MOTIVE POWER OF HEAT. Ill 



They will be numberless. The motion produced 

 by the dilatation and compression of solid or liquid 

 bodies would only be very slight. In order to give 

 them sufficient amplitude we should be forced to 

 make use of complicated mechanisms. It would 

 be necessary to employ materials of the greatest 

 strength to transmit enormous pressure ; finally, 

 the successive operations would be executed very 

 slowly compared to those of the ordinary steam- 

 engine, so that apparatus of large dimensions and 

 heavy cost would produce but very ordinary re- 

 sults. 



The elastic fluids, gases or vapors, are the means 

 really adapted to the development of the motive 

 power of heat. They combine all the conditions 

 necessary to fulfil this office. They are easy to 

 compress ; they can be almost infinitely expanded ; 

 variations of volume occasion in them great 

 changes of temperature; and, lastly, they are very 

 mobile, easy to heat and to cool, easy to transport 

 from one place to another, which enables them to 

 produce rapidly the desired effects. We can easily 

 conceive a multitude of machines fitted to develop 

 the motive power of heat through the use of 

 elastic fluids ; but in whatever way we look at it, 

 we should not lose sight of the following prin- 

 ciples: 



