cylinder as would reconvert the steam contained in it into water,
then we should have in the bottom of the cylinder a quantity of
warm water with a vacuum above it: but the entire mass of metal
composing the cylinder itself, which was previously at the
temperature of boiling water, would still be at the same
temperature. The warm water, resting in contact with this metal in
the bottom of the cylinder, would be immediately heated by it, and
would rise in its temperature, while the metal of the cylinder
itself would be somewhat lowered in temperature by the heat which
it would thus impart to the warm water contained in it. Under
these circumstances, as we shall presently explain, steam would be
produced from the water, which would fill the cylinder; and
although such steam would not have a mechanical pressure equal in
amount to the atmosphere, and therefore would not altogether
prevent the piston from descending if it had no load to move, yet
it would deprive the engine of so great a portion of its
legitimate power as to render it altogether inefficient. But this
defect would be removed by throwing into the cylinder a sufficient
quantity of cold water, not only to destroy the steam contained in
it, but also to cool the entire mass of metal composing the
cylinder itself, until it would be reduced to such a temperature
that the vapour proceeding from the water contained in it would
have so small a pressure that it would not seriously or
injuriously obstruct the descent of the piston.
The piston being made to descend with such force as to render the
machine practically efficient, it would then be necessary again to
make it ascend; and to accomplish this, Watt found that the boiler
should supply a quantity of steam many times greater than was
necessary to fill the cylinder. Mature reflection on the
circumstances which have been just explained, enabled him to
discover how this undue quantity of steam was rendered necessary.
Let it be recollected, that when the piston has reached the bottom
of the cylinder, the whole mass of the cylinder, and [Pg089] the
piston itself, are reduced to so low a temperature that the vapour
of water, having the same temperature, has no pressure sufficiently
great to obstruct the action of the machine. When, in order to make
the piston ascend, steam is introduced from the boiler into the
cylinder under the piston, this steam encounters, in the first
instance, the cold surfaces of the metal forming the bottom of the
cylinder and the bottom of the piston. The first effect of this is
to convert the steam which comes from the boiler into water, an
effect which is produced by that steam imparting its heat to the
metal with which it comes into contact. This destruction of steam
continues until the metal exposed to contact with it has been heated
up to the temperature of boiling water. Then, and not till then, the
steam below the piston will have a pressure equal to that of the
atmosphere above it, and the piston will begin to ascend. As it
ascends, however, the sides of the cylinder which it exposes to the
contact of the steam are cold, and partially destroy the steam.
Steam, therefore, must be supplied from the boiler to replace the
steam thus destroyed; nor can the piston reach the top of the
cylinder until such a quantity of steam shall have flowed from the
boiler into the cylinder, as shall be sufficient not only to fill
the cylinder under the piston, but likewise, by its condensation, to
raise the whole mass of the cylinder and piston to the temperature
of boiling water.
Such were the circumstances which forced themselves upon the
attention of Watt, in the course of repairing, and subsequently
trying, the model of the atmospheric engine, at Glasgow. Being
informed generally of the uses of the engine in the drainage of
mines, and of the vast expense attending its operation, by reason
of the quantity of fuel which it consumed, he saw how important
any improvement would be by which the extensive sources of waste
which had thus presented themselves could be removed. He saw also,
that all that portion of steam which was expended, not in filling
the cylinder under the piston, but in heating the great mass of
metal composing the cylinder and piston, from a low temperature to
that of boiling water, upon each stroke of the piston, was so much
heat lost, and that the proportion of the fuel expended in
evaporating the steam thus wasted would be saved, if by any
[Pg090] expedient _he could make the piston descend without
cooling the cylinder_. But in order to estimate the full amount of
this waste, and to discover the most effectual means of preventing
it, it was necessary to investigate the quantity of heat necessary
for the evaporation of a given quantity of water; also, the
quantity of steam which a given quantity of water would produce,
as well as other circumstances connected with the temperature and
pressure of steam. He, therefore, applied himself to make
experiments with a view to elucidate these questions; and
succeeded in obtaining results which led to the discovery of some
of the most important of those physical phenomena, on the due
application of which, the efficacy of the steam engine, which he
afterwards invented, depended, and which also form striking facts
in the general physics of heat.
