on a stand S, under which is placed a large spirit lamp L, or other
means of heating it. In the top of this vessel are three apertures,
in two of which are screwed a [Pg110] thermometer T, the bulb of
which enters the hollow brass sphere, and a stop-cock C, which may
be closed or opened at pleasure, to confine the steam, or allow it
to escape. In the third aperture at the top, is screwed a long
barometer tube, open at both ends. The lower end of this tube
extends nearly to the bottom of the spherical vessel B. In the
bottom of this vessel is placed a quantity of mercury, the surface
of which rises to some height above the lower end of the tube A.
Over the mercury is poured a quantity of water, so as to half fill
the vessel B. Matters being thus arranged, the screws are made
tight, so as to confine the water, and the lamp is allowed to act on
the vessel; the temperature of the water is raised, and steam is
produced, which, being confined within the vessel, exerts its
pressure on the surface of the water, and resists its ebullition.
The pressure of the steam acting on the surface of the water is
communicated to the surface of the mercury, and it forces a portion
of the mercury into the tube A, which presently rises above the
point where the tube is screwed into the top of the vessel B. As the
action of the lamp continues, the thermometer T exhibits a gradually
increasing temperature; while the column of mercury in A shows the
force with which the steam presses on the surface of the water in
B,—this column being balanced by the pressure of the steam. Thus,
the temperature and pressure of the steam at the same moment may
always be observed by inspecting the thermometer T and the tube A.
When the column in the tube A has risen to the height of 30 inches
above the level of the mercury in the vessel B, then the pressure of
the steam will be equivalent to double the pressure of the
atmosphere, because, the tube A being open at the top, the
atmosphere presses on the [Pg111] surface of the mercury in it. The
thermometer T will be observed gradually to rise until it attains
the temperature of 212°; but it will not stop there, as it would do
if immersed in water boiled in an open vessel. It will, on the other
hand, continue to rise; and when the column of mercury in A has
attained the height of 30 inches, the thermometer T will have risen
to 251°,—being 39° above the ordinary boiling point.
During the whole of this process, the surface of the water being
submitted to a constantly increasing pressure, its ebullition is
prevented, and it continues to receive heat without boiling. That
it is the increased pressure which resists its ebullition, and
causes it to receive a temperature above 212°, may be easily
shown. Let the stop-cock C be opened; immediately the steam in B,
having a pressure considerably greater than that of the
atmosphere, will rush out, and will continue to issue from C,
until its pressure is balanced by the atmosphere. At the same time
the column of mercury in A will be observed rapidly to fall, and
to sink below the orifice by which it is inserted in the vessel B.
The thermometer T will also fall until it attains the temperature
of 212°. At that point, however, it will remain stationary; and
the water will now be distinctly heard to be in a state of rapid
ebullition. If the stop-cock C be once more closed, the
thermometer will begin to rise, and the column of mercury
ascending in A will be again visible.
If, instead of a stop-cock being at C, the aperture were made to
communicate with a valve, like the safety-valve of a steam engine,
loaded with a certain weight,—say at the rate of 15 lbs. on the
square inch,—then the thermometer T, and the mercury in the tube
A, would not rise indefinitely as before. The thermometer would
continue to rise till it attained the temperature of 251°; and the
mercury in the tube A would rise to the height of 30 inches. At
this limit the resistance of the valve would be balanced by the
pressure of the steam; and as fast as the water would have a
tendency to produce steam of a higher pressure, the valve would be
raised and the steam suffered to escape; the thermometer T and the
column of mercury in A remaining stationary during this process.
If the valve were loaded more heavily, the phenomena would be
[Pg112] the same, only that the mercury in T and A would become
stationary at certain heights. But, on the other hand, if the
valve were loaded at a less pressure than 15 lbs. on the square
inch, then the mercury in the two tubes would become stationary at
lower points.
(59.) These experiments show that every increase of pressure above
the ordinary pressure of the atmosphere causes an increase in the
temperature at which water boils. We shall now inquire whether a
diminution of pressure will produce a corresponding effect on the
boiling point.
