212 THE POPULAR SCIENCE MONTHLY. 



tion. The transformation of mechanical effect into heat involves no 

 losses except those resulting from imperfect installation, and these 

 may be so completely avoided that Dr. Joule was able by this method 

 to determine the equivalent values of the two forms of energy. But, 

 in attempting the inverse operation of effecting the conversion of heat 

 into mechanical energy, we find ourselves confronted by the second 

 law of thermo-dynamics, which says that, whenever a given amount of 

 heat is converted into mechanical effect, another but variable amount 

 descends from a higher to a lower potential, and is thus rendered un- 

 available. 



In the condensing steam-engine this waste heat comprises that com- 

 municated to the condensing-water, while the useful heat, or that con- 

 verted into mechanical effect, depends upon the difference of temper- 

 ature 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 fourteen atmospheres, has been 

 adopted with considerable promise of success, as appears from an able 

 report on this engine by Sir Frederick Bramwell. To obtain more ad- 

 vantageous primary conditions we have to turn to the caloric or gas- 

 engine, because in them the co-efficient 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 combustion, 

 and t' reduced to atmosj)heric tempei'ature, and these maximum limits 

 can be much more nearly approached in the gas-engine worked by a 

 combustible mixture of air and hydrocarbons than in the steam-engine. 



Assuming, then, in an explosive gas-engine a temperature of 1,500 

 C, at a pressure of four 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 work- 

 ing range of 1500 600 = 900, and a theoretical efficiency of 



900 

 i &f\n o^< = a ^out one half, contrasting very favorably with that 



of a good expansive condensing steam-engine, in which the range is 



ion 2 



150 - 30 = 120 C, and the efficiency = =-. A good ex- 



pansive steam-engine is therefore capable of yielding as mechanical 

 work two-seventh part of the heat communicated to the boiler, which 

 does not include the heat lost by imperfect combustion, and that car- 

 ried away in the chimney. Adding to these the losses by friction and 

 radiation in the engine, we find that the best steam-engine yet con- 

 structed does not yield in mechanical effect more than one seventh 

 part of the heat-energy residing in the fuel consumed. In the gas- 

 engine we have also to make reductions from the theoretical efficiency, 

 on account of the rather serious loss of heat by absorption into the 



