of air for the combustion of 100 lbs. of coal. A cubic foot of air at
ordinary temperature weighs about .075 lbs.; so that 100 lbs. of coal
require 15,524 cubic feet of air, or 1 lb. of coal requires about 155 cubic
feet of air, supposing every atom of the oxygen to enter into combination.
If, then, from one-third to one-half of the air passes unconsumed through
the fire, an allowance of 240 cubic feet of air for each pound of coal will
be a small enough allowance to answer the requirements of practice, and in
some cases as much as 300 cubic feet will be required,--the difference
depending mainly on the peculiar configuration of the furnace.

144. _Q._--Can you state the evaporative efficacy of a pound of coal?

_A._--The evaporative efficacy of a pound of carbon has been found
experimentally to be equivalent to that necessary to raise 14,000 lbs. of
water through 1 degree, or 14 lbs. of water through 1000 degrees, supposing
the whole heat generated to be absorbed by the water. Now, if the water be
raised into steam from a temperature of 60 deg., then 1118.9 deg. of heat will have
to be imparted to it to convert it into steam of 15 lbs. pressure per
square inch. 14,000 / 1118.9 = 12.512 Lbs. will be the number of pounds of
water, therefore, which a pound of carbon can raise into steam of 15 lbs.
pressure from a temperature of 60 deg.. This, however, is a considerably larger
result than can be expected in practice.

145. _Q._--Then what is the result that may be expected in practice?

_A._--The evaporative powers of different coals appear to be nearly
proportional to the quantity of carbon in them; and bituminous coal is,
therefore, less efficacious than coal consisting chiefly of pure carbon. A
pound of the best Welsh or anthracite coal is capable of raising from 9-1/2
to 10 lbs. of water from 212 deg. into steam, whereas a pound of the best
Newcastle is not capable of raising more than about 8-1/2 lbs. of water
from 212 deg. into steam; and inferior coals will not raise more than 6-1/2
lbs. of water into steam. In America it has been found that 1 lb. of the
best coal is equal to 2-1/2 lbs. of pine wood, or, in some cases to 3 lbs.;
and a pound of pine wood will not usually evaporate more than about 2 1/2
lbs. of water, though, by careful management, it may be made to evaporate 4
1/2 lbs. Turf will generate rather more steam than wood. Coke is equal or
somewhat superior to the best coal in evaporative effect.

146. _Q