lb. of air heated 1° Fahr. would be 53.2 f.p. The quantity of heat
Ky necessary to heat air under constant volume is to Kv, or that
necessary to heat it under constant pressure, as 71:100, or as
1:1.408, or very nearly as 1:SQRT(2)--a result which was arrived
at by Masson from theoretical considerations. The 71 per cent.
escaping as heat may be utilized in place of other fuel; and with the
first hot-air engine I ever saw, it was employed for drying blocks of
wood. In the same way, the unconverted heat of the exhaust steam from
a high-pressure engine, or the heated gases and water passing away
from a gas-engine, may be employed.

[Illustration]

We are now in a position to judge what is the practical efficiency of
the gas-engine. Some years since, in a letter which I addressed to
_Engineering_, and which also appeared in the _Journal of Gas
Lighting_,[2] I showed (I believe for the first time) that, in the
Otto-Crossley engine, 18 per cent. of the total heat was converted
into power, as against the 8 per cent. given by a very good
steam-engine. About the end of 1883 a very elaborate essay, by M.
Witz, appeared in the _Annales de Chimie et de Physique_, reporting
experiments on a similar engine, which gave an efficiency somewhat
lower. Early in 1884 there appeared in _Van Nostrand's Engineering
Magazine_ a most valuable paper, by Messrs. Brooks and Steward, with a
preface by Professor Thurston,[3] in which the efficiency was estimated
at 17 to 18 per cent. of the total heat of combustion. Both these
papers show what I had no opportunity of ascertaining, that is, what
becomes of the 82 per cent. of heat which is not utilized--information
of the greatest importance, as it indicates in what direction
improvement may be sought for, and how loss may be avoided. But, short