This may be easily accomplished by the aid of an air pump. Let
water at the temperature of 200° be placed in a glass vessel under
the receiver of an air pump, and let the air be gradually
withdrawn. After a few strokes of the pump, the water will boil;
and if the mercurial gauge of the pump be observed, it will be
found that its altitude will be about 23-1/2 inches. Thus the
pressure to which the water is submitted has been reduced from the
ordinary pressure of the atmosphere expressed by the column of 30
inches of mercury, to a diminished pressure expressed by 23-1/2
inches; and we find that the temperature at which the water boils
has been lowered from 212° to 200°. Let the same experiment be
repeated with water at the temperature of 180°, and it will be
found that a further rarefaction of the air is necessary, but the
water will at length boil. If the gauge of the pump be now
observed, it will be found to stand at about fifteen inches,
showing, that at the temperature of 180° water will boil under
half the ordinary pressure of the atmosphere. These experiments
may be varied and repeated; and it will be always found, that, as
the pressure is diminished or increased, the temperature at which
the water will boil will be also diminished or increased.
(60.) The same effects may be exhibited in a striking manner
without an air pump, by producing a vacuum by the condensation of
steam. Let a small quantity of water be placed in a thin glass
flask, and let it be boiled by holding it over a spirit lamp. When
the steam is observed to issue abundantly from the mouth of the
flask, let it be quickly corked and removed from the lamp. The
process of boiling will then cease, and the water will become
quiescent; but if the flask be plunged [Pg113] in a vessel of
cold water, the water it contains will again pass into a state of
violent ebullition, thus exhibiting the singular fact of water
being boiled by cooling it. This effect is produced by the cold
medium in which the flask is immersed, causing the steam above the
surface of the water in it to be condensed, and therefore
relieving the water from its pressure. The water, under these
circumstances, boils at a lower temperature than when submitted to
the pressure of the uncondensed vapour.
(61.) There is no limit to the temperature to which water may be
raised, if it be submitted to a sufficient pressure to resist its
tendency to take the vaporous form. If a strong metallic vessel be
nearly filled with water, so as to prevent the liquid from
escaping by any force which it can exert, the water thus inclosed
may be heated to any temperature whatever without boiling; in
fact, it may be made red-hot; and the temperature to which it may
be raised will have no limit, except the strength of the vessel
containing it, or the point at which the metal of which it is
formed may begin to soften or to be fused.
(62.) The following table will show the temperature at which water
will boil under different pressures of the atmosphere corresponding
to the altitudes of the barometer between 26 and 31 inches.
Barometer. Boiling Point.
26 inches 204°·91
26·5 205°·79
27 206°·67
27·5 207°·55
28 208°·43
28·5 209°·31
29 210°·19
29·5 211°·07
30 212°
30·5 212°·88
31 213°·76
From this table it appears, that, for every tenth of an inch which
the barometric column varies between these limits, the boiling
temperature changes by the fraction of a degree expressed by the
decimal ·176, or nearly by the vulgar fraction 1/6.
(63.) In the experiment already described, by which the latent
[Pg114] heat of steam was determined, the water was supposed to be
boiled under the ordinary pressure of the atmosphere. Having seen,
however, that water may boil at different temperatures, under
different pressures, the inquiry presents itself, whether the heat
absorbed in vaporisation at different temperatures, and under
different pressures, is subject to any variation? Experiments of
the same nature as those already described, instituted upon water
in a state of ebullition at different temperatures, as well below
as above 212°, have led to the discovery of a very remarkable fact
in the theory of vapour. It has been found that the heat absorbed
by vaporisation is always less, the higher the temperature at
which the ebullition takes place; and less, by the same amount as
the temperature of ebullition is increased. Thus, if water boil at
312°, the heat absorbed in ebullition will be less by 100° than if
it boiled at 212°; and again, if water be boiled under a
diminished pressure, at 112°, the heat absorbed in vaporisation
will be 100° more than the heat absorbed by water boiled at 212°.
It follows, therefore, that the actual consumption of heat in the
process of vaporisation must be the same, whatever be the
temperature at which the vaporisation takes place; for whatever
heat is saved in the sensible form, is consumed in the latent
form, and _vice versâ_.
