THE CONQUEST OF NATURE 



throughout the cycle than could be accomplished by 

 the action of a single cylinder. 



To understand this it must be recalled that when using 

 the expansive property of steam, the piston thrust could 

 not possibly be uniform, since the greatest pressure 

 exerted by the steam would be exerted at the moment 

 before it was shut off from the boiler, and its pressure 

 must then decrease progressively, as it exerts more 

 and more work upon the piston and becomes more 

 expanded, thus obviously retaining less elastic energy. 

 The operation of the fly-wheel largely compensates 

 this difference of pressure in practise, but it would be 

 obviously advantageous could the pressure be equalized; 

 and, as just stated, the compound engine tends to pro- 

 duce this result. 



The second, and perhaps the more important merit 

 of the compound engine is, that it is found in practise to 

 keep the cylinders at a more uniform temperature. A 

 moment's reflection makes it clear why this should be 

 the case, since in a single-cylinder engine the exhaust 

 connects with the cool condenser, whereas in the com- 

 pound engine the exhaust from the first cylinder con- 

 nects with the second cylinder at only slightly lower 

 temperature. 



In many modern engines a third cylinder and some- 

 times even a fourth is added, constituting what are 

 called respectively triple-expansion and quadruple- 

 expansion engines. The triple-expansion system is 

 very generally employed, especially where it is peculiarly 

 desirable to economize fuel, as, for example, in the case 

 of ships. 



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