243 



(4.) Comparison of the Relative Advantages of the Steam-Engine 

 and Air-Engine. 



In the steam-engine, with the expansive principle pushed to the 

 utmost, as Carnot points out, the effective range of temperature, 

 or the fall utilised, is from the temperature of the boiler to that of 

 the condenser. The superior limit of temperature is restricted by 

 the circumstance, that the pressure of saturated steam is enormously 

 great for high temperatures ; so that in practice, the temperature in 

 the boiler is not in any ordinary engines so high as 150° per cent., but 

 is in general very much below this limit. Carnot points out, that 

 in this respect, the air-engine has a vast advantage over the steam- 

 engine ; as there is no limit to the temperature in the hot part, ex- 

 cept such as the preservation of the materials requires ; and, there- 

 fore, in it an enormously greater portion of the whole fall, from the 

 temperature of the coals to that of the atmosphere, may be made 

 use of. In other respects, we have no reason a priori for giving a 

 preference to one kind of engine above the other. We cannot, how- 

 ever, feel confident that any air-engine has yet been constructed, 

 which is capable of economising the fall actually used, as well as is 

 done by steam-engines, with their comparatively limited range of 

 temperature, or even that the duty for fuel consumed has in any 

 actual air-engine exceeded or even come up to the duty performed 

 by the best steam-engines. 



(5.) On the Economy of Actual Steam-Engines. 



The following table affords a synoptic view of the performances 

 and theoretical duties, in various actual cases.* 



When heat is transmitted from a body at 140°,f through an 

 engine, to a body at 30°, the work due to each unit of heat is 439 

 foot-pounds. This is the " theoretical duty" referred to in the last 

 column in the table. 



* I am indebted to the kindness of Professor Gordon, of Glasgow, for the 

 experimental data. 



t Pressure 3J atmospheres; 37 lb. on the square inch of the safety-valve. 



