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



power which would have been expended in compressing the 

 steam is partly saved ; but the saving of power bears a 

 small proportion to the mechanical equivalent of the heat 

 wasted. 



The amount of waste thus occasioned is comparatively un- 

 important in practice, provided it be not increased by unskil- 

 ful methods of heating the feed-water ; for, under ordinary 

 circumstances, the heat required for that purpose seldom ex- 

 ceeds one-seventh part of the latent heat of evaporation, and 

 it may be considered to reduce the efficiency of the engine 

 below the theoretical maximum by about one-sixteenth. 



Another and a more important point in which the conditions 

 prescribed by theory cannot be exactly fulfilled, is the extent 

 of the expansion during the process C : if this expansion were 

 carried in practice down to the pressure of condensation, the 

 cylinder and every part of the engine would be bulky, heavy, 

 and costly, and the action of the steam upon the piston, dur- 

 ing the latter portion of the stroke, would be so feeble as to 

 cause an unsteadiness of motion unsuitable for the driving of 

 machinery. The expansion, therefore, cannot be fully carried 

 out. The diminution of efficiency from" this cause depends 

 upon the extent to which the expansive working is carried. 

 Should the expansive working be wholly omitted, the efficiency 

 may be reduced to one-third or one-fourth of its theoretical 

 value, or even less, according to circumstances. 



16. Actual Efficiency of well-constructed Steam-Engines. 

 — In single-acting engines for pumping water, in which the 

 difficulties of employing a great extent of expansive working 

 are the least, the actual efficiency has already, in some cases, 

 attained a value nearly approximating to its maximum theo- 

 retical value. In double-acting engines, however, so long a 

 range of expansive working cannot be employed ; and their 

 ordinary average consumption of coal, when skilfully made 

 and worked, is four pounds per horse-power per hour, the coal 

 being of the evaporating power already specified. This cor- 

 responds to an efficiency represented by 0-0825, being about 

 0-46 of the theoretical maximum. 



Considering that the causes of waste of heat and power in 

 the steam-engine are, as has been already explained, incapable 



