MOTIVE POWER OF HEAT. 47 



in some engines;* the atmosphere would not re- 

 ceive it. It does receive it under the actual con- 

 dition of things, only because it fulfils the office 

 of a vast condenser, because it is at a lower tem- 

 perature; otherwise it would soon become fully 

 charged, or rather would be already saturated, f 



* Certain engines at high pressure throw the steam out 

 iuto the atmosphere instead of the condenser. They are 

 used specially in places where it would be difficult to 

 procure a stream of cold water sufficient to produce 

 condensation. 



f The existence of water in the liquid state here 

 necessarily assumed, since without it the steam-engine 

 could not be fed, supposes the existence of a pressure 

 capable of preventing this water from vaporizing, con- 

 sequently of a pressure equal or superior to the tension 

 of vapor at that temperature. If such a pressure were 

 not exerted by the atmospheric air, there would be in- 

 stantly produced a quantity of steam sufficient to give 

 rise to that tension, and it would be necessary always 

 to overcome this pressure iu order to throw out the 

 steam from the engines into the new atmosphere. Now 

 this is evidently equivalent to overcoming the tension 

 which the steam retains after its condensation, as effected 

 by ordinary means. 



If a very high temperature existed at the surface of 

 our globe, as it seems certain that it exists in its interior, 

 all the waters of the ocean would be in a state of vapor 

 in the atmosphere, and no portion of it would be found 

 in a liquid state. 