(49.) The first question to which he directed his experiments, was
the determination of the extent to which water enlarged its
volume, or magnitude, when it passed into steam. To ascertain
this, he filled a thin Florence flask with steam, of a pressure
equal to the atmosphere, and weighed it accurately. The same flask
was then filled with water, and weighed again. Finally, the weight
of the flask itself was ascertained. It is evident, that by such
means, the exact weight of the steam which filled the flask, and
of the same bulk of water, would be obtained. He found that the
water weighed about eighteen hundred times more than the steam;
from whence he inferred that the steam which filled the flask
contained about eighteen hundred times less water than the flask
would contain.[17]
[Pg091] Having once ascertained this point, he was able, by
observing the quantity of water evaporated in the boiler of the
atmospheric model, to compute the volume of steam which was
supplied to the cylinder. It was evident, that for every cubic
inch of water evaporated in the boiler, eighteen hundred cubic
inches of steam were supplied to the cylinder. Having accurately
observed the evaporation of the boiler for a short time, and the
number of strokes made by the piston in the same time, he found
that the quantity of water evaporated in the boiler would supply
about four times as much steam as the cylinder would require. He
consequently inferred, that about three-fourths of the steam
produced was wasted.
The next question to which he directed his experiments, was to
ascertain the quantity of cold water necessary to be injected into
the cylinder, in order to condense the steam contained in it. To
ascertain this, he attached a pipe to a boiler, by which he was
enabled to conduct the steam from the boiler into a glass jar
containing cold water at fifty-two degrees of temperature. The
steam, as it passed from the boiler through the pipe, was
condensed by the cold water, and continued to be so condensed,
until, by the heat which it imparted to the water, the latter
began to boil, and would then condense no more steam. On comparing
the water in the glass jar, when boiling, with the water
originally contained in it at fifty-two [Pg092] degrees, the
quantity was found to be increased in the proportion of six to
seven, very nearly; from which he inferred, that to reduce one
ounce of steam to water, it was necessary to mix about six ounces
of cold water with it.
He was further led to the conclusion, that steam contains a vast
quantity of heat, by the following experiment. He heated, in a
close digester, a quantity of water several degrees above the
common boiling point. When thus heated, by opening a stop-cock, he
allowed the compressed steam to escape into a cold vessel; in
three or four seconds, he found that the heat of the water in the
digester was reduced from a very high temperature to the common
boiling point; yet, that all the steam which escaped from it, and
which carried off with it the superabundant heat, formed only a
few drops of water when condensed; from which he inferred, that
this small quantity of water, in the form of steam, contained as
much heat as was sufficient to raise all the water in the digester
from the boiling point to the temperature at which it was before
the steam was allowed to escape.
Having thus ascertained the exact quantity of cold water which
ought to be injected into the cylinder in order to condense the
steam which filled the cylinder, he found, on comparing the
quantity necessary to be injected in order to enable the piston to
descend, that this quantity was about four times as great as that
which was necessary to condense the steam. This led him to the
conclusion, that about four times as much heat was destroyed in
the cylinder as needed to be destroyed, if the object were the
mere condensation of the steam. This result fully corroborated the
other conclusion, deduced, from the proportion which he found
between the quantity of steam supplied by the boiler and the
actual contents of the cylinder.
(50.) Watt was forcibly struck with these circumstances, not only
on account of their importance in an economical point of view,
when their relation to steam power was considered, but still more
so, as indicating phenomena in the physics of heat altogether
novel to him.
He, therefore, eagerly sought his friend Dr. Black, to whom he
communicated these results. Then, for the first time, he [Pg093]
was informed, by Black, of the theory of LATENT HEAT, which had
recently been discovered by him, and of which these very phenomena
formed the basis.
