MOTIVE POWER OF HEAT. 191 



Hence the total quantity added is equal to 



But, since B denotes the specific heat under con- 

 stant pressure, the quantity of heat requisite to 

 bring the gas into this state, from its primitive 

 condition, is equal to Br\ and hence we have 



IV. Comparison of the Relative Advantages of the 

 Air-engine and Steam-engine. 



54. In the use of water-wheels for motive power, 

 the economy of the engine depends not only upon 

 the excellence of its adaptation for actually trans- 

 mitting any given quantity of water through it, 

 and producing the equivalent of work, but upon 

 turning to account the entire available fall; so, as 

 we are taught by Carnot, the object of a thermo- 

 dynamic engine is to economize in the best possible 

 way the transference of all the heat evolved, from 

 bodies at the temperature of the source, to bodies 

 at the lowest temperature at which the herat can be 

 discharged. With reference, then, to any engine of 

 the kind, there will be two points to be considered: 



(1) The extent of ihefall utilized. 



