device at the back of the mortar records the length of swing which, by a
vernier, may be read to 1/200 in.

_Bichel Pressure Gauges._--Pressure gauges are constructed for the
purpose of determining the unit disruptive force of explosives
detonating at different rates of velocity, by measuring pressures
developed in an enclosed space from which the generated gases cannot
escape. The apparatus consists of a stout steel cylinder, which may be
made absolutely air-tight; an air-pump and proper connections for
exhausting the air in the cylinder to a pressure equivalent to 10 mm. of
mercury; an insulated plug for providing the means of igniting the
charge; a valve by which the gaseous products of combustion may be
removed for subsequent analysis; and an indicator drum (Fig. 1, Plate
VII) with proper connections for driving it at a determinable speed.

This apparatus is in the southeast corner of Building No. 17. The
cylinder is 31½ in. long, 19¾ in. in diameter, and is anchored to
a solid concrete footing at a convenient height for handling. The
explosion chamber is 19 in. long and 7⅞in. in diameter, with a capacity
of exactly 15 liters. The cover of the cylinder is a heavy piece of
steel held in place by stout screw-bolts and a heavy steel clamp.

The charge is placed on a small wire tripod, and connections are made
with a fuse to an electric firing battery for igniting the charges. The
cover is drawn tight, with the twelve heavy bolts against lead washers.
The air in the cylinder is exhausted to 10 mm., mercury column, in order
to approach more closely the conditions of a stemmed charge exploding in
a bore-hole inaccessible to air; the indicator drum is placed in
position and set in motion; and, finally, the shot is fired. The record
shown on the indicator card is a rapidly ascending curve for quick