in different forms of the Air-E’ngine. 165 
cylinder may then be transferred to that body, and the piston 
pressed down, the temperature remaining at 'T°, until a third po- 
sition, e’” f’’’, is reached, which is such that, on again placing the 
cylinder on K, and completing the downward stroke, the temper- 
ature will again rise to S°. It thus appears that, of a certain de- 
finite amount of heat drawn from A, a certain less amount is 
given up to B, and the difference is converted into mechanical 
effect.* 
It is evident that, by reversing this whole process, that is, by 
first placing the cylinder on K, and allowing the piston to rise to 
e” f'", thereby depressing the temperature from S° to 'T°, then 
Placing the cylinder on B, and allowing a further expansion to 
e” f’, the temperature being kept at T° by heat received from B, 
then transferring the cylinder to K again, and forcing down the 
piston to e’ f’, so as to raise the temperature to S°, we may finally, 
y completing the downward stroke in contact with A, transfer 
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also the additional amount expended in compression, above what 
was received during the expansion. ae 
_4 gas or vapor engine constructed in conformity with this prin- 
ciple, is capable, therefore, if worked backward by force, of gen- 
erating as much heat as is expended in its direct action, and of 
testoring the heat so generated to the original source. Such an 
engine is what Carnot defines to be a perfect thermo-dynamic en- 
gine. It is obvious from the nature of things, that no other de- 
scription of engine, moved by heat, can convert a larger fraction 
of the heat drawn from the source into available power, than one 
of this kind: for, if there could be such an one, it would be ca- 
pable of driving this one backward, and of thus constantly re- 
funding to the source as much heat as it withdraws, while still 
Preserving a balance of positive power—or, in other words, a 
balance of effect without a cause—which is impossible. We are 
expression for this effect, for the perfect engine. The illustra- 
ons which immediately follow are substantially derived from 
hompson and Clansius, and would be superfluous, had the in- 
Vestigations of those writers been republished in this country. 
. i identical -wi hich has hitherto 
rd which was identical with that w. 
been Universally received, the amount of heat given up toB it af pina nee 
éd from A, but exactly equal to it: hence the m orce Set 
umed to be a natural concomitant of the transfer of heat from a ho 2 A 
— body, and not a conyersion of the heat, or of any part of it, into mechani 
t Clausius, on th ing F f Heat, &c., from Poggendorff’s Annalen, vol. 
letix, republished in she nied, Ed. Phil. Mag. July and August, 1861. ht 8 
— paper above quoted—also Lond. Phil. Trans., part 1, 1852, and elsewhere. 
