226

SMEATON'S EXPERIMENTS.

using a larger quantity of injection water; but then the cylinder by this means was cooled so much, as to require steam to heat it again, in quantity out of proportion to the power gained by the more perfect condensation: in ordinary cases it required a quantity of steam equal to several times the capacity of the cylinder to heat and fill it every stroke; he saw that it was in consequence of these effects that the old engineers did not reduce the internal heat of their cylinders below 142° or 174°, by which they could not place more than six or seven pounds weight on each inch square area of their pistons*. Suspecting that there was some

The same things had occurred to Smeaton in an engine he erected for the New River Company, in 1767.

"He considered, in calculating its proportions, that the stoppage of the water at every stroke, as well as putting the lever-beam, piston, heavy-rods, and chains from a state of rest into motion twice at every stroke, was a great loss of power in the common engines; he therefore determined to work the engine slower, and with larger pumps, and to put upon the piston all the load it would bear. To reduce the velocity of the column of water still more, he would place the fulcrum of the beam out of the centre, and make the stroke of the piston nine feet, whilst the pump which lifted thirty-six feet, should work with only a six feet stroke. This arrangement obliged him to employ a long narrow cylinder of only eighteen inches diameter, and from this he also expected to obtain other advantages, viz., that every part of the steam, being nearer the surface of the cylinder, would be more readily condensed, and in consequence that a less quantity of injection-water would serve the cylinder, which would itself be more heated. He therefore placed a load of ten and one-third pounds on each inch on the piston, and having once seen a common engine struggle under this burden, he thought himself quite secure under those advantages; but how great was his surprise and mortification to find, that instead of requiring less injection-water, it required more, and that two men were obliged to assist to raise additional injection-water by hand, to keep the engine in motion at the same time that the cylinder was so cold, that he could keep his hand upon it, and bear it for a length of time in the hot well. He tried many experiments, without any good effect,

WATT'S EXPERIMENTS.

227

error in the usual estimate of the expansion of water into steam, he ascertained by a few simple experiments, that a measure of water is dilated into eighteen hundred measures of steam, capable of resisting the pressure of the atmosphere*; and that when water boils under pressures greater than that of the atmosphere, if the temperatures proceeded in an arithmetical ratio, the expansive power of the vapour increased in some geometrical one.

During these experiments, his curiosity had been excited by the great quantity of injectionwater required to condense the steam; and also by the great heat the water acquired by the condenuntil he altered the fulcrum of the beam so much as to re duce the load on the piston to eight pounds and a quarter: though this shortened the stroke at the pump, and the engine went so much quicker, as to raise more water, consume less coals, take less injection-water, the cylinder became hot, and the injection-water came out at 180° Fahr., and the engine did its work better and more pleasantly. This at once convinced me," says he, " that a considerable degree of condensation of the steam took place in the cylinder, and that I had lost more this way by the coldness of the cylinder, than I had gained by the increase of load. In short, this single alteration seemed to have unfettered the engine."Rees's Cyclop. vol. xxxiv. Art. STEAM ENGINE.

Smeaton weighed a Florence flask of four inches diameter, first when it was perfectly dry and empty, and afterwards when it was filled with water; then pouring out all the water except a small quantity, he put the flask on the fire, and made it boil until the last drop had disappeared, and at that instant he closed the mouth of the flask, to retain the steam which was in it. He weighed the flask to ascer tain the difference of weight between it when filled with water, and by steam equal to the pressure of the atmo sphere; deducting the weight of the flask from each experiment, it gave the proportion of the quantity of water converted into steam, and the content of the flask gave its expansion. His first trial made it ; but, suspecting a source of error, he reduced his estimate to T.-Ibid. Art. STEAM ENGINE.

228

DOCTOR BLACK.

sation. By means of a glass tube inserted into the spout of a tea-kettle, he allowed the steam to flow into a glass filled with cold water, until it was boiling hot. The water was then found to have gained nearly a sixth part by the steam which had been condensed to heat it, and he drew the conclusion, that a measure of water converted into steam can raise about six measures of water to its own heat, or eighteen hundred measures of steam can heat six measures of water. "Being struck," says he, "with this remarkable fact, and not understanding the reason of it, I mentioned it to my friend Dr. Black*, who then explained to me his doctrine of latent heat, which he had taught some time before this period, (summer of 1764); but having been occupied with the pursuits of business, if I had heard of it I had not attended to it, when I thus stumbled upon one of the material facts by which that beautiful theory is supported."

These interesting pursuits were not, however, those in which he was at this moment most deeply interested. The influence of a tender attachment drew him from retirement, and for a time from philosophy. A single life, and a college room,

"Dr. Robison," says Mr. Watt, "qualifies me as the pupil and intimate friend of Dr. Black, and goes the length of supposing me to have professed to owe my improvements upon the steam-engine to the instruction and information I had received from him, which certainly was a misapprehension; he is also mistaken in his assertion, that I had attended two courses of the Doctor's lectures. Unfortunately for me, the necessary avocations of my business prevented me from attending his, or any other lectures at college." "The 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 at heats below 1000, and that a vacuum could not be obtained, unless the cylinder and its contents were cooled every stroke to below that heat."-Robison's Mech. Philos., vol. ii., p. 6.

were now exchanged for the society of his maternal cousin, and a shop in the salt-market; and his wife being the daughter of a freeman, the opposition he experienced at his outset was also at an end. His civility and attention gained him friends, and his business increasing, he was soon in a situation to require the labours of an assistant and apprentice.