per cent.) has been lost by imperfect combustion, radiation, or
convection. The water required for condensing this quantity of steam
is 550 lb.; and, taking the temperature in the hot well as 102°, 550
lb. have been raised 40° from 62°. Thus we account for 550 × 40 =
22,000, or (say) 71½ per cent. still remaining as heat. If we add this
71½ per cent. to 18½ per cent. we have 90 per cent., and there remain
only 10 per cent. of the heat that can possibly have been converted
into power. But some of this has been lost by radiation from
steam-pipes, cylinder, etc. Allowing but 1 per cent. for this, we have
only 9 per cent. as the efficiency of a really good condensing engine.
This estimate agrees very closely with the actual result; for the 2.2
lb. of coal would develop 30,800[theta]; and this, multiplied by
Joule's equivalent, amounts to nearly 24 millions of foot-pounds. As 1
horse power is a little less than 2 million foot-pounds per hour, only
one-twelfth, or a little more than 8 per cent. of the total heat is
converted; so that whether we look at the total quantity of heat which
we show unconverted, or the total heat converted, we find that each
supplements and corroborates the other. If we take the efficiency of
the engine alone, without considering the loss caused by the boiler,
we find that the 25,168[theta] which entered the boiler should have
given 19,429,696 foot-pounds; so that the 2 millions given by the
engine represent about 10 per cent. of the heat which has left the
boiler. The foregoing figures refer to large stationary or marine
engines, with first-rate boilers. When, however, we come to
high-pressure engines of the best type, the consumption of coal is
twice as much; and for those of any ordinary type it is usual to
calculate 1 cubic foot, or 62½ lb., of water evaporated per horse
power. This would reduce the efficiency to about 6 per cent. for the
best, and 3 per cent. for the ordinary non-condensing engines; and if
to this we add the inefficiency of some boilers, it is certain that