570 PROFESSOR 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 Srir- 
LING of Galston, may be considered as an excellent beginning for the air-engine ;+ 
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 the Economy of actual Steam-Engines. 
56. The steam-engine being universally employed at present as the means 
for deriving motive power from heat, it is extremely interesting to examine, ac- 
cording to Carnov’s theory, the economy actually attained in its use. In the first 
place, we remark that, out of the entire “ fall” from the temperature of the coals to 
that of the atmosphere, it is only part—that from the temperature of the boiler to 
the temperature of the condenser—that is made available ; while the very great 
fall from the temperature of the burning coals to that of the boiler, 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, while 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.t 
* 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. 
+ It is probably this invention to which Carnor alludes in the following passage (p. 112) :— 
“ Tl a été fait, dit-on, tout recemment en Angleterre des essais heureux sur le développement de la 
puissance motrice par l’action de la chaleur sur l’air atmosphérique. Nous ignorons entiérement 
ne quoi ces essais ont consisté, si toutefois ils sont réels.”’ 
{ From this point of view, we see very clearly how imperfect is the steam-engine, even after all 
Warr’s improvements. For to “‘ push the principle of expansion to the utmost,’’ we must allow the 
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