392 f be Steam [Book VII. 



the fnifting valve ; then by turning the key of the in- 

 jection-cock, a jet of cold water is admitted into the 

 cylinder, which condenfes the fleam and makes a va- 

 cuum ; and the atmofphere then preffing upon the pif- 

 ton, forces it down to the lower part of the cylinder, 

 and makes a ftroke by railing the column of water at 

 the other end of the beam. After two or three ftrokes 

 are made in this manner, by a man opening and 

 fhutting the cocks to try if they are right, then the 

 pins may be put into the pin-holes in the working 

 plug, and the engine left to turn the cocks of itfelf, 

 which it will do with greater exactnefs than any 

 man. 



Many important improvements have been lately 

 made in the fleam engine by the ingenious Mr. Watt, 

 of Birmingham. Pie has contrived to preferve an 

 uniform heat in the cylinder of his engines, by fuf- 

 fering no cold water to touch it, and by protecting 

 it from the air, fir ether cold bodies, by a furrounding 

 cafe filled with fteam, or with hot air or water, arid 

 by coating it over with fubftances that tranfmit heat 

 (lowly. He makes his vacuum to approach nearly 

 to that of the barometer, by condenfing the fteam in 

 a feparate vefiel, called the condenfer, which may be 

 cooled at pleafure without cooling the cylinder, either 

 by an injection of cold water, or by furrounding the 

 condenfer. with it, and generally by both. He ex- 

 tracts the injection water and detached air from the 

 cylinder or condenfer by pumps, which are wrought 

 by the engine itfelf, or blows them out by the fteam. 

 As the entrance of air into the cylinder would ftop 

 the operation of die engines, and as it is hardly to 

 be expected that fucji enormous piftons as thofe of 

 fteam engines can move up and down, and yet be ab- 

 ibJutdy tight in the common engines, a dream of 



water 



