MOTIVE POWER OF HEAT. 113 



density, which requires also that the gas be first 

 taken under a very heavy pressure, or that it 

 acquire by its dilatation an enormous volume 

 conditions both difficult to fulfil. The first neces- 

 sitates the employment of very strong vessels to 

 contain the gas at a very high temperature and 

 under very heavy pressure. The second necessi- 

 tates the use of vessels of large dimensions. These 

 are, in a word, the principal obstacles which pre- 

 vent the utilization in steam-engines of a great 

 part of the motive power of the heat. We are 

 obliged to limit ourselves to the use of a slight fall 

 of caloric, while the combustion of the coal fur- 

 nishes the means of procuring a very great one. 



It is seldom that in steam-engines the elastic 

 fluid is produced under a higher pressure than six 

 atmospheres a pressure corresponding to about 

 160 Centigrade, and it is seldom that condensa- 

 tion takes place at a temperature much under 40. 

 The fall of caloric from 160 to 40 is 120, while 

 by combustion we can procure a fall of 1000 to 

 2000. 



In order to comprehend this more clearly, let us 

 recall what we have termed the fall of caloric. 

 This is the passage of the heat from one body, A, 

 having an elevated temperature, to another, B, 

 where it is lower. We say that the fall of the 



