MOTIVE POWER OF HEAT. 65 



to the position ef ; the temperature remains un- 

 changed. 



(7) The step described under number 3 is re- 

 newed, then successively the steps 4, 5, 6, 3, 4, 5, 

 6, 3, 4, 5 ; and so on. 



In these various operations the piston is subject 

 to an effort of greater or less magnitude, exerted 

 by the air enclosed in the cylinder; the elastic 

 force of this air varies as much by reason of the 

 changes in volume as of changes of temperature. 

 But it should -be remarked that with equal 

 volumes, that is, for the similar positions of the 

 piston, the temperature is higher during the move- 

 ments of dilatation than during the movements of 

 compression. During the former the elastic force 

 of the air is found to be greater, and consequently 

 the quantity of motive power produced by the 

 movements of dilatation is more considerable than 

 that consumed to produce the movements of com- 

 pression.. Thus we should obtain an excess of 

 motive power an excess which we could employ 

 for any purpose whatever. The air, then, has 

 served as a heat-engine ; we have, in fact, employed 

 it in the most advantageous manner possible, for 

 no useless re-establishment of equilibrium has 

 been effected in the caloric. 



All the above-described operations may be 



