humidifiers to moisten the atmosphere after the temperature of the air
outside the gallery has been raised to mine temperature and drawn
through the humidifiers. It has been found that if a relative humidity
of 90%, at a temperature of 60° Fahr., is maintained for 48 hours,
simulating summer conditions in a mine, the absorption of moisture by
the dust and the blanketing effect of the humid air prevent the general
ignition of the dust.

These humidity tests have been run in Gas and Dust Gallery No. 1 with
special equipment consisting of a Koerting exhauster having a capacity
of 240,000 cu. ft. per hour, which draws the air out of the gallery
through the first doorway, or that next the concrete head in which the
cannon is embedded.

The other end of the gallery is closed by means of brattice cloth and
paper diaphragms, the entire gallery being made practically air-tight.
The air enters the fifteenth doorway through a box, passing over steam
radiators to increase its temperature, and then through the humidifier
heads.


EXPLOSIVES TESTING APPARATUS.

There is no exposed woodwork in Building No. 17, which is 40 by 60 ft.,
two stories high, and substantially constructed of heavy stone masonry,
with a slate roof. The structure within is entirely fire-proof. Iron
columns and girders, and wooden girders heavily encased in cement,
support the floors which are either of cement slab construction or of
wooden flooring protected by expanded metal and cement mortar, both
above and beneath. At one end, on the ground floor, is the exposing and