570 TEOFESSOR WILLIAM THOMSON S ACCOUNT OF 



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 " per- 

 fect steam-engine."* 



Neither steam-engines nor air-engines, however, are nearly perfect ; and we 

 do not know in which of the two kinds of machine the nearest approach to per- 

 fection may be actually attained. The beautiful engine invented by Mr Stir- 

 ling of Galston, may be considered as an excellent beginning for the air-engine ;f 

 and it is only necessary to compare this with Newcomen's steam-engine, and 

 consider what Watt has effected, to give rise to the most sanguine anticipations 

 of improvement. 



V. On ike Economy of actual Steam-Engines. 



5(5. The steam-engine being universally employed at present as the means 

 for deriving motive poAver from heat, it is extremely interesting to examine, ac- 

 cording to Carnot's theory, the economy actually attained in its use. In the first 

 place, we remark that, out of the entire " fall" from the temperatm-e of the coals to 

 that of the atmosphere, it is only part — that from the temperature of the boiler to 

 the temperature of the condeuser — that is made available ; while the very great 

 fall from the temperature of the burning coals to that of the boUer, and the com- 

 paratively small fall from the temperature of the condenser to that of the atmo- 

 sphere, are entirely lost as far as regards the mechanical effect which it is desired 

 to obtain. We infer from this, that the temperature of the boiler ought to be 

 kept as high as, according to the strength, is consistent with safety, whUe that of 

 the condenser ought to be kept as nearly down at the atmospheric tempera- 

 ture as possible. To take the entire benefit of the actual fall, Carnot shewed 

 that the " principle of expansion" must be pushed to the utmost.| 



* 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 ; and a second medium, alternately in the state 

 of saturated vapour and liquid water, to receive the heat, discharged at an intermediate temperature 

 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 temperature not in contact 

 with water in the hottest part of the apparatus, instead of, as in the steam-engine, always in a satu- 

 rated state. 



t It is probably this invention to which Carnot alludes in the following passage (p. 112) : — 

 " II a ete fait, dit-on, tout recemment en Angleterre des essais heureux sur le developpement de la 

 puissance motrice par Taction de la chaleur sur I'air atmospherique. Nous ignorons entierement 

 ne quoi ces essais ont consiste, si toutefois ils sont reels." 



J From this point of view, we see very clearly how imperfect is the steam-engine, even after all 

 Watt's improvements. For to " push the principle of expansion to the utmost," we must allow the 



