horizontal, starting on a line 1½ in. from the back. The point is
rounded to conform to a radius of ¼ in. The back of each is 2 in. longer
than the edge, making a total length of 10 in., and is 1 in. deep and 12
in. wide. This shoulder gives bolting surface to the beam from which the
mortar is hung. The beam is of solid steel, has a 4 by 8-in. section,
and is 87 in. long. Heavy steel castings are bolted to it to take the
threads of the machine-steel rods which form the saddles on which the
mortar is suspended. The radius of the swing, measured from the point of
the knife-edges to the center of the trunnions, is 89¾ in.

The cannon consists of two parts, a jacket and a liner. The jacket is 36
in. long, has an external diameter of 24 in., and internal diameters of
9½ and 7½ in. It is made of the best cast steel or of forged steel.

The liner is 36½ in. long, with a 1-in. shoulder, 7¾ in. from the back,
changing the diameter from 9½ to 7½ in. The bore is smooth, being 2¼ in.
in diameter and 21½ in. long. The cannon rests on a 4-wheel truck, to
which it is well braced by straps and rods. A track of 30-in. gauge
extends about 9 ft. from the muzzle of the mortar to the bumper for the
cannon.

The shot is fired by an electric firing battery, from the first floor of
Building No. 17, about 10 yd. away. To insure the safety of the operator
and the charger, the man who loads the cannon carries a safety plug
without which the charge cannot be exploded. The wires for connecting to
the fuse after charging are placed conveniently, and the safety plug is
then inserted in a box at the end of the west wall. The completion of
the firing battery by the switch at the firing place is indicated by the
flashing of a red light, after which all that is necessary to set off
the charge is to press a button on the battery. An automatic recording