the whole of the jacket heat can be utilized where hot water is
required for other purposes; and this, with the exhaust gases, has
been used for drying and heating purposes.

With such advantages, it may be asked: Why does not the gas-engine
everywhere supersede the steam-engine? My answer is a simple one: The
gas we manufacture is a dear fuel compared with coal. Ordinary coal
gas measures 30 cubic feet to the pound; and 1,000 cubic feet,
therefore, weigh 33 lb. Taking the price at 2s. 9d. per 1,000 cubic
feet, it costs 1d. per lb. The 30 cubic feet at 630[theta] give
19,000[theta] all available heat. Although good coal may yield 14,000
units by its combustion, only about 11,000 of these reach the boiler;
so that the ratio of the useful heat is 11/19. The thermal efficiency
of the best non-condensing engine to that of the gas-engine is in the
ratio 4/22. Multiplying together these two ratios, we get

11 4 44
-- x ------- = ----
19 22_{1/2} 4.28

That is, speaking roughly, 1 lb. of gas gives about ten times as much
power as 1 lb. of coal does in a good non-condensing engine. But at
18s. 8d. a ton we get 10 lb. of coal for 1d.; so that with these
figures the cheapness of the coal would just compensate for the
efficiency of the gas. As to the waste heat passing away from the
engine being utilized, here the gas-engine has no advantage; and, so
far as this is concerned, the gas is about eight times dearer than
coal. The prices of gas and coal vary so much in different places that
it is hard to determine in what cases gas or coal will be the dearer
fuel, considering this point alone.