears to have
had of the expansion which water undergoes in passing into steam. The
following is extracted from the MS.:
"The principles of the new force of fire invented by Chevalier Morland
in 1682, and presented to his Most Christian Majesty in 1683:--'Water
being converted into vapour by the force of fire, these vapours
shortly require a greater space (about 2000 times) than the water
before occupied, and sooner than be constantly confined would split a
piece of cannon. But being duly regulated according to the rules of
statics, and by science reduced to measure, weight, and balance, then
they bear their load peaceably (like good horses,) and thus become of
great use to mankind, particularly for raising water, according to the
following table, which shows the number of pounds that may be raised
1800 times per hour to a height of six inches by cylinders half filled
with water, as well as the different diameters and depths of the said
cylinders.'"
DENIS PAPIN, 1695.
(29.) Denis Papin, a native of Blois in France, and professor of
mathematics at Marbourg, had been engaged about this period in the
contrivance of a machine in which the atmospheric pressure should be
made available as a mechanical agent by creating a partial vacuum in a
cylinder under a piston. His first attempts were directed to the
production of this vacuum by mechanical means, having proposed to
apply a water-wheel to work an air-pump, and so maintain the degree of
rarefaction required. This, however, would eventually have amounted to
nothing more than a mode of transmitting the power of the water-wheel
to another engine, since the vacuum produced in this way could only
give back the power exerted by the water-wheel diminished by the
friction of the pumps; still this would attain the end first proposed
by Papin, which was merely to transmit the force of the stream of a
river, or a fall of water, to a distant point, by partially exhausted
pipes or tubes. He next, however, attempted to produce a partial
vacuum by the explosion of gunpowder; but this was found to be
insufficient, since so much air remained in the cylinder under the
piston, that at least half the power due to a vacuum would have been
lost. "I have, therefore," proceeds Papin, "attempted to attain this
end by another method. Since water being converted into steam by heat
acquires the property of elasticity like air, and may afterwards be
recondensed so perfectly by cold that there will no longer remain the
appearance of elasticity in it, I have thought that it would not be
difficult to construct machines in which, by means of a moderate heat,
and at a small expense, water would produce that perfect vacuum which
has been vainly sought by means of gunpowder."
Papin accordingly constructed the model of a machine, consisting of a
small pump, in which was placed a solid piston, and in the bottom of
the cylinder under the piston was contained 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.[6]
[Footnote 6: Recueil de diverses pièces touchant quelques nouvelles
machines, p. 38.]
THOMAS SAVERY, 1698.
(30.) The discovery of the method of producing a vacuum by the
condensation of steam was reproduced before 1688, 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, in
1695, and states that his discovery of the condensing principle arises
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 flask, and the moment he plunged its mouth in the water,
the liquid immediately rushed up into the flask and filled it. (21.)
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 34 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 repeat 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.
CHAPTER III.
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 Jet. -- Potter's Discovery of the
Method of Working the Valves.
(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 generate the steam, which is
called the boiler, and the second, that in which the steam is applied
as a moving power.
The former apparatus in Savery's engine consists of two strong
boilers, sections of which are represented at D and E in fig. 7.; 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 greater 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 constantly wrought by the hand of the attendant.
Two _gauge-pipes_ 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 them, 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 G should be at a level of a little less than one-third of
the whole depth, and the mouth of G´ at a level a 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. 8.) 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. 8.) 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. 8.) be two steam vessels communicating by the tubes T
T´ (marked by the same letters in fig. 7.) with the greater boiler D.
Let S be a pipe, called the _suction pipe_, descending into 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
of 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 upward, 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. 7.) by the cock M.[7]
[Footnote 7: This pipe in fig. 9. is represented as proceeding from
the force-pipe above the cistern C.]
The communication of the pipes T T´ with the boiler can be opened and
closed, alternately, by the regulator R, (fig. 7.) 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 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.
When this has taken place, suppose the regulator be moved so as to
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 (6.) and occupy the vessel V. The air in the
suction pipe S, being thus allowed an increased space, will be
proportionably diminished in its elastic force (6.), and its pressure
will no longer balance that of the atmosphere acting on the external
surface L[8] 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 on L (7.). When this has taken place, the water will cease to
ascend.
[Footnote 8: Not in the diagram.]
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 on L 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. 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 on the surface of the water in V, so as to form a
surface as hot as boiling water.[9] 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.
[Footnote 9: Hot water being lighter than cold, it floats on the
surface.]
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 on L
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 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. 7.) 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 on L, 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. 7.) 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
at L.
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 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.[10] A perspective view of this engine is presented in fig. 9.
