[Illustration: Fig. 2.

{cannon as described in text} ]

The buildings near the gallery are protected by two barricades near the
open end, each 10 ft. high and 30 ft. long. A back-stop, consisting of a
swinging steel plate, 6 ft. high and 9 ft. long, 50 ft. from the end of
the gallery, prevents any of the stemming from doing damage.

Tests are witnessed from an observation room, a protected position about
60 ft. from the gallery. The walls of the room are 18 in. thick, and the
line of vision passes through a ½-in. plate glass, 6 in. wide and 37 ft.
long, and is further confined by two external guards, each 37 ft. long
and 3 ft. wide.

In this gallery a series of experiments has been undertaken to determine
the amount of moisture necessary with different coal dusts, in order to
reduce the likelihood of a coal-dust explosion from a blown-out shot of
one of the dangerous types of explosives.

Coal dust taken from the roads of one of the coal mines in the Pittsburg
district required at least 12% of water to prevent an ignition. It has
also been proven that the finer the dust the more water is required, and
when it was 100-mesh fine, 30% of water was required to prevent its
ignition by the flame of a blown-out shot in direct contact. The methods
now used in sprinkling have been proven entirely insufficient for
thoroughly moistening the dust, and hence are unreliable in preventing a
general dust explosion.

At this station successful experiments have been carried out by using