MOTIVE POWER OF HEAT. 99 



operation will be TI7 + ^T of the primitive vol- 

 ume ; this is a very slight increase, absolutely 

 speaking, but great relatively to the difference of 

 temperature between the bodies A and B. 



The motive power developed by the whole of 

 the two operations described (page 70) will be very 

 nearly proportional to the increase of volume and 

 to the difference between the two pressures exer- 

 cised by the air, when it is found at the tempera- 

 tures 0.001 and zero. 



This difference is, according to the law of M. 

 Gay-Lussac, ^Wo^o ^ * ne elastic force of the gas, 

 or very nearly ^g^VinF f * ne atmospheric pressure. 



The atmospheric pressure balances at 10.40 

 metres head of water ; wfop$ f this pressure 

 equals -g-^VoFo X 10 m .40 of head of water. 



As to the increase of volume, it is, by supposi- 

 tion, yj-g- + ^- T of the original volume, that is, of 

 the volume occupied by one kilogram of air at 

 zero, a volume equal to O mc .77, allowing for the 

 specific weight of the air. So then the product, 



will express the motive power developed. This 

 ] ower is estimated here in cubic metres of water 

 raised one metre, 



