as is the time that has elapsed since the appearance of these papers,
you will find that progress has been made, and that a still higher
efficiency is now claimed.

[Footnote 2: See _Journal_, vol. xxxv, pp. 91, 133.]

[Footnote 3: Ibid., vol. xliii., pp. 703, 744.]

When I first wrote on this subject, I relied upon some data which led
me to suppose that the heating power of ordinary coal gas was higher
than it really is. At our last meeting, Mr. Hartley proved, by
experiments with his calorimeter, that gas of 16 or 17 candles gave
only about 630 units of heat per cubic foot. Now, if all this heat
could be converted into power, it would yield 630 × 772, or 486,360
f.p.; and it would require only 1,980,000 / 486,360 = 4.07 cubic feet to
produce 1 indicated horse power. Some recent tests have shown that,
with gas of similar heating power, 18 cubic feet have given 1
indicated horse power, and therefore 4.07 / 18 = 22.6 of the whole heat
has been converted--a truly wonderful proportion when compared with
steam-engines of a similar power, showing only an efficiency of 2 to 4
per cent.

The first gas-engine which came into practical use was Lenoir's,
invented about 1866, in which the mixture of gas and air drawn in for
part of the stroke at atmospheric pressure was inflamed by the spark
from an induction coil. This required a couple of cells of a strong
Bunsen battery, was apt to miss fire, and used about 90 cubic feet of
gas per horse power. This was succeeded by Hugon's engine, in which
the ignition was caused by a small gas flame, and the consumption was
reduced to 80 cubic feet. In 1864 Otto's atmospheric engine was