support of coal dust.

The cannon in which the explosive is fired is placed in the concrete
head, the axial line of the bore-hole being coincident with that of the
gallery. This cannon (Fig. 2) is similar to that used in the ballistic
pendulum. The charge is fired electrically from the observation room. To
minimize the risk of loading the cannon, the charger carries in his
pocket the plug of a stage switch (the only plug of its kind on the
ground), so that it is impossible to complete the circuit until the
charger has left the gallery. That portion of the first division of the
gallery which is not embedded in concrete, has a 3-in. covering made up
of blocks of magnesia, asbestos fiber, asbestos, cement, a thin layer of
8-oz. duck, and strips of water-proof roofing paper, the whole being
covered with a thick coat of graphite paint. The object of this covering
is to assist in maintaining a constant temperature.

[Illustration: PLATE VII.

Fig. 1.--Bichel Pressure Gauges.

Fig. 2.--Rate of Detonation Recorder.]

The entire gallery rests on a concrete foundation 10 ft. wide, which has
a maximum height of 4½ ft. and a minimum height of 2 ft.

The concrete head in which the cannon is placed completely closes that
end of the gallery. A narrow drain extends under the entire length of
the gallery, and a tapped hole at the bottom of each section provides an
efficient means of drainage.