MOTIVE POWER OF HEAT. 97 



Thus, for example, more caloric is necessary to 

 maintain at 100 the temperature of a certain 

 quantity of air the volume of which is doubled, 

 than to maintain at 1 the temperature of this 

 same air during a dilatation exactly equal. 



These unequal quantities of heat would produce, 

 however, as we have seen, equal quantities of 

 motive power for equal fall of caloric taken at dif- 

 ferent heights on the thermometric scale; whence 

 we draw the following conclusion : 



The fall of caloric produces more motive power at 

 inferior than at superior temperatures. 



Thus a given quantity of heat will develop more 

 motive power in passing from a body kept at 1 

 degree to another maintained at zero, than if these 

 two bodies were at the temperature of 101 and 

 100. 



The difference, however, should be very slight. 

 It would be nothing if the capacity of the air for 

 heat remained constant, in spite of changes of 

 density. According to the experiments of MM. 

 Delaroche and Berard, this capacity varies little 

 so little even, that the differences noticed might 

 strictly have been attributed to errors of observa- 

 tion or to some circumstances of which we have 

 failed to take account. 



We are not prepared to determine precisely, 



