WILLIAM SIEMENS, F.K.S. 41 



by Fig. 8, Plate 5 ; whereas the power actually imparted to the 

 working piston is represented by the portion of the diagram 

 marked b b b, the corners of which are rounded off, in the ratio 

 produced by the two cranks working over right angles, and is 

 equivalent to 27'5 units, being about -Jth parts of the entire area. 

 The field b b b y z, immediately below the sectioned portion, 

 represents the back pressure on the working piston, or the power 

 required to force the air from the working cylinders back into the 

 displacing vessels. 



The theoretical effect produced by a Stirling's engine, is to 

 that of a perfect engine, as the units of heat expended in the 

 entire expansion to the units producing useful effect, or as 123 

 to 27-5. 



There remains to be considered the necessary mechanical loss 

 of heat, owing to the difference of temperature between the air 

 on entering the cool ends of the metallic plates (the regenerators), 

 and on returning from the same, which loss has- been proved, by 

 experiments hereafter to be explained, to amount to about 25 

 Fahr., or to f x 25 = 19 Fahr. if distributed upon the entire 

 quantity of air employed, because only three-quarters of the same 

 are actually heated each time. 



The real effect of a Stirling's engine, as compared with a perfect 

 engine, is therefore as 123 + 19 : 27'5 = 5'1G : 1, or it yields 



770 



the power of - r == 149 Ibs. lifted 1 foot high, for every water 

 O'lb 



unit of heat expended. 



It follows from the above, that the Stirling's engine con- 

 verts into dynamical effect about one-fifth of the heat supplied, 

 which is about equal to the performance of the best Cornish 

 engines. 



For calculating the actual power of a Stirling's engine of given 

 proportions, the formula assumes the following more simple 

 form : 



Ind - H - p - = x iQ which A = 10 and 



t - 1 1 the decrease of temperature, by the expansion of the air 

 from the greatest to the lowest pressure. 



The cause of the failure of Mr. Stirling's engine in practice 

 may apparently be traced, chiefly to insufficiency of heating 



