products of combustion at constant pressure, is 0.238, that only
sufficient air is passed through the fire to supply the quantity of
oxygen theoretically required for the combustion of the carbon, and
that the temperature of the air is at 60° Fahrenheit = 520° absolute.
The symbol T represents the absolute temperature of the furnace, a
value which is easily calculated in the following manner: 1 lb. of
carbon requires 2-2/3 lb. of oxygen to convert it into carbonic acid,
and this quantity is furnished by 12.2 lb. of air, the result being
13.2 lb. of gases, heated by 14,544 units of heat due to the energy of
combustion; therefore:

14,544 units
T = 520° + ------------------ = 5,150° absolute.
13.2 lb. X 0.238

The lower temperature, _t_, we may take as that of the feed water, say
at 100° or 560° absolute, for by means of artificial draught and
sufficiently extending the heating surface, the temperature of the
smoke may be reduced to very nearly that of the feed water. Under such
circumstances the proportion of heat which can be realized is

5,150° - 560°
= --------------- = 0.891;
5,150°

that is to say, under the extremely favorable if not impracticable
conditions assumed, there must be a loss of 11 per cent. Next, to give
a numerical value to the potential energy, H, to be derived from a
pound of carbon, calculating from absolute zero, the specific heat of
carbon being 0.25, and absolute temperature of air 520°: