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



communicates to the latter mechanical power. During the 

 diminution of volume, on the contrary, the piston expends me- 

 chanical power in compressing the working substance. The 

 increase of volume takes place at a higher temperature, and 

 therefore at a higher pressure, than the diminution of volume ; 

 consequently, the mechanical power communicated to the piston, 

 exceeds that taken away from it. The surplus is the power of 

 the engine, available for performing mechanical work. 



7. Efficiency of Therm o-dynamic Engines. — The efficiency 

 of a thermo-dynamic engine is the ratio which the available 

 power bears to the mechanical equivalent of the whole heat 

 expended. 



If it were possible to construct an engine such, that the 

 heat communicated to the working substance should entirely 

 disappear, the power produced by that engine would be the 

 exact equivalent of the heat expended : that is to say, 772 foot- 

 pounds for each British unit of heat ; and its efficiency would 

 be represented by unity. According to the average esti- 

 mate already stated, it would produce power to the amount of 

 6,000,000 foot-pounds for each pound of coal consumed ; and, 

 as a horse-power is 1,980,000 foot-pounds per hour, the con- 

 sumption of coal would be 0*33 lb. per horse-power per hour. 

 It is impossible, however, by any engine, to realize anything 

 approaching to this degree of efficiency. This arises from 

 two causes : — first, the necessary loss of heat, depending on 

 the limits of temperature between which the engine works, 

 according to a law which has been already referred to, and 

 which will shortly be stated ; and, secondly, the waste of heat 

 and power arising from the engine's not fulfilling exactly the 

 conditions prescribed by theory. "When the necessary loss of 

 heat alone is taken into account, the efficiency as determined 

 by calculation, may be called the Theoretical Maximum 

 Efficiency of the kind of engine under consideration. When 

 the waste of heat and power is also allowed for, the result is 

 the actual efficiency of the engine. 



8. Theoretical Conditions of Maximum Efficiency. — The 

 latent heat of increase of volume at an elevated temperature, 

 being the direct source of the power of a thermo-dynamic en- 

 gine, it is obvious, that, cceteris paribus, the more we reduce 