Some passages in the works of Dr. Robison produced an erroneous
impression, that a large share of the merit of the discoveries of
Watt which have been just explained was due to Dr. Black, to whose
instructions on the subject of latent heat Watt was represented to
have owed the knowledge of those facts which led to his principal
inventions and improvements. We shall here give, in the words of
Watt himself, his explanation of the circumstances which led to
this error. This explanation is given in a letter addressed by
Watt to Dr. Brewster, in May 1814, and prefixed to the third
volume of Brewster's edition of Robison's Mechanical Philosophy:—
"The representations of friends whose opinions I highly value
induce me to avail myself of this opportunity of noticing an error
into which not only Dr. Robison, but apparently also Dr. Black,
has fallen, in relation to the _origin_ of my improvements upon
the steam engine, and which not having been publicly controverted
by me, has, I am informed, been adopted by almost every subsequent
writer upon the subject of latent heat.
"Dr. Robison, in the article Steam Engine, after passing an
encomium upon me, dictated by the partiality of friendship,
qualifies me as the '_pupil_ and intimate friend of Dr. Black,'—a
description which not being there accompanied with any inference,
did not particularly strike me at the time of its first perusal.
He afterwards, in the dedication to me of his edition of Dr.
Black's lectures upon chemistry, goes the length of supposing me
to have professed to owe my improvements upon the steam engine to
the instructions and information I had received from that
gentleman, which certainly was a misapprehension; as, though I
have always felt and acknowledged my obligations to him for the
information I had received from his conversation, and particularly
for the knowledge of the doctrine of latent heat, I never did nor
_could_ consider my improvements as originating in those
communications. He is also mistaken in his assertion (p. 8. of the
preface to the above work), that 'I had attended two courses
[Pg094] of the doctor's lectures;' for, unfortunately for me, the
necessary avocations of my business prevented me from attending
his or any other lectures at college; and as Dr. Robison was
himself absent from Scotland for four years at the period referred
to, he must have been misled by erroneous information. In p. 184.
of the lectures, Dr. Black says, 'I have the pleasure of thinking
that the knowledge we have acquired concerning the nature of
elastic vapours, in consequence of my fortunate observation of
what happens in its formation and condensation, has contributed in
no inconsiderable degree to the public good by _suggesting_ to my
friend Mr. Watt of Birmingham, then of Glasgow, his improvement on
this useful engine' (meaning the steam engine of which he is then
speaking). There can be no doubt from what follows in his
description of the engine, and from the very honourable mention
which he has made of me in various parts of his lectures, that he
did not mean to lessen any merit that might attach to me as an
inventor; but, on the contrary, he was always disposed to give me
fully as much praise as I deserved.
"And were that otherwise doubtful, it would, I think, be evident
from the following quotation from a letter of his to me, dated
13th February 1783, where, speaking of an intended publication by
a friend of mine, on subjects connected with the history of steam,
he says, 'I think it is very proper for you to give him a short
account of your discoveries and speculations; _and particularly_
_to assert clearly and fully your sole right to the honour of_
_the improvements of the steam engine_.' And in a written
testimonial which he very kindly gave me, on the occasion of a
trial at law against a piracy of my invention in 1796-7, after
giving a short account of the invention, he adds, '_Mr. Watt was_
_the sole inventor of the capital improvement and contrivance_
_above mentioned._'
"Under this conviction of his candour and friendship, it is very
painful to me to controvert any assertion or opinion of my revered
friend; yet, in the present case I find it necessary to say, that
he appears to me to have fallen into an error; and I hope, in
addition to my assertion, to make that appear by the short history
I have given of my invention, in my [Pg095] notes upon Dr.
Robison's essay, as well as by the following account of the state
of my knowledge previous to my receiving any explanation of the
doctrine of latent heat; and also from that of the facts which
principally guided me in the invention.
"It was known very long before my time, that steam was condensed
by coming into contact with cold bodies, and that it communicated
heat to them; witness the common still, &c. &c.
"It was known, by some experiments of Dr. Cullen and others, that
water and other liquids boiled in vacuo at very low heats; water
below 100°.
"It was known to some philosophers that the capacity or
equilibrium of heat, as we then called it, was much smaller in
mercury and tin than in water.
