MOTIVE POWER OF HEAT. 53 



ing on it a constant pressure until it is entirely 

 liquefied. The body B fills here the place of the 

 injection-water in ordinary engines, with this dif- 

 ference, that it condenses the vapor without 

 mingling with it, and without changing its own 

 temperature.* 



* We may perhaps wonder here that the body B being 

 at the same temperature as the steam is able to condense 

 it. Doubtless this is not strictly possible, but the slightest 

 difference of temperature will determine the condensation, 

 which suffices to establish the justice of our reasoning. It 

 is thus that, in the differential calculus, it is sufficient that 

 we can conceive the neglected quantities indefinitely re- 

 ducible in proportion to the quantities retained in the 

 equations, to make certain of the exact result. 



The body B condenses the steam without changing its 

 own temperature this results from our supposition. We 

 have admitted that this body may be maintained at a con- 

 stant temperature. We take away the caloric as the steam 

 furnishes it. This is the condition in which the metal of 

 the condenser is found when the liquefaction of the steam 

 is accomplished by applying cold water externally, as was 

 formerly done in several engines. Similarly, the water of 

 a reservoir can be maintained at a constant level if the 

 liquid flows out at one side as it flows in at the other. 



One could even conceive the bodies J.and B maintaining 

 the same temperature, although they might lose or gain 

 certain quantities of heat. If, for example, the body A 

 were a mass of steam ready to become liquid, and the body 



