192 THOMSON ON CARNOT'8 



(2) The economy of the engine, with the fall 

 which it actually uses. 



55. In the first respect, the air-engine, as Carnot 

 himself points out, has a vast advantage over the 

 steam-engine; since the temperature of the hot 

 part of the machine may be made very much 

 higher in the air-engine than would be possible in 

 the steam-engine, on account of the very high 

 pressure produced in the boiler, by elevating the 

 temperature of the water which it contains to any 

 considerable extent above the atmospheric boiling- 

 point. On this account a "perfect air-engine " 

 would be a much more valuable instrument than a 

 " perfect steam-engine." * 



* Carnot suggests a combination of the two principles, 

 with air as the medium for receiving the heat at a very 

 high temperature from the furnace; ^nd a second medium, 

 alternately in the state of saturated vapor and liquid water, 

 to receive the heat, discharged at aii intermediate temper- 

 ature from the air, and transmit it to the coldest part of 

 the apparatus. It is possible that a complex arrangement 

 of this kind might be invented which would enable us to 

 take the heat at a higher temperature, and discharge it at a 

 lower temperature than would be practicable in any simple 

 air-engine or simple steam-engine. If so, it would no 

 doubt be equally possible, and perhaps more convenient, 

 to employ steam alone, but to use it at a very high tem- 

 perature not in contact with water in the hottest part of 