The different parts already described will easily be recognised, being
marked with the same letters as in figs. 6, 7.
[Footnote 10: In the diagrams used for explaining the principles and
operations of machines, I have found it contribute much to the
clearness of the description to adopt an arrangement of parts somewhat
different from that of the real machine. When once the nature and
principles on which the machine acts are well understood, the reader
will find no difficulty in transferring every part to its proper
place, which is represented in the perspective drawings.]
(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
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 15 lbs. (3.) on every square inch. Now, a column of
water, whose base is one square inch, and whose height is 34 feet,
weighs about 15lbs. If we suppose that a perfect vacuum were produced
in the steam-vessels V V´ (fig. 8.) by condensation, the atmospheric
pressure on L would fail to force up the water, if the height of the
top of these vessels exceeded 34 feet. It is plain, therefore, that
the engine cannot be more than 34 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 (17.); 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, it was found that the engine
could never be placed with practical advantage at a greater height
than 26 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 15lbs.
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 34 feet in height; and if steam of triple the force be used, it
will raise a column of 68 feet high, which, added to 26 feet raised by
the atmosphere, gives a total lift of 94 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 of the pressure is balanced by the pressure of
the atmosphere on the external surface of the vessels; but an
effective pressure of 30lbs. 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 90
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 90 feet of the depth; so that the water raised by one through
the first 90 feet should be received in a reservoir, from which it was
to be elevated the next 90 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 enhanced
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 cool 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
begin to act until a quantity of hot water, formed by condensed steam,
is collected on the surface of the cold water which fills the vessel V
V´. 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 vessel 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 water in F, and in producing a vacuum, bears a very small
proportion indeed to what is consumed in heating the apparatus after
condensation.
[Illustration: Pl. I.]
(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 68 feet
of water. Steam is therefore required of a pressure of 45lbs. 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° (17.),
even so high as 267°; 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.
Besides the drainage of mines, Savery proposed to apply his steam
engine to a variety of other purposes; such as supplying cities with
water, forming ornamental waterworks in pleasure grounds, turning
mills, &c.
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 8 hours a day, while the
engine works for 24 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
great work of drainage, from the causes which have been just mentioned
and the increasing necessity for effecting this object arising from
the circumstance of the large property in mines, which became every
year unproductive by it, stimulated the ingenuity of mechanics to
contrive some means of rendering those powers of steam exhibited in
Savery's engine practically available. Among others, Thomas Newcomen,
a blacksmith of Dartmouth, and John Cawley, a plumber of the same
place, turned their attention to this inquiry.
Newcomen appears to have resumed the old method of raising the water
from the mines by ordinary pumps, but conceived the idea of working
these pumps by some moving power less expensive than that of horses.
The means whereby he proposed effecting this was by connecting the end
of a pump rod D (fig. 10.), by a chain, with the arch-head A of a
working beam A B, playing on an axis C. The other arch-head B of this
beam was connected by a chain with the rod E of a solid piston P,
which moved air-tight in a cylinder F. If a vacuum be created beneath
the piston P, the atmospheric pressure acting upon it will press it
down with a force of 15 lbs. per square inch; and the end A of the
beam being thus raised, the pump-rod D will be drawn up. If a pressure
equivalent to the atmosphere be then introduced below the piston, so
as to neutralize the downward pressure, the piston will be in a state
of indifference as to rising or falling; and if in this case the rod D
be made heavier than the piston and its rod, so as to overcome the
friction, &c. it will descend, and elevate the piston again to the top
of the cylinder. The vacuum being again produced, another descent of
the piston, and consequent elevation of the pump-rod, will take place;
and so the process may be continued.
[Illustration: Pl. II. SAVERY'S STEAM ENGINE.]
Such was Newcomen's first conception of the _atmospheric engine_; and
the contrivance had much, even at the first view, to recommend it. The
power of such a machine would depend entirely on the magnitude of the
piston; and being independent of a highly elastic steam, would not
expose the materials to the destructive heat which was necessary for
working Savery's engine. Supposing a perfect vacuum to be produced
under the piston in the cylinder, an effective downward pressure would
be obtained, amounting to 15 times as many pounds as there are square
inches in the section of the piston.[11] Thus, if the base of the
piston were 100 square inches, a pressure equal to 1500lbs. would be
obtained.
[Footnote 11: As the calculation of the power of an engine depends on
the number of square inches in the section of the piston, it may be
useful to give a rule for computing the number of square inches in a
circle. The following rule will always give the dimensions with
sufficient accuracy:--_Multiply the number of inches in the diameter
by itself; divide the product by 14, and multiply the quotient thus
obtained by 11, and the result will be the number of square inches in
the circle._ Thus if there be 12 inches in the diameter, this
multiplied by itself gives 144, which divided by 14 gives 10-4/14,
which multiplied by 11 gives 115, neglecting fractions. There are,
therefore, 115 square inches in a circle whose diameter is 12 inches.]
