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
recording apparatus for flame tests of explosives, also pressure gauges,
and a calorimeter, and, at the other end, is a gallery for testing
safety lamps.
The larger portion of the second floor is occupied by a gas-tight
training room for rescue work, and an audience chamber, from which
persons interested in such work may observe the methods of procedure. A
storage room for rescue apparatus and different models of safety lamps
is also on this floor.
The disruptive force of explosives is determined in three ways, namely,
by the ballistic pendulum, by the Bichel pressure gauge, and by Trauzl
lead blocks.
_Ballistic Pendulum._--The disruptive force of explosives, as tested by
the ballistic pendulum, is measured by the amount of oscillation. The
standard unit of comparison is a charge of ½ lb. of 40% nitro-glycerine
dynamite. The apparatus consists essentially of a 12-in. mortar (Fig. 3,
Plate VI), weighing 31,600 lb., and suspended as a pendulum from a beam
having knife-edges. A steel cannon is mounted on a truck set on a track
laid in line with the direction of the swing of the mortar. At the time
of firing the cannon may be placed 1/16-in. from the muzzle of the
mortar. The beam, from which the mortar is suspended, rests on concrete
walls, 51 by 120 in. at the base and 139 in. high. On top of each wall
is a 1-in. base-plate, 7 by 48 in., anchored to the wall by ⅝-in. bolts,
28 in. long. The knife-edges rest on bearing-plates placed on these
base-plates. The bearing-plates are provided with small grooves for the
purpose of keeping the knife-edges in oil and protected from the
weather. The knife-edges are each 6 in. long, 2-11/16 in. deep from
point to back, 2 in. wide at the back, and taper 50° with the
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
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
explosives and a shallower, slowly rising curve for explosives of slow
detonation. When the gases cool, the curve merges into a straight line,
which indicates the pressures of the cooled gases on the sides of the
chamber.
[Illustration: PLATE VIII.
Fig. 1.--Explosives Calorimeter.
Fig. 2.--Building No. 17, and Flame-Test Apparatus.