342 THE ADDRESSES, LECTURES, ETC., OF 



of temperature between the boiler and condenser. The boiler 

 pressure is limited, however, by considerations of safety and 

 convenience of construction, and the range of working temperature 

 rarely exceeds 120 C. except in the engines constructed by 

 Mr. Perkins, in which a range of 160 C., or an expansive action 

 commencing at 14 atmospheres, has been adopted with con- 

 siderable success, as appears from an able report on this engine 

 by Sir Frederick Bramwell. To obtain more advantageous 

 primary conditions we have to turn to the caloric or gas engine, 



T T 1 ' 



because in them the coefficient of efficiency expressed by - - may 



be greatly increased. This value would reach a maximum if the 

 initial absolute temperature T could be raised to that of com- 

 bustion, and T' reduced to atmospheric temperature, and these 

 maximum limits can be much more nearly approached in the gas 

 engine, worked by a combustible mixture of air and hydro-carbons, 

 than in the steam engine. 



Assuming, then, in an explosive gas engine a temperature of 

 1,500 G. at a pressure of 4 atmospheres, we should, in accordance 

 with the second law of thermo-dynamics, find a temperature after 

 expansion to atmospheric pressure of 600 C., and therefore a 

 working range of 1500 - 600 = 900, and a theoretical efficiency 



of - - = about one-half, contrasting very favourably with 



1000 T ^74 



that of a good expansive condensing steam engine, in which the 



120 2 

 range is 150-30 = 120 C., and the efficiency 



good expansive steam engine is therefore capable of yielding as 

 mechanical work fth parts of the heat communicated to the boiler, 

 which does not include the heat lost by imperfect combustion and 

 that carried away in the chimney. Adding to these the losses by 

 friction and radiation in the engine, we find that the best steam 

 engine yet constructed does not yield in mechanical effect more 

 than -fth part of the heat energy residing in the fuel consumed. 

 In the gas engine we have also to make redactions from the 

 theoretical efficiency, on account of the rather serious loss of heat 

 by absorption into the working cylinder, which has to be cooled 

 artificially in order to keep its temperature down to a point at 

 which lubrication is possible ; this, together with frictional loss, 