(38.) In order to accomplish this design, two things were necessary:
1. To make a speedy and effectual vacuum below the piston in the
descent; and 2. To contrive a counterpoise for the atmosphere in the
ascent.
The condensation of steam immediately presented itself as the most
effectual means of accomplishing the former; and the elastic force of
the same steam previous to condensation an obvious method of effecting
the latter. Nothing now remained to carry the design into execution,
but the contrivance of means for the alternate introduction and
condensation of the steam; and Newcomen and Cawley were accordingly
granted a patent in 1707, in which Savery was united, in consequence
of the principle of condensation for which he had previously received
a patent being necessary to the projected machine. We shall now
describe the _atmospheric engine_, as first constructed by Newcomen:--
The boiler K is placed over a furnace I, the flue of which winds round
it, so as to communicate heat to every part of the bottom of it. In
the top, which is hemispherical, two gauge-pipes G G´ are placed, as
in Savery's engine, and a _puppet valve_ V, which opens upward, and is
loaded at one pound per square inch; so that when the steam produced
in the boiler exceeds the pressure of the atmosphere by more than one
pound on the square inch, the valve V is lifted, and the steam escapes
through it, and continues to escape until its pressure is sufficiently
diminished, when the valve V again falls into its seat.
The great steam-tube is represented at S, which conducts steam from
the boiler to the cylinder; and a feeding pipe T furnished with a
cock, which is opened and closed at pleasure, proceeds from a cistern
L to the boiler. By this pipe the boiler may be replenished from the
cistern, when the gauge cock G´ indicates that the level has fallen
below it. The cistern L is supplied with hot water by means which we
shall presently explain.
(39.) To understand the mechanism necessary to work the piston, let us
consider how the supply and condensation of steam must be regulated.
When the piston has been forced to the bottom of the cylinder by the
atmospheric pressure acting against a vacuum, in order to balance that
pressure, and enable it to be drawn up by the weight of the pump-rod,
it is necessary to introduce steam from the boiler. This is
accomplished by opening the cock R in the steam-pipe S. The steam
being thus introduced from the boiler, its pressure balances the
action of the atmosphere upon the piston, which is immediately drawn
to the top of the cylinder by the weight of the pump-rod D. It then
becomes necessary to condense this steam, in order to produce a
vacuum. To accomplish this the further supply of steam must be cut
off, which is done by closing the cock R. The supply of steam from the
boiler being thus suspended, the diffusion of cold water on the
external surface of the cylinder becomes necessary to condense the
steam within it. This was done by enclosing the cylinder within
another, leaving a space between them.[12] Into this space cold water
is allowed to flow from a cock M placed over it, which is supplied by
a pipe from the cistern N. This cistern is supplied with water by a
pump O, which is worked by the engine itself, from the beam above it.
[Footnote 12: The external cylinder is not represented in the
diagram.]
The cold water supplied from M, having filled the space between the
two cylinders, abstracts the heat from the inner one; and condensing
the steam, produces a vacuum, into which the piston is immediately
forced by the atmospheric pressure. Preparatory to the next descent,
the water which thus fills the space between the cylinders, and which
is warmed by the heat it has abstracted from the steam, must be
discharged, in order to give room for a fresh supply of cold water
from M. An aperture, furnished with a cock, is accordingly provided in
the bottom of the cylinder, through which the water is discharged into
the cistern L; and being warm, is adapted for the supply of the boiler
through T, as already mentioned.
The cock R being now again opened, steam is admitted below the piston,
which, as before, ascends, and the descent is again accomplished by
opening the cock M, admitting cold water between the cylinders, and
thereby condensing the steam below the piston.
The condensed steam, thus reduced to water, will collect in the bottom
of the cylinder, and resist the descent of the piston. It is,
therefore, necessary to provide an exit for it, which is done by a
valve opening _outwards_ into a tube which leads to the feeding
cistern L, into which the condensed steam is driven.
That the piston should continue to be air-tight, it was necessary to
keep a constant supply of water over it; this was done by a cock
similar to M, which allowed water to flow from the pipe M on the
piston.
(40.) Soon after the first construction of these engines, an
accidental circumstance suggested to Newcomen a much better method of
condensation than the effusion of cold water on the external surface
of the cylinder. An engine was observed to work several strokes with
unusual rapidity, and without the regular supply of the condensing
water. Upon examining the piston, a hole was found in it, through
which the water, which was poured on to keep it air-tight, flowed, and
instantly condensed the steam under it.
