very small powers), Robson's (at present undergoing transformation in
the able hands of Messrs. Tangye), Korting's, and others--are in use;
but, so far as I can learn, all require a larger quantity of gas than
those previously referred to.

[Illustration: OTTO ATMOSPHERIC GAS ENGINE.]

[Illustration: CLERCK'S GAS ENGINE, 6 HORSE POWER.]

[Illustration: OTTO-CROSSLEY GAS ENGINE, 16 H.P.

Consumption 17.6 cubic feet of 16-candle gas per
theoretical horse power per hour.

Average pressure, 90.4 × constant, .568 theoretical
horse power per pound = 50.8 theoretical horse power.]

[Illustration: ATKINSON'S DIFFERENTIAL GAS ENGINE, 8 H.P.]

I have all along spoken of efficiency as a percentage of the total
quantity of heat evolved by the fuel; and this is, in the eyes of a
manufacturer, the essential question. Other things being equal, that
engine is the most economical which requires the smallest quantity of
coal or of gas. But men of science often employ the term efficiency in
another sense, which I will explain. If I wind a clock, I have spent a
certain amount of energy lifting the weight. This is called "energy of
position;" and it is returned by the fall of the weight to its
original level. In the same way if I heat air or water, I communicate
to it energy of heat, which remains potential as long as the
temperature does not fall, but which can be spent again by a decrease