The specific heat of air at constant pressure has been proved by the
experiments of Regnault to be 0.2378, or something less than
one-fourth of that of water--a result arrived at by Rankine from
totally different data. In the case we have taken, there have been
expended 500 × 0.2378, or (say) 118.9[theta] to produce 26,450 f.p.
Each unit has therefore produced (26,450 / 118.0) = 222.5 f.p.,
instead of 772 f.p., which would have been rendered if every unit had
been converted into power. We therefore conclude that (222.5 / 772) = 29
per cent. of the total heat has been converted. The residue, or 71 per
cent., remains unchanged as heat, and may be partly saved by a
regenerator, or applied to other purposes for which a moderate heat is
required.

The quantity of heat necessary to raise the heat of air at a constant
volume is only 71 per cent. of that required to raise to the same
temperature the same weight of air under constant pressure. This is
exactly the result which Laplace arrived at from observations on the
velocity of sound, and may be stated thus--

Specific Foot- Per
heat. pounds. cent.

Kp = 1 lb. of air at constant pressure 0.2378 × 772 = 183.5 = 100
Kv = 1 lb. of air at constant volume 0.1688 × 772 = 130.3 = 71
------ --- ----- ---
Difference, being heat converted into power 0.0690 × 772 = 53.2 = 29


Or, in a hot-air engine without regeneration, the maximum effect of 1