HYDRAULICS. C 21J 



are more compressed than the atmosphere, a degree 

 of heat above 212 degrees is necessary for the 

 production of steam. 



When the vacuum is produced by throwing cold 

 water into the cylinder to condense the steam, that 

 water becomes hot ; and, being in a vessel partially 

 exhausted, it produces a steam, which in part re- 

 sists the pressure of the atmosphere upon the 

 piston, and lessens the power of the engine. Ano- 

 ther defect is the destruction of steam, which un- 

 avoidably happens upon attempting to fill a cold 

 cylinder with it ; for the injection -water, at the 

 same time that it condenses the steam, not only 

 cools the cylinder, but remains there until it be 

 forced out at the eduction-pipe by the steam which 

 is let in to fill the cylinder for the next stroke ; 

 and that steam will be condensed into water as 

 fast as it enters, until all the parts it comes in con- 

 tact with be nearly as hot as itself. 



Watt preserved an uniform heat in the cylinder 

 of his engines, by not suffering cold water to touch 

 it, and by protecting it from the air or other cold 

 bodies, by a surrounding case filled with the steam, 

 or with hot air or water, and by coating it over 

 with substances that transmit heat slowly. He 

 made his vacuum more perfect, by condensing the 

 steam in a separate vessel, called the condenser ; 

 which may be cooled at pleasure without cooling 

 the cylinder, either by injection of cold water, or 

 by surrounding the condenser with it ; and gene- 

 rally by both. He extracted the injection-water 

 and detached air from the cylinder or condenser, 

 by pumps which were wrought by the engine 

 itself. 



As the inside of the cylinder was, in the old 

 engine, exposed to the air at every stroke when 

 the piston descended, and was thus considerably 



