MOTIVE POWER OF HEAT. 95 



ence is the same in both cases, as any one can 

 prove by simple reasoning, which it seems un- 

 necessary to give here in detail ; hence the motive 

 power produced is the same. 



Let us now compare the quantities of heat em- 

 ployed in the two cases. In the first, the quantity 

 of heat employed is that which the body A fur- 

 nishes to the air to maintain it at the temperature 

 of 100 during its expansion. In the second, it is 

 the quantity of heat which this same body should 

 furnish to it, to keep its temperature at one degree 

 during an exactly similar change of volume. If 

 these two quantities of heat were equal, there 

 would evidently result the law that we have already 

 assumed. But nothing proves that it is so, and we 

 shall find that these quantities are not equal. 



The air that we shall first consider as occupying 

 the space abed (Fig. 2), and having 1 degree of 

 temperature, can be made to occupy the space abef, 

 and to acquire the temperature of 100 degrees by 

 two different means: 



(1) We may heat it without changing its vol- 

 ume, then expand it, keeping its temperature 

 constant. 



(2) We may begin by expanding it, maintaining 

 the temperature constant, then heat it, when it 

 has acquired its greater volume. 



