26 W. J. M. Rankine on the Means of 



which latter mass of air, passing through the nozzle, lifts the 

 piston. 



Process C. — The plunger descends and forces the air below 

 it through the holes of the inner bottom, and through the 

 metal or glass plates of the economizer, which absorb, more or 

 less completely, the sensible heat of the ' air. This air, by 

 passing amongst the cold-water tubes, enters the space above 

 the plunger. Should it leave the economizer at a temperature 

 higher than that of the cold-water tubes, the latter abstract 

 an additional portion of its sensible heat. 



Process D. — The piston descends, compressing the whole 

 mass of air ; the heat so generated is abstracted by the cold- 

 water tubes. 



Process A. — The plunger partially rises, as before ; a por- 

 tion of air descends through the economizer, and recovers the 

 heat remaining stored up there. Should its temperature, on 

 leaving the economizer, not have attained its original elevation, 

 the additional sensible heat required is supplied from the fur- 

 nace through the bottom of the receiver. 



The cycle of processes is now finished, and may be repeated 

 ad infinitum. 



Thus it appears that the air confined in the receiver and cy- 

 linder of Stirling's air-engine consists of two portions : one por- 

 tion, which always remains above the plunger, and which serves 

 merely as a perfectly elastic cushion, to transmit pressure and 

 motion between the piston and the other portion of the air, and 

 not as a means of developing power ; and another portion of air, 

 which, being driven by the plunger to the bottom and top of 

 the receiver alternately, is successively heated, expanded, 

 cooled, and compressed ; and, as the expansion takes place at a 

 high temperature, and the compression at a low one, more power 

 is produced by the former than is consumed by the latter, and 

 thus there remains a surplus of available power for the en- 

 gine.* The existence of the cushion of air before-mentioned, 



* In calculating the space to be traversed by the piston of an air-engine, in 

 which part of the air acts as a cushion, allowance must be made for the space 

 through which this cushion-air expands and contracts, with the variation of 

 pressure, as well as for the space required for the changes of volume of the 

 workiny-air. The total space traversed is thus increased in a certain propor- 



