x PRACTICAL PURPOSE 263 



as steam was an elastic body, it would rush into a vacuum, and 

 if a communication were made between the cylinder and an 

 exhausted vessel, it would rush into it, and might be there con- 

 densed without cooling the cylinder. James Watt. 



It must not be supposed, however, that Watt's 

 improvements of Newcomen's engine were due only to 

 a happy thought put into action. He approached the 

 question of perfecting the steam-engine as a scientific 

 problem, and under the inspiration of the principle of 

 " latent heat " discovered by Joseph Black, his friend 

 and guide in the many researches. Black, who was a 

 professor in the University of Glasgow, measured the 

 amount of heat required to transform ice into water 

 and water into steam, and found that a great quantity of 

 heat was used up in each case without producing any rise 

 of temperature. Conversely, when steam is condensed 

 into water, or water is frozen into ice, the change of 

 state involves the liberation of heat. A pound of steam, 

 for example, contains five hundred times more heat 

 than a pound of boiling water. 



Watt discovered, by numerous experiments, that the 

 quantity of heat thus contained in steam was nearly 

 constant for different pressures within the ranges used 

 in steam-engines a conclusion which showed that 

 decided advantage could be gained by using steam at 

 high pressure. He proved also by experiment that, 

 with the Newcomen engine, three-quarters of the heat 

 of the steam was lost in making the cylinder hot at each 

 stroke, and he aimed at lessening this waste of steam 

 and consequently of fuel, with the result that he gave 

 to the world an engine which consumed per horse-power 

 only one- quarter of the fuel previously used by any 

 engine. Though now the consumption of coal per horse- 

 power is only from one-third to one-quarter what it was 



