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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

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 44 grains. When the flask was filled with water, it was found to contain about 17 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

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

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.

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"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

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.'

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"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

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 mercury and tin than in water.

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"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:

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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

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