MOTIVE POWER OF HEAT. 59 



the water to the original temperature. This can 

 undoubtedly be done by at once putting it again 

 in contact with the body A ; but there is then 

 contact between bodies of different temperatures, 

 and loss of motive power.* It would be impossi- 

 ble to execute the inverse operation, that is, to 

 return to the body A the caloric employed to raise 

 the temperature of the liquid. 



This difficulty may be removed by supposing the 

 difference of temperature between the body A and 

 the body B indefinitely small. The quantity of 

 heat necessary to raise the liquid to its former 



* This kind of loss is found in all steam-engines. In 

 fact, the water destined to feed the boiler is always cooler 

 than the water which it already contains. There occurs 

 between them a useless re-establishment of equilibrium of 

 caloric. We are easily convinced, a posteriori, that this re- 

 establishment of equilibrium causes a loss of motive power 

 if we reflect that it would have been possible to previously 

 heat the feed-water by using it as condensing-water in a 

 small accessory engine, when the steam drawn from the 

 large boiler might have been used, and where the conden- 

 sation might be produced at. a temperature intermediate 

 between that of the boiler and that of the principal con- 

 denser. The power produced by the small engine would 

 have cost no loss of heat, since all that which had been 

 used would have returned into the boiler with the water of 

 condensation. 



