and on the sides. It is connected with a dark room in Building No. 17 by
a light-tight conduit of rectangular section, 12 in. wide, horizontal on
the bottom, and sloping on the top from a height of 8¼ ft. at the stack
to 21 in. at the inside of the wall of the building.

The conduit is carefully insulated from the light at all joints, and is
riveted to the stack. A vertical slit, 2 in. wide and 8 ft. long,
coincident with the center line of the conduit, is cut in the stack. A
vertical plane drawn through the center line of the bore-hole of the
cannon and that of the slit, if produced, intersects the center line of
a quartz lens, and coincides with the center of a stenopaic slit and the
axis of the revolving drum carrying the film. The photographing
apparatus consists of a shutter, a quartz lens, and a stenopaic slit, 76
by 1.7 mm., between the lens and the sensitized film on the rotary drum.
The quartz lens is used because it will focus the ultra-violet rays,
which are those attending extreme heat.

The drum is 50 cm. in circumference and 10 cm. deep. It is driven by a
220-volt motor connected to a tachometer which reads both meters per
second and revolutions per minute. A maximum peripheral speed of 20 m.
per sec. may be obtained.

When the cannon is charged, the operator retires to the dark room in
which the recording apparatus is located, starts the drum, obtains the
desired speed, and fires the shot by means of a battery. When developed,
the film shows a blur of certain dimensions, produced by the flame from
the charge. From the two dimensions--height and lateral
displacement--the length and duration of the flame of the explosive are
determined.