+----------+------------+--------+------------+
Total cubic feet of | | | | |
products per 1 lb. | | | | |
of coal........... | -- | -- | 226.4 | 100.0 |
---------------------+----------+------------+--------+------------+

The temperature of the furnace not having been determined, we must
calculate it on the supposition, which will be justified later on,
that 50 per cent more air was admitted than was theoretically
necessary to supply the oxygen required for perfect combustion. This
would make 18 lb. of air per 1 lb. of coal; consequently 19 lb. of
gases would be heated by 14,727 units of heat. Hence:

14,727 u.
T = ---------------- = 3,257°
19 lb. × 0.238

above the temperatures of the air, or 3,777° absolute. The temperature
of the smoke, _t_, was 849° absolute; hence the maximum duty would be

3,777° - 849°
--------------- = 0.7752.
3,777°

The specific heat of coal is very nearly that of gases at constant
pressure, and may, without sensible error, be taken as such. The
potential energy of 1 lb. of coal, therefore, with reference to the
oxygen with which it will combine, and calculated from absolute zero,
is: