112 MOTIVE POWER OF HEAT. 



(1) The temperature of the fluid should be made 

 as high as possible, in order to obtain a great fall 

 of caloric,, and consequently a large production of 

 motive power. 



(2) For the same reason the cooling should be 

 carried as far as possible. 



(3) It should be so arranged that the passage 

 of the elastic fluid from the highest to the lowest 

 temperature should be due to increase of volume; 

 that is, it should be so arranged that the cooling of 

 the gas should occur spontaneously as the effect of 

 rarefaction. The limits of the temperature to 

 which it is possible to bring the fluid primarily,, 

 are simply the limits of the temperature obtainable 

 by combustion ; they are very high. 



The limits of cooling are found in the tempera- 

 ture of the coldest body of which we can easily and 

 freely make use ; this body is usually the water of 

 the locality. 



As to the third condition, it involves difficulties 

 in the realization of the motive power of heat 

 when the attempt is made to take advantage of 

 great differences of temperature, to utilize great 

 falls of heat. In short, it is necessary then that 

 the gas, by reason of its rarefaction, should pass 

 from a very high temperature to a very low one, 

 which requires a great change of volume and of 