"It was also known that evaporation caused the cooling of the
evaporating liquid, and bodies in contact with it.
"I had myself made experiments to determine the following facts:—
"First, the capacities of heat for iron, copper, and some sorts of
wood, comparatively with water.
"Second, the bulk of steam compared with that of water.
"Third, the quantity of water evaporated in a certain boiler by a
pound of coals.
"Fourth, the elasticities of steam at various temperatures greater
than that of boiling water, and an approximation to the law which
it followed at other temperatures.
"Fifth, how much water in the form of steam was required every
stroke by a small Newcomen's engine, with a wooden cylinder six
inches diameter, and twelve inches stroke.
"Sixth, the quantity of cold water required in every stroke to
condense the steam in that cylinder, so as to give it a working
power of about 7 lb. on the inch.
"Here I was at a loss to understand how so much cold water could
be heated so much by so small a quantity of water in the form of
steam; and I accordingly applied to Dr. Black, and then first
understood what was called latent heat.
"But this theory, though useful in determining the quantity of
injection necessary where the quantity of water [Pg096]
evaporated by the boiler, and used by the cylinder, was known, and
in determining, by the quantity and heat of the hot water emitted
by Newcomen's engines, the quantity of steam required to work them
did not lead to the improvements I afterwards made in the engine.
These improvements proceeded upon the old established fact, that
steam was condensed by the contact of cold bodies; and the later
known one, that water boiled in vacuo at heats below 100°, and
consequently that a vacuum could not be obtained unless the
cylinder and its contents were cooled every stroke to below that
heat."
[Illustration: LOCH LOMOND.]
FOOTNOTES:
[16] We are indebted for many of the anecdotes of the life of
Watt to the _Eloge Historique_, recently published by M. Arago,
who was furnished with all the documents and circumstances
relating to this celebrated person which were considered proper
for publication, by his son, the present James Watt, Esq., of
Aston Hall, near Birmingham, and to the notes added to this
memoir by Mr. Muirhead, a relative of Mr. Watt.
[17] The following is the account of these experiments given
in Watt's own words:—
"It being evident that there was a great error in Dr.
Desagulier's calculations of Mr. Beighton's experiments on the
bulk of steam, a Florence flask, capable of containing about a
pound of water, had about one ounce of distilled water put
into it; a glass tube was fitted into its mouth, and the
joining made tight by lapping that part of the tube with
packthread covered with glazier's putty. When the flask was
set upright, the tube reached down near to the surface of the
water, and in that position the whole was placed in a tin
reflecting oven before a fire until the water was wholly
evaporated, which happened in about an hour, and might have
been done sooner, had I not wished the heat not much to exceed
that of boiling water. As the air in the flask was heavier
than the steam, the latter ascended to the top, and expelled
the air through the tube. When the water was all evaporated,
the oven and flask were removed from the fire, and a blast of
cold air was directed against one side of the flask, to
collect the condensed steam in one place. When all was cold,
the tube was removed, the flask and its contents were weighed
with care; and the flask being made hot, it was dried by
blowing into it by bellows, and when weighed again was found
to have lost rather more than four grains, estimated at 4-1/3
grains. When the flask was filled with water, it was found to
contain about 17-1/8 ounces avoirdupois of that fluid which
gave about 1800 for the expansion of water converted into
steam of the heat of boiling water.
"This experiment was repeated with nearly the same result, and
in order to ascertain whether the flask had been wholly filled
with steam, a similar quantity of water was for the third time
evaporated; and, while the flask was still cold, it was placed
inverted with its mouth (contracted by the tube) immersed in a
vessel of water, which it sucked in as it cooled, until in the
temperature of the atmosphere it was filled to within half an
ounce measure of water.
"In repetitions of this experiment at a later date, I
simplified the apparatus by omitting the tube, and laying the
flask upon its side in the oven, partly closing its mouth by a
cork, having a notch on one side, and otherwise proceeding as
has been mentioned."
[Pg097]
[Illustration: GLASGOW.]
CHAP. IV.
