VII 



GAS AND OIL ENGINES 



JUST at the time when the type of piston-and- 

 cylinder engine has thus been challenged, it has 

 chanced that a new motive power has been applied 

 to the old type of engine, through the medium of heated 

 gas. The idea of such utilization of a gas other than 

 water vapor is by no means new, but there have been 

 practical difficulties in the way of the construction of a 

 commercial engine to make use of the expansive power 

 of ordinary gases. 



The principle involved is based on the familiar fact 

 that a gas expands on being heated and contracts when 

 cool. Theoretically, then, all that is necessary is to heat 

 a portion of air confined in a cylinder, to secure the ad- 

 vantage of its expansion, precisely as the expansion of 

 steam is utilized, by thrusting forward a piston. Such 

 an apparatus constitutes a so-called " caloric" or hot- 

 air engine. As long ago as the year 1807 Sir G. Cayley 

 in England produced a motor of this type, in which the 

 heated air passed directly from the furnace to the 

 cylinder, where it did work while expanding until its 

 pressure was not greater than that of the atmosphere, 

 when it was discharged. The chief mechanical diffi- 

 culty encountered resulted from the necessity for the 

 employment of very high temperatures; and for a long 



