MOTIVE POWER OF HEAT. 157 



Taking the mean of the pressures at the beginning 

 and end of each operation, we find 



(4) j> - ^ G*, 



which, as we are neglecting infinitely small quan- 

 tities of the second order, will be the expressions 

 for the mean pressures during the four successive 

 operations. Now, the mechanical effect gained or 

 spent, during any of the operations, will be found 

 by multiplying the mean pressure by the increase 

 or diminution of volume which takes place; and 

 we thus find 



(4) (p - \ 



