366 WAIT. 



been given up. In 1760 and the two following years 

 Watt had been in familiar intercourse with Professor 

 Black, had witnessed his experiments on heat, and 

 had learnt from him the true cause of evaporation and 

 condensation. When, therefore, he began to experi- 

 ment upon the mechanical application of steam, its 

 expansion, and its condensation, he enjoyed that ines- 

 timable advantage of thoroughly knowing the princi- 

 ples on which its changes and its action depended. His 

 own experiments now put him in possession of the 

 causes which determine the rapidity of evaporation, 

 the proportion which it bears to the surface exposed to 

 the fire, the effects of pressure upon the boiling-point, 

 and the quantity of fuel required to convert a given 

 quantity of water into steam circumstances which 

 had hitherto been only vaguely and generally examined, 

 but which he now reduced to mathematical precision. 



The first discovery which he made upon the atmo- 

 spheric engine and its waste of fuel, was that the in- 

 jection of cold water which condenses the steam also 

 cools the cylinder to a degree which requires a great 

 expenditure of fuel again to give it the necessary heat 

 for keeping the steam expanded to fill it. He found 

 that three-fourths of the fuel employed were thus con- 

 sumed ; in other words, that if the cylinder could be 

 kept at the temperature which it has before the jet is 

 thrown in, one-fourth of the fuel would suffice for the 

 operation. 



The next defect of the process was scarcely less im- 

 portant. The water injected, coming in contact with 

 the steam, was itself heated ; the evolution of the 

 latent heat, which Black's discovery showed Watt 