Let us suppose a given weight of water at the temperature of 32°
to be exposed to any regular source by which heat may be supplied
to it. If it be under the ordinary atmospheric pressure, the first
180° of heat which it receives will raise it to the boiling point,
and the next 1000° will convert it into steam. Thus, in addition
to the heat which it contains at 32°, the steam at 212° contains
1180° of heat. But if the same water be submitted to a pressure
equal to half the atmospheric pressure, then the first 148° of
heat which it receives will cause it to boil, and the next 1032°
will convert it into vapour. Thus, steam at the temperature of
180° contains a quantity of heat more than the same quantity of
water at 32°, by 1032° added to 148°, which gives a sum of 1180°.
Steam, therefore, raised under the ordinary pressure of the
atmosphere at 212°, and steam raised under half that pressure at
180°, contain the same quantity of heat,—with this difference
[Pg115] only—that the one has more latent heat, and less sensible
heat, than the other.
From this fact, that the sum of the latent and sensible heats of
the vapour of water is constant, it follows that the same quantity
of heat is necessary to convert a given weight of water into
steam, at whatever temperature, or under whatever pressure, the
water may be boiled. It follows, also, that, in the steam engine,
equal weights of high-pressure and low-pressure steam are produced
by the same consumption of fuel; and that, in general, the
consumption of fuel is proportional to the quantity of water
vaporised, whatever the pressure of the steam may be.[18]
(64.) Having explained the conditions under which, by supplying
heat to water, it is converted into steam, and, by abstracting
heat from steam, it may be reconverted into water, let us now
consider the mechanical force which is developed in these
phenomena.
[Illustration: _Fig._ 18.]
Let A B (_fig._ 18.) be a tube, or cylinder, the base of which is
equal to a square inch, and let a piston P move in it so as to be
steam-tight. Let it be supposed, that under this piston there is,
in the bottom of the cylinder, a cubic inch of water between the
bottom of the piston and the bottom of the tube; let the piston be
counterbalanced by a weight W acting over a pulley, which will be
just sufficient to counterpoise the weight of the piston, so as
leave no force tending to keep the piston down, except the force
of the atmosphere acting above it. Under the circumstances here
supposed, the piston being in contact with the water, and all air
being excluded, it will be pressed down by the weight of the
atmosphere, which we will suppose to be fifteen pounds, the
magnitude of the piston being a square inch. [Pg116]
Now let the flame of a lamp be applied at the bottom of the tube;
the water under the piston having its temperature thereby
gradually raised, and being submitted to no pressure save that of
the atmosphere above the piston, it will begin to be converted
into steam when it has attained the temperature of 212°. According
as it is converted into steam, it will cause the piston to ascend
in the tube until all the water has been evaporated. If the tube
were constructed of sufficient length, the piston then would be
found to have risen to the height of about seventeen hundred
inches, or one hundred and forty-two feet; since, as has been
already explained, water passing into steam under the ordinary
pressure of the atmosphere undergoes an increase of bulk in the
proportion of about seventeen hundred to one.
Now in this process, the air above the piston, which presses on it
with a force equal to fifteen pounds, has been raised one hundred
and forty-two feet. It appears, therefore, that, by the
evaporation of a cubic inch of water under a pressure equal to
fifteen pounds per square inch, a mechanical force of this amount
is developed.
It is evident that fifteen pounds raised one hundred and forty-two
feet successively, is equivalent to one hundred and forty-two
times fifteen pounds raised one foot. Now, one hundred and
forty-two times fifteen is two thousand one hundred and thirty,
and therefore the force thus obtained is equal to two thousand one
hundred and thirty pounds raised one foot high. This being within
about 110 pounds of a ton, it may be stated, in round numbers,
that, by the evaporation of a cubic inch of water under these
circumstances, a force is obtained equal to that which would raise
a ton weight a foot high.
The augmentation of volume which water undergoes in passing into
steam under the pressure here supposed, may be easily retained in
the memory, from the accidental circumstance that a cubic inch of
water is converted into a cubic foot of steam, very nearly. A
cubic foot contains one thousand seven hundred and twenty-eight
cubic inches,—which is little different from the proportion which
steam bears to water, when raised under the atmospheric pressure.
