MOTIVE POWER OF HEAT, 121 



latter out into the atmosphere, and there would be 

 also the advantage of avoiding the use of a refrig- 

 erant, which is not always available, but it would be 

 requisite that the increase of the volume of the air 

 should not reduce its pressure below that_ of the 

 atmosphere. 



(5) One of the gravest inconveniences of steam 

 is that it cannot be used at high temperatures with- 

 out necessitating the use of vessels of extraordinary 

 strength. It is not so with air for which there ex- 

 ists no necessary relation between the elastic force 

 and the temperature. Air, then, would seem more 

 suitable than steam to realize the motive power of 

 falls of caloric from high temperatures. Perhaps 

 in low temperatures steam may be more conven- 

 ient. "We might conceive even the possibility of 

 making the same heat act successively upon air and 

 vapor of water. It would be only necessary that 

 the air should have, after its use, an elevated tem- 

 perature, and instead of throwing it out immedi- 

 ately into the atmosphere, to make it envelop a 

 steam-boiler, as if it issued directly from a 

 furnace. 



The use of atmospheric air for the development 

 of the motive power of heat presents in practice 

 very great, but perhaps not insurmountable, diffi- 

 culties. If we should succeed in overcoming them, 



