102 MOTIVE POWER OF HEAT. 



Let us suppose one kilogram of liquid water en- 

 closed in the cylindrical vessel abed (Fig. 4), be- 

 tween the bottom ab and the piston 

 cd. Let us suppose, also, the two 

 bodies A, B maintained each at a 

 constant temperature, that of A being 

 a very little above that of B. Let us 

 imagine now the following operations : 

 (1) Contact of the water with the 

 body A, movement of the piston from 

 the position cd to the position ef, for- 

 mation of steam at the temperature 

 of the body A to fill the vacuum pro- 

 duced by the extension of volume. We will sup- 

 pose the space abef large enough to contain all the 

 water in a state of vapor. 



(2) Removal of the body A, contact of the vapor 

 with the body B, precipitation of a part of this 

 vapor, diminution of its elastic force, return of 

 the piston from ef to ab, liquefaction of the rest of 

 the vapor through the effect of the pressure com- 

 bined with the contact of the body B. 



(3) Removal of the body B, fresh contact of 

 the water with the body A, return of the water 

 to the temperature of this body, renewal of the 

 former period, and so on. 



The quantity of motive power developed in a 



