realizing the Advantages of the Air-Engine. 23 



of being wholly removed in practice, it may be estimated that 

 the greatest amount of actual efficiency to be expected in 

 double-acting steam-engines by any probable improvement, 

 is about three-fourths of the theoretical maximum, or 0-133, — 

 corresponding to a consumption of coal, calculated as before, 

 of 2J lb. per horse-power per hour. 



Supplement to Section II. — On the Steam-and-Ether Engine of 

 M. du Trembley. 



(16 A.) This engine exemplifies one means of diminishing 

 that difficulty attending the fulfilment of the conditions of 

 theoretical perfection in the steam-engine, which arises from 

 the impracticability of expanding the steam until its pressure 

 has fallen to that corresponding to a low temperature of con- 

 densation. 



Instead of carrying the expansion of the steam to the great 

 extent required by theory, it is carried to such an extent only 

 as is convenient in practice. The steam is then liquefied at 

 the pressure attained at the end of its expansion, and the 

 heat given out during its liquefaction is employed to evaporate 

 ether, which works an auxiliary engine. By this process, 

 after the expansion of the steam has been carried to a certain 

 extent, vapour of ether is in fact substituted for the steam 

 and jnade to perform the remainder of its work in its stead ; 

 and as the vapour of ether, at a given temperature, exerts a 

 higher pressure and occupies a less volume than steam does, 

 the cylinder of the auxiliary ether-engine occupies much less 

 space, and gives a more steady action than would be required 

 for the performance of the same work by continuing the ex- 

 pansion of the steam. 



The maximum theoretical efficiency of the steam-and-ether 

 engine is the same with that of any other thermo-dynamic en- 

 gine working between the temperature of evaporation of the 

 water, and that of liquefaction of the ether. 



Its advantage consists in obtaining a nearer approximation 

 to that theoretical efficiency within given limits as to the bulk 

 and cost of the engine, than is practicable with an engine 

 worked by steam alone. 



