52 MOTIVE POWER OF HEAT. 



two bodies A and B, kept each at a constant tem- 

 perature, that of A being higher than that of B. 

 These two bodies, to which we can give or from 

 which we can remove the heat without causing 

 their temperatures to vary, exercise the functions 

 of two unlimited reservoirs of caloric. We will 

 call the first the furnace and the second the re- 

 frigerator. 



If we wish to produce motive power by carrying 

 a certain quantity of heat from the body A to the 

 body B we shall proceed as follows : 



(1) To borrow caloric from the body A to make 

 steam with it that is, to make this body fulfil 

 the function of a furnace, or rather of the metal 

 composing the boiler in ordinary engines we here 

 assume that the steam is produced at the same 

 temperature as the body A. 



(2) The steam having been received in a space 

 capable of expansion, such as a cylinder furnished 

 with a piston, to increase the volume of this space, 

 and consequently also that of the steam. Thus rare- 

 fied, the temperature will fall spontaneously, as 

 occurs with all elastic fluids ; admit that the rare- 

 faction may be continued to the point where the 

 temperature becomes precisely that of the body B. 



(3) To condense the steam by putting it in con- 

 tact with the body B, and at the same time exert- 



