MOTIVE POWER OF HEAT. 119 



atmosphere. It is evident that this mode of work- 

 ing is fully equivalent, in respect to the motive 

 power produced, to condensing the steam at 100, 

 and that a portion of the useful effect is lost. But 

 the engines working thus dispense with condenser 

 and air-pump. They are less costly than the 

 others, less complicated, occupy less space, and can 

 be used in places where there is not sufficient water 

 for condensation. In such places they are of in- 

 estimable advantage, since no others could take 

 their place. These engines are principally em- 

 ployed in England to move coal-wagons on rail- 

 roads laid either in the interior of mines or outside 

 of them. 



We have, further, only a few remarks to make 

 upon the use of permanent gases and other vapors 

 than that of water in the development of the mo- 

 tive power of heat. 



Various attempts have been made to produce 

 motive power by the action of heat on atmospheric 

 air. This gas presents, as compared with vapor of 

 water, both advantages and disadvantages, which 

 we will proceed to examine. 



(1) It presents, as compared with vapor of water, 

 a notable advantage in that, having for equal vol- 

 ume a much less capacity for heat, it would cool 

 more rapidly by an equal increase of vohime. 



