WATT. 37 



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 necessarily 

 took place on its condensation, had the effect, together 

 with the absorption of the steam's sensible heat, of con- 

 verting a portion of the injected water itself into steam. 

 Hence the vacuum was very far from perfect; and 

 the resistance which the piston thus met with in its 

 descent was found to be equal to one-fourth part of 

 the atmospheric pressure, that is to say, the working 

 power of the machine was diminished one-fourth. 



From the distinct view thus obtained of the evil 

 arose the suggestion of the remedy. The whole mis- 

 chief proceeded from the condensation being performed 

 in the cylinder, where the steam was thrown and the 

 piston worked. It occurred to Watt, that if the con- 

 densation could be performed in a separate vessel, com- 

 municating with the cylinder, the latter could be kept 

 hot while the former was cooled, and the vapour aris- 

 ing from the injected water could also be prevented 