On this suggestion Newcomen abandoned the external cylinder, and
introduced a pipe H furnished with a cock Q into the bottom of the
cylinder, so that on turning the cock the pressure of the water in the
pipe H, from the level of the water in the cistern N, would force the
water to rise as a jet into the cylinder, and would instantly condense
the steam. This method of condensing by a jet formed a very important
improvement in the engine, and is the method still used.
(41.) Having taken a general view of the parts of the atmospheric
engine, let us now consider more particularly its operation.
When the engine is not working the weight of the pump-rod D draws down
the beam A, and draws the piston to the top of the cylinder, where it
rests. Let us suppose all the cocks and valves closed, and the boiler
filled to the proper depth. The fire being lighted beneath it, the
water is boiled until the steam acquires sufficient force to lift the
valve V. When this takes place, the engine may be started. For this
purpose the regulating valve R is opened. The steam rushes in and is
first condensed by the cold cylinder. After a short time the cylinder
acquires the temperature of the steam, which then ceases to be
condensed, and mixes with the air which filled the cylinder. The steam
and heated air, having a greater force than the atmospheric pressure,
will open a valve placed at the end X of a small tube in the bottom of
the cylinder, and which opens outwards. From this (which is called the
_blowing valve_[13]) the steam and air rush in a constant stream until
all the air has been expelled, and the cylinder is filled with the
pure vapour of water. This process is called _blowing_ the engine
preparatory to starting it.
[Footnote 13: Also called the _snifting_ valve, from the peculiar
noise made by the air and steam escaping from it.]
When it is about to be started, the engine-man closes the regulator R,
and thereby suspends the supply of steam from the boiler. At the same
time he opens the _condensing valve_ H,[14] and thereby throws up a
jet of cold water into the cylinder. This immediately condenses the
steam contained in the cylinder, and produces the vacuum. (The
atmosphere cannot enter the _blowing_ valve, because it opens
_outwards_, so that no air can enter to vitiate the vacuum.) The
atmospheric pressure above the piston now takes effect, and forces it
down in the cylinder. The descent being completed, the engine-man
closes the condensing valve H, and opens the regulator R. By this
means he stops the play of the jet within the cylinder, and admits the
steam from the boiler. The first effect of the steam is to expel the
condensing water and condensed steam which are collected in the bottom
of the cylinder through the tube Y, containing a valve which opens
_outwards_, (called the _eduction valve_,) which leads to the hot
cistern L, into which this water is therefore discharged.
[Footnote 14: Also called the _injection valve_.]
When the steam admitted through R ceases to be condensed, it balances
the atmospheric pressure above the piston, and thus permits it to be
drawn to the top of the cylinder by the weight of the rod D. This
ascent of the piston is also assisted by the circumstance of the steam
being somewhat stronger than the atmosphere.
When the piston has reached the top, the regulating valve r is closed,
and the condensing valve H opened, and another descent produced as
before, and so the process is continued.
The manipulation necessary in working this engine was, therefore, the
alternate opening and closing of two valves; the regulating and
condensing valves. When the piston reached the top of the cylinder,
the former was to be closed, and the latter opened; and, on reaching
the bottom, the former was to be opened, and the latter closed.
(42.) From the imperfect attention which even an assiduous attendant
could give to the management of these valves, the performance of the
engines was very irregular, and the waste of fuel very great, until a
boy named _Humphrey Potter_ contrived means of making the engine work
its own valves. This contrivance, although made with no other design
than the indulgence of an idle disposition, nevertheless constituted a
most important step in the progressive improvement of the
steam-engine; for by its means, not only the irregularity arising from
the negligence of attendants was avoided, but the speed of the engine
was doubled.
Potter attached strings to the levers which worked the valves, and
carrying these strings to the working beam, fastened them upon it in
such a manner that as the beam ascended and descended, it pulled the
strings so as to open and close the proper valves with the most
perfect regularity and certainty. This contrivance was afterwards much
improved by an engineer named _Beighton_, who attached to the working
beam a straight beam called a _plug frame_, carrying pins which, in
the ascent and descent of the beam, struck the levers attached to the
valves, and opened and closed them exactly at the proper moment.
The engine thus improved required no other attendance except to feed
the boiler occasionally by the cock T, and to attend the furnace.
CHAP. IV.
ENGINE OF JAMES WATT.
Advantages of the Atmospheric Engine over that of Captain Savery.
-- It contained no new Principle. -- Papin's Engine. -- James
Watt. -- Particulars of his Life. -- His first conceptions of the
means of Economising Heat. -- Principle of his projected
Improvements.