THE CONQUEST OF NATURE 



ing and cooling it. A six horse-power engine of this 

 type is said to have a cylinder four and one-half inches 

 in diameter and a stroke of four and seven-eighth inches, 

 and makes four hundred and fifty revolutions per min- 

 ute. The heater is twenty inches in diameter, sixteen 

 inches long, and has a heating surface of sixty square 

 feet. The total weight of heater and engine complete 

 is four hundred pounds for a half horse-power Ericsson 

 engine. 



"The Svea heater," says Mr. Hesse, "absorbs the 

 heat as perfectly as an ordinary steam boiler, and the 

 heat-radiating surface of both heater and engine is not 

 larger than that of a steam plant of the same power, 

 thereby placing the two motors on the same basis, as far 

 as the utilization of the heat in the fuel itself is concerned. 



"The advantage which every hot-air engine has over 

 the steam engine is the amount of heat saved in the va- 

 porization of the water. It is now well known that one 

 gas is as efficient as another for the conversion of heat 

 into power. Air and steam at 100 C. are consequently 

 on the same footing and ready to be superheated. The 

 amount of heat required to bring the two gases to this 

 temperature is, however, very different. 



"With an initial temperature of 10 C. for both air 

 and water, we find that one kilogram of steam requires 

 9 + 537 = 627 thermal units, and one kilogram of air 

 0.24 X 90 = 21.6 thermal units. Some heat is re- 

 covered if the feed water is heated and the steam con- 

 densed, but the difference is still so great as to altogether 

 exclude steam as a competitor, provided air can be as 

 readily handled. 



