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



Section III. — On the Actual Efficiency of Air-Engines. 



17. As the object of this paper, in referring to the actual 

 performance of previous air-engines, is to illustrate the waste 

 by which that performance falls short of the theoretical maxi- 

 mum, I shall refer to those engines only which have actually 

 been at work, and the details of whose performance have been 

 made public with tolerable precision, namely, the engine of 

 the Messrs Stirling, and that of Captain Ericsson, which latter 

 was used for marine propulsion about the year 1852. 



18. Stirling's Air-Engine. — In describing generally the 

 air-engine which was invented by the Rev. Robert Stirling 

 in the year 1816, and improved by him and Mr James Stir- 

 ling at subsequent periods, it will be sufficient to speak as of 

 a single-acting engine only ; a double-acting engine having 

 simply a similar apparatus for each side of the piston. 



Suppose a cylindrical cast-iron air-receiver, of sufficient 

 strength to be safe with a working pressure of sixteen atmo- 

 spheres, with a convex hemispherical bottom, and a concave 

 hemispherical top, to be placed in a vertical position over a 

 flue connected with a furnace, but screened from the radiant 

 heat ; the hemispherical bottom of this receiver constitutes the 

 surface for the reception of heat ; I believe it was 3 inches 

 thick in the engine last erected. Within this vertical receiver 

 there is a hollow metal plunger, filled with some non-conduct- 

 ing substance, and capable of being moved up and down by 

 means of a rod. This plunger is of precisely the same form 

 with the receiver, but considerably less in height, and some- 

 what less in diameter. The effect of raising this plunger is to 

 displace the air from the upper part of the receiver, and to 

 send it down to the bottom, where it is exposed to heat ; the 

 air passing through the space between the plunger and the 

 sides of the receiver : the effect of lowering the plunger is to 

 cause the air to return to the top of the receiver. In the in- 

 terior of the uppermost part of the receiver is a coil of 

 small tubes, in which cold water is made to circulate, and 

 amongst which the air must pass whenever it is displaced. 

 Lower down, and occupying the annular space between the 



