realizing the Advantages of the Air-Engine. 29 



temperature is not, as it is in the steam-engine, of secon- 

 dary importance. If the temperature of the air in an air- 

 engine were elevated altogether by means of heat supplied 

 from the furnace, the waste from this cause would be from 

 three to four times greater than the latent heat of expansion 

 which performs the work, and the economy of the engine 

 would be entirely destroyed. Some persons, founding their 

 calculations upon this supposition, have pronounced the air- 

 engine to be necessarily wasteful and inefficient. 



The sensible heat in question might be entirely produced 

 by an additional compression of the air performed during the 

 process A, the power employed to effect such compression 

 being developed by an additional expansion performed during 

 the process C, in which the temperature of the air falls. To 

 afford room, however, for the additional expansion, the bulk 

 of the engine would have to be increased about five-fold, which 

 would render it inconvenient in practice, especially for propel- 

 ling ships. 



The process actually pursued in Stirling's engine, of storing 

 up the sensible heat by means of the economizer or regene- 

 rator, and using it over and over again, has already been 

 generally described. In the original engine of the Rev. Robert 

 Stirling, the regenerator consisted simply of the sides of the 

 receiver and plunger, the latter being covered with a network 

 of wires, in order to increase the surface ; in the engine, as 

 improved by Mr James Stirling, it is composed of thin parallel 

 plates of metal or glass. In Captain Ericsson's engine it con- 

 sists of several sheets of wire gauze. 



The efficacy of a regenerator to prevent expenditure of heat 

 in raising the temperature of the air increases with its mass 

 and surface ; but no amount of mass and surface, how large 

 soever, is sufficient to make it act with theoretical perfection. 

 There is reason to believe, however, that both in Stirling's and 

 in Ericsson's engines the masses and surfaces of the regenera- 

 tors were sufficient to reduce the waste of heat, in raising the 

 temperature of the air, to a very small quantity. 



Some persons, overlooking the latent heat of expansion — 

 the real source of power — appear at one time to have imagined 

 that a theoretically perfect regenerator would prevent all ex- 



