Page:Popular Science Monthly Volume 12.djvu/148

136 measures of temperatures less than 212°, and of pressures less than atmospheric.

He thus discovered that, with the amount of injection-water used in the Newcomen engine, bringing the temperature of the interior, as he found, down to from 140° to 175° Fahr., a very considerable back-pressure would be met with.

Continuing his research still further, he measured the amount of steam used at each stroke; and, comparing it with the quantity that would just fill the cylinder, he found that at least three-fourths was wasted.

The quantity of cold water necessary to produce condensation of a given weight of steam was next determined, and he found that one pound of steam contained enough heat to raise about six pounds of cold water, as used for condensation, from the temperature of 52 Fahr. to the boiling-point; and, going still further, he found that he was compelled to use, at each stroke of the Newcomen engine, four times as much injection-water as should suffice to condense a cylinder full of steam. Thus was confirmed his previous conclusion that three-fourths of the heat supplied to the engine was wasted.

His experiments having revealed to him the now well-known fact of the existence of latent heat, he went to his friend Dr. Black, of the university, with this intelligence; and the latter then informed him of the Theory of Latent Heat which had but a short time earlier been discovered by Dr. Black himself.

33. Watt had now, therefore, determined by his own researches, as he himself enumerates them, the following facts:

(1.) The capacities for heat of iron, copper, and of some sorts of wood, as compared with water.

(2.) The bulk of steam compared with that of water.

(3.) The quantity of water evaporated in a certain boiler by a pound of coal.

(4.) The elasticities of steam, at various temperatures greater than that of boiling water, and an approximation to the law which it follows at other temperatures.

(5.) How much water, in the form of steam, was required, at every stroke, by a small Newcomen engine, with a wooden cylinder six inches in diameter and twelve inches stroke.

(6.) The quantity of cold water required, at every stroke, to condense the steam in that cylinder, so as to give it a working power of about seven pounds on the square inch.

34. After these well-devised and truly scientific investigations, Watt was enabled to enter upon his work of improving the steam-engine with an intelligent understanding of its existing defects, and with a knowledge of their cause.

It was on a Sunday afternoon, in the spring of 1765, that he