WATT. 367 



necessarily took place on its condensation, had the 

 effect, together with the absorption of the steam's 

 sensible heat, of converting 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 

 from impairing the vacuum. The communication 

 could easily be effected by a tube, and the water could 

 be pumped out. This is the first and the grand inven- 

 tion by which he at once saved three-fourths of the fuel, 

 and increased the power one-fourth, thus making every 

 pound of coal consumed produce five times the force 

 formerly obtained from it. But this was not all. He 

 found it expedient to remove the air from the upper 

 part of the cylinder, as it tended to diminish the heat. 

 In effecting this he was, secondly, led to open a com- 

 munication with the boiler, and introduce steam above 

 the piston while it descended, thus making the upper 

 chamber of the cylinder air-tight. The steam thus 

 acted in aiding the descent of the piston, instead 



