THE STEAM-ENGINE. 



495 



to be consumed. It must be such that it shall supply sufficient oxygen to the 

 fuel to maintain the combustion, and not more than sufficient, since any excess 

 would be attended with the effect of absorbing the heat of combustion, without 

 contributing to the maintenance of that effect. 



Heat is communicated from body to body in two ways, by radiation and by 

 contact. 



Rays of heat issue from a heated body, and are dispersed through the sur- 

 rounding space in a manner, and according to laws, similar to those which 

 govern the radiation of light. The heat thus radiated meeting other bodies is 

 imparted to them, and penetrates them with more or less facility according to 

 their physical qualities. 



A heated body also brought into contact with another body of lower tem- 

 perature, communicates heat to that other body, and will continue to do so 

 until the temperature of the two bodies in contact shall be equalized. Heat 

 proceeds from fuel in a state of combustion in both these ways : the heated 

 fuel radiates heat in all directions around it, and the heat thus radiated will be 

 imparted to all parts of the furnace which are exposed to the fuel. 



The gases, which are the products of the combustion, escape from the fuel 

 at a very high temperature, and consequently, in acquiring that temperature 

 they absorb a considerable quantity of the heat of combustion. But besides 

 the gases actually formed in the process of combustion, the azote forming four 

 fifths of the air carried through the fuel to support the combustion, absorbs heat 

 from the combustible, and rises into the upper part of the furnace at a high 

 temperature. These various gases, if conducted directly to the chimney, would 



Fig. 52. 