EXPOSITION OF PHYSICAL PRINCIPLES. — THERMOMETER. — METHOD OF
GRADUATING IT. — FREEZING AND BOILING POINTS. — LATENT HEAT OF
WATER. — QUANTITY OF HEAT NECESSARY TO CONVERT ICE INTO WATER.
— QUANTITY OF HEAT GIVEN OUT BY WATER IN BEING CONVERTED INTO
ICE. — PROCESS OF BOILING. — OF RECONVERSION OF STEAM INTO
WATER. — QUANTITY OF HEAT NECESSARY TO CONVERT WATER INTO
STEAM. — BOILING POINT OF WATER. — DIFFERENT IN DIFFERENT
PLACES. — DEPENDS ON THE BAROMETER. — VARIES WITH THE
PRESSURE. — EXPERIMENTAL PROOF OF THIS. — BOILS AT LOWER
TEMPERATURES THAN 212° UNDER PRESSURES LESS THAN THE
ATMOSPHERE. — SUM OF LATENT AND SENSIBLE HEAT OF STEAM ALWAYS
THE SAME. — THE FUEL NECESSARY TO EVAPORATE WATER THE SAME,
WHATEVER BE THE TEMPERATURE OR PRESSURE AT WHICH IT IS
EVAPORATED. — MECHANICAL FORCE OBTAINED BY EVAPORATION. — THIS
FORCE NEARLY THE SAME UNDER ALL CIRCUMSTANCES.
(51.) We shall pause here to put the reader in possession of the
physical and mechanical principles connected with the evaporation
of water and other liquids, which are necessary to enable him to
understand the full extent of the value and the merit of the
discoveries of Watt, and to comprehend the [Pg098] structure and
operation of the steam engine in its improved form, as it has
passed to us from his hands.
As we shall frequently have occasion to refer to the indications
of a thermometer, we shall first explain the principle of that
instrument as it is commonly used in this country.
The thermometer is an instrument used for the purpose of measuring
and indicating the temperature or sensible heat of material
substances.
Heat, like all other physical agents, can only be measured by its
effects. One of these effects best suited for this purpose, is the
change of dimension which all bodies undergo in consequence of
their change of temperature. In general, when heat is applied to a
material substance, that substance undergoes an enlargement of
bulk; and if heat be abstracted from it, it suffers a diminution
of bulk. This variation of magnitude is not always in the same
proportion as the increase or diminution of temperature; but it is
so when applied to certain substances and between certain limits.
One of the substances whose expansion and contraction through an
extensive range of temperature has been found to be nearly
uniform, and which is attended with other convenient qualities for
a thermometer, is the liquid called _mercury_ or _quicksilver_. A
mercurial thermometer is constructed in the following way:—
A glass tube is made with a small and uniform bore: upon the end
of this tube, a bulb is blown, having a magnitude very great
compared with the bore of the tube. Let us suppose this bulb and a
part of the tube to be filled with mercury. If the mercury
contained in the bulb be heated, it will expand, and being more
susceptible of expansion than the glass which contains it, the
bulb will be too small for its augmented volume: the mercury in
the bulb can only, therefore, obtain room for its increased bulk
by pressing the mercury in the tube upwards, which it will
accordingly do. The increase of volume which the mercury in the
bulb therefore undergoes, will be exhibited by the increased
length of the column in the tube. Since the bore of the tube is
made so exceedingly minute compared with the magnitude of the
bulb, a very small quantity of mercury forced [Pg099] from the
bulb into the tube, will cause a considerable increase of the
length of the column. Small degrees of expansion will therefore be
rendered very apparent, and may be accurately measured. The
following is the method by which the thermometer called
_Fahrenheit's thermometer_ is graduated.
The tube and bulb being prepared and supplied with mercury, as
already explained, let the instrument be plunged in a vessel of
melting ice. It will be found that the mercury will stand in the
tube at a certain point, from which it will not vary so long as
any ice remains not completely melted in the vessel. Let a mark be
made on the tube, or on a scale attached to the tube, at the point
corresponding to the top of the column: the point thus marked is
called the _freezing point_.