quantity may be used by such an apparatus than is required for this
purpose. Combustion arches sprung above the grates have proved of
advantage in maintaining a high furnace temperature and in assisting in
the ignition of fresh coal.

Stacks used with forced blast should be of such size as to insure a
slight suction in the furnace under any conditions of operation. A blast
up to 3 inches of water should be available for the finer sizes supplied
by engine driven fans, automatically controlled by the boiler pressure.
The blast required will increase as the depth of the fuel bed increases,
and the slight suction should be maintained in the furnace by damper
regulation.

The use of blast with the finer sizes causes rapid fouling of the
heating surfaces of the boiler, the dust often amounting to over 10 per
cent of the total fuel fired. Economical disposal of dust and ashes is
of the utmost importance in burning fuel of this nature. Provision
should be made in the baffling of the boiler to accommodate and dispose
of this dust. Whenever conditions permit, the ashes can be economically
disposed of by flushing them out with water.

Bituminous Coals--There is no classification of bituminous coal as to
size that holds good in all localities. The American Society of
Mechanical Engineers suggests the following grading:

_Eastern Bituminous Coals_--

(A) Run of mine coal; the unscreened coal taken from the mine.

(B) Lump coal; that which passes over a bar-screen with openings 1-1/4
inches wide.

(C) Nut coal; that which passes through a bar-screen with 1-1/4-inch
openings and over one with 3/4-inch openings.

(D) Slack coal; that which passes through a bar-screen with 3/4-inch
openings.

_Western Bituminous Coals_--

(E) Run of mine coal; the unscreened coal taken from the mine.

(F) Lump coal; divided into 6-inch, 3-inch and 1-1/4-inch lump, according
to the diameter of the circular openings over which the respective
grades pass; also 6 x 3-inch lump and 3 x 1-1/4-inch lump, according as
the coal passes through a circular opening having the diameter of
the larger figure and over that of the smaller diameter.

(G) Nut coal; divided into 3-inch steam nut, which passes through an
opening 3 inches diameter and over 1-1/4 inches; 1-1/4 inch nut, which
passes through a 1-1/4-inch diameter opening and over a 3/4-inch
diameter opening; 3/4-inch nut, which passes through a 3/4-inc