[Pg117]
(65.) It will, therefore, be an advantage to retain in memory the
following general facts:—
1. _A cubic inch of water evaporated under the ordinary
atmospheric pressure, is converted into a cubic foot of steam._
2. _A cubic inch of water evaporated under the atmospheric
pressure, gives a mechanical force equal to what would raise about
a ton weight a foot high._
(66.) Let us, again, suppose the piston P (_fig._ 23.) to be
restored to its original position, with the liquid water beneath
it; and, in addition to the weight of the atmosphere which before
pressed it down, let us suppose another weight of fifteen pounds
laid upon it, so that the water below shall be pressed by double
the weight of the atmosphere. If the lamp were now applied, and at
the same time a thermometer were immersed in the water, it would
be found that the water would not begin to be converted into steam
until it attained the temperature of about 250°. The piston would
then begin, as before, to ascend, and the water to be gradually
converted into vapour. The water being completely evaporated, it
would be found that the piston would be raised to a height little
more than half its former height, or 72 feet. The mechanical
effect, therefore, thus obtained, will be equivalent to double the
former weight raised half the former height.
In like manner, if the piston were loaded with thirty pounds in
addition to the atmosphere, the whole pressure on the water being
then three times the pressure first supposed, the piston would be
raised to somewhat more than one third of its first height by the
evaporation of the water. This would give a mechanical force
equivalent to three times the original weight raised a little more
than one third of the original height.
In general, as the pressure on the piston is increased, the height
to which the piston would be raised by the evaporation of the
water will be diminished in a proportion somewhat less than the
proportion in which the pressure on the piston is increased. If
the temperature at which the water is converted into steam under
these different pressures were the same, then the height to which
the piston would be raised by the evaporation of the water would
be diminished in precisely [Pg118] the same proportion as the
pressure on the piston is increased; and, in that case, the whole
mechanical force developed by the evaporation of the water would
remain exactly the same under whatever pressure the water might be
boiled. We shall explain hereafter the extent to which the
variation of temperature in the water and steam corresponding to
the variation of pressure modifies this law; but, as the effect of
the difference of temperatures is not considerable, it will be
convenient to register in the memory the following important
practical conclusion:—
(67.) _A cubic inch of water converted into steam will supply a
mechanical force very nearly equal to a ton weight raised a foot
high; and this force will not be subject to considerable
variation, whatever be the temperature or pressure at which the
water may be evaporated._
[Illustration: GLASGOW.]
FOOTNOTES:
[18] The preceding paragraphs, and some other parts of the
present volume on the general properties of Heat, are taken
from my Treatise on Heat, in the _Cabinet Cyclopœdia_, to
which those who desire more detailed explanation and more
copious illustration should refer.
[Pg119]
[Illustration: GLASGOW COLLEGE.]
CHAP. V.
WATT FINDS THAT CONDENSATION IN THE CYLINDER IS INCOMPATIBLE
WITH A DUE ECONOMY OF FUEL. — CONCEIVES THE NOTION OF
CONDENSING OUT OF THE CYLINDER. — DISCOVERS SEPARATE
CONDENSATION. — INVENTS THE AIR-PUMP. — SUBSTITUTES STEAM
PRESSURE FOR ATMOSPHERIC PRESSURE. — INVENTS THE STEAM CASE,
OR JACKET. — HIS FIRST EXPERIMENTS TO REALISE THESE
INVENTIONS. — HIS EXPERIMENTAL APPARATUS. — DIFFICULTIES OF
BRINGING THE IMPROVED ENGINES INTO USE. — WATT PRACTISES AS A
CIVIL ENGINEER. — HIS PARTNERSHIP WITH ROEBUCK. — HIS FIRST
PATENT. — DESCRIPTION OF HIS SINGLE-ACTING STEAM ENGINE.
(68.) At the period to which we have now brought the history of
the invention of the steam engine, Watt had obtained, chiefly by
his own experiments, a sufficient knowledge of the phenomena which
have been just explained, to enable him to arrive at the
conclusion that a very small proportion of the whole mechanical
effect attending the evaporation was really rendered available by
the atmospheric engine; and that, [Pg120] therefore, extensive
and injurious sources of waste existed in its machinery.
He perceived that the principal source of this wasteful
expenditure of power consisted in the quantity of steam which was
condensed at each stroke of the piston, in heating the cylinder
previous to the ascent of the piston. Yet, as it was evident that
that ascent could not be accomplished in a cold cylinder, it was
apparent that this waste of power must be inevitable, unless some
expedient could be devised, by which _a vacuum could be maintained
in the cylinder, without cooling it_. But, to produce such a
vacuum, the steam must be condensed; and, to condense the steam,
its temperature must be lowered to such a point that the vapour
proceeding from it shall have no injurious pressure; yet, if
condensed steam be contained in a cylinder at a high temperature,
it will return to the temperature of the cylinder, recover its
elasticity, and resist the descent of the piston.
Having reflected on these circumstances, it became apparent to
Watt, that a vice was inherent in the structure of the atmospheric
engine, which rendered a large waste of power inevitable; this
vice arising from the fact, that the condensation of the steam was
incompatible with the condition of maintaining the elevated
temperature of the cylinder in which that condensation took place.
It followed, therefore, either that the steam must be imperfectly
condensed, or that the condensation could not take place in the
cylinder. It was in 1765, that, pondering on these circumstances,
the happy idea occurred to him, that the production of a vacuum
could be equally effected, though _the place_ where the
condensation of the steam took place were not the cylinder itself.
He saw, that if a vessel in which a vacuum was produced were put
into communication with another containing an elastic fluid, the
elastic fluid would rush into the vacuum, and diffuse itself
through the two vessels; but if, on rushing into such vacuum, this
elastic fluid, being vapour, were there condensed, or restored to
the liquid form, that then the space within the two vessels would
be equally rendered a vacuum;—that, under such circumstances, one
of the vessels might be maintained at any temperature, however
high, while [Pg121] the other might be kept at any temperature,
however low. This felicitous conception formed the first step in
that splendid career of invention and discovery which has
conferred immortality on the name of Watt. He used to say, that
the moment the idea of separate condensation occurred to
him,—that is, of condensing, in one vessel kept cold, the steam
coming from another vessel kept hot,—all the details of his
improved engine rushed into his mind in such rapid succession,
that, in the course of a day, his invention was so complete that
he proceeded to submit it to experiment.
[Illustration: _Fig._ 19.]
To explain the first conception of this memorable invention; let a
tube or pipe, S (_fig._ 19.), be imagined to proceed from the
bottom of the cylinder A B to a vessel, C, having a stop-cock, D,
by which the communication between the cylinder and the vessel C
may be opened or closed at pleasure. If we suppose the piston P at
the top of the cylinder, and the space below it filled with steam,
the cylinder and steam being at the usual temperature, while the
vessel C is a vacuum, and maintained at a low temperature. Then,
on opening the cock D, the steam will rush from the cylinder A B
through the tube S, and, passing into the cold vessel C, will be
condensed by contact with its cold sides. This process of
condensation will be rendered instantaneous if a jet of cold water
is allowed to play in the vessel C. When the steam thus rushing
into C, has been destroyed, and the space in the cylinder A B
becomes a vacuum, then the pressure of the atmosphere being
unobstructed, the piston will descend with the force due to the
excess of the pressure of the atmosphere above the friction. When
it has descended, suppose the stop-cock D closed, and steam
admitted from [Pg122] the boiler through a proper cock or valve
below the piston, the cylinder and piston being still at the same
temperature as before. The steam on entering the cylinder, not
being exposed to contact with any surface below its own
temperature, will not be condensed, and therefore will immediately
cause the piston to rise, and the piston will have attained the
top of the cylinder when as much steam shall have been supplied by
the boiler as will fill the cylinder. When this has taken place,
suppose the communication with the boiler cut off, and the cock D
once more opened: the steam will again rush through the pipe S
into the vessel C, where encountering the cold surface and the jet
of cold water, it will be condensed, and the vacuum, as before,
will be produced in the cylinder A B; that cylinder still
maintaining its temperature, the piston will again descend, and so
the process may be continued.
(69.) Having carried the invention to this point, Watt saw that
the vessel C would gradually become heated by the steam which
would be continually condensed in it. To prevent this, as well as
to supply a constant jet of cold water, he proposed to keep the
vessel C submerged in a cistern of cold water, from which a pipe
should conduct a jet to play within the vessel, so as to condense
the steam as it would pass from the cylinder.