that they
be brought into contact with air sufficiently heated to cause them to
ignite, that sufficient space be allowed for their mixture with the air,
and that sufficient time be allowed for their complete combustion before
they strike the boiler heating surfaces, since these surfaces are
comparatively cool and will lower the temperature of the gases below
their ignition point. The air drawn through the fire by the draft
suction is heated in its passage and heat is added by radiation from the
hot brick surfaces of the furnace, the air and volatile gases mixing as
this increase in temperature is taking place. Thus in most instances is
the first requirement fulfilled. The element of space for the proper
mixture of the gases with the air, and of time in which combustion is to
take place, should be taken care of by sufficiently large combustion
chambers.

Certain bituminous coals, owing to their high volatile content, require
that the air be heated to a higher temperature than it is possible for
it to attain simply in its passage through the fire and by absorption
from the side walls of the furnace. Such coals can be burned with the
best results under fire brick arches. Such arches increase the
temperature of the furnace and in this way maintain the heat that must
be present for ignition and complete combustion of the fuels in
question. These fuels too, sometimes require additional combustion
space, and an extension furnace will give this in addition to the
required arches.

As stated, the difficulty of burning bituminous coals successfully will
increase with the increase in volatile matter. This percentage of
volatile will affect directly the depth of coal bed to be carried and
the intervals of firing for the most satisfactory results. The variation
in the fuel over such wide ranges makes it impossible to definitely
state the thickness of fires for all classes, and experiment with the
class of fuel in use is the best method of determining how that
particular fuel should be handled. The following suggestions, which are
not to be considered in any sense hard and fast rules, may be of service
for general operating conditions for hand firing:

Semi-bituminous coals, such as Pocahontas, New River, Clearfield, etc.,
require fires from 10 to 14 inches thick; fresh coal should be fired at
intervals of 10 to 20 minutes and sufficient coal charged at each firing
to maintain a uniform thickness. Bituminous coals from Pittsburgh