i 3 6 THE POPULAR SCIENCE MONTHLY. 



mate 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, 1 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 fol- 

 lows at other temperatures. 



(5.) How much water, in the form of steam, was required, at 

 every stroke, by a small New T comen engine, with a wooden cylinder 

 six inches in diameter and twelve inches stroke. 



(6.) The quantity of cold water required, at every stroke, to con- 

 dense 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 de- 



1 Robinson's " Mechanical Philosophy," edited by Brewster. 



