carbon and the hydrogen of the coal with a sufficient quantity of the
oxygen of the air to effect complete combustion into carbonic acid and
water. In order to do this, we have to use a quantity of air much
larger than is theoretically necessary, and also to heat an amount of
inert nitrogen five times greater than the necessary oxygen; and we
are therefore obliged to create a draught which carries away to the
chimney a considerable portion of the heat developed. The combustion,
moreover, is never perfect; and some heat is lost by conduction and
radiation. The principal loss is by hot gases escaping from the flues
to the chimney. Even with well-set boilers, the temperature in the
chimney varies from 400° to 600° Fahr. Taking the mean of 500°, this
would represent a large proportion of the total heat, even if the
combustion were perfect; for, as a general rule, the supply of air to
a furnace is double that which is theoretically necessary. For our
present purpose, it will be sufficient to see how much the whole loss
is, without dividing it under the several heads of "imperfect
combustion," "radiation," and "convection," by the heated gases
passing to the chimney.

With a very good boiler and furnace each pound of coal evaporates 10
pounds of water from 62° Fahr., changing it into steam of 65 lb.
pressure at a temperature of 312°, or 250° above that of the water
from which it is generated. Besides these 250°, each pound of steam
contains 894 units of latent heat, or 1,144 units in all. A very good
condensing engine will work with 2.2 lb. of coal and 22 lb. of steam
per horse power per hour. Now. 1 lb. of good coal will, by its
combustion, produce 14,000 heat-units; and the 2.2 lb. of coal
multiplied by 14,000 represent 30,800[theta]. Of these we find in the
boiler 22 × 1,144, or 25,168 units, or about 81½ per cent., of the
whole heat of combustion; so that the difference (5,632 units, or 18½