MOTIVE POWER OF HEAT. 103 



complete cycle of operations is measured by the 

 product of the volume of the vapor multiplied by 

 the difference between the tensions that it pos- 

 sesses at the temperature of the body A and at 

 that of the body B. As to the heat employed, 

 that is to say, transported from the body A to the 

 body B, it is evidently that which was necessary 

 to turn the water into vapor, disregarding always 

 the small quantity required to restore the tempera- 

 ture of the liquid water from that of B to that 

 of A. 



Suppose the temperature of the body A 100 de- 

 grees, and that of the body .Z? 99 degrees: the 

 difference of the tensions will be, according to the 

 table of M. Dalton, 26 millimetres of mercury or 

 O m .36 head of water. 



The volume of the vapor is 1700 times that of 

 the water. If we operate on one kilogram, that 

 will be 1700 litres, or l rac .700. 



Thus the value of the motive power developed 

 is the product 



1.700 X 0.36 =0.611 units, 



of the kind of which we have previously made use. 



The quantity of heat employed is the quantity 



required to turn into vapor water already heated to 



100. This quantity is found by experiment. "We 



