realizing the Advantages of the Air-Engine. 9 



the other part of the expenditure of heat, namely, the heat 

 which is expended in elevating the temperature of the working 

 substance, the more nearly shall we attain to the maximum 

 theoretical efficiency of the engine. It is theoretically possible 

 to produce the required elevation of temperature, without any 

 expenditure of heat. This is to be accomplished in two ways : — 

 either, by elevating the temperature of the substance by com- 

 pression during the process A of the cycle : the power re- 

 quired for effecting such compression being obtained, during 

 the process C, by depressing the temperature of the substance 

 entirely by expansion ; or otherwise, by storing up in a mass of 

 some solid conducting material (called an economizer or regene- 

 rator), the heat given out by the working substance, while its 

 temperature is being depressed, during the process C, and em- 

 ploying the heat, so stored up, to produce the required eleva- 

 tion of temperature during the process A. This method of 

 economizing heat was invented in 1816, by the Rev. Robert 

 Stirling. 



By one or other of those methods, it is theoretically possible 

 to limit the expenditure of heat in a thermo-dynamic engine to 

 that amount which disappears during the process B of the 

 cycle, in producing increase of volume at an elevated tempera- 

 ture. The heat which reappears during the process D, by the 

 compression of the working substance at a low temperature, 

 and which is carried away by refrigeration, constitutes the ne- 

 cessary loss of heat ; and, if this be deducted from the whole 

 heat expended, the remainder will be the theoretical maximum 

 value of the heat which is permanently converted into mecha- 

 nical power, and its ratio to the whole heat expended will be 

 the theoretical maximum efficiency of the engine. 



9. Absolute Temperatures. — The theoretical maximum 

 efficiency of a thermo-dynamic engine, depends upon what are 

 called the absolute temperatures of the working substance, 

 during the second and fourth processes of the cycle ; that is to 

 say, the absolute temperatures at which heat in a theoretically 

 perfect engine is received and abstracted respectively. Abso- 

 lute temperatures are measured by the product of the pressure 

 and volume of a given weight of a given perfect gas. A per- 

 fect gas is one in which the mutual attraction of the particles 

 is insensible. 



