r lead cylinders.

[Footnote A: Invented by General Rodman, United States Engineers.]

[Illustration: FIG. 55.--PRESSURE GAUGE.]

Berthelot uses a cylinder of copper, as also did the English Commission,
but in the simpler form of apparatus mostly used by manufacturers lead
cylinders are used. This form of apparatus (Fig. 55) consists of a base of
iron to which four uprights _a_ are fixed, set round the circumference of
a 4-inch circle; the lead plug rests upon the steel base let into the
solid iron block. A ring _c_ holds the uprights _d_ together at the top.
The piston _b_, which rests upon the lead plug, is a cylinder of tempered
steel 4 inches in diameter and 5 inches in length; it is turned away at
the sides to lighten it as much as possible. It should move freely between
the uprights _d_. In the top of this cylinder is a cavity to hold the
charge of explosive. The weight of this piston is 12-1/4 lbs. The shot _e_
is of tempered steel, and 4 inches in diameter and 10 inches in length,
and weighs 34-1/2 lbs. It is bored through its axis to receive a capped
fuse.

The instrument is used in the following manner:--A plug of lead 1 inch
long and 1 inch in diameter, and of a cylindrical form, is placed upon the
steel plate between the uprights _a_, the piston placed upon it, the
carefully weighed explosive placed in the cavity, and the shot lowered
gently upon the piston. A piece of fuse, with a detonator fixed at one
end, is then pushed through the hole in the shot until it reaches the
explosive contained in the cavity in the piston. The fuse is lighted. When
the charge is exploded, the shot is thrown out, and the lead cylinder is
more or less compressed. The lead plugs must be of a uniform density and
homogeneous structure, and should be cut from lead rods that have been
drawn, and not cast separately from small masses of metal.

[Illustration: FIG. 56.--_b_, STEEL PUNCH; _c_, LEAD CYLINDER FOR USE WITH
PRESSURE GAUGE.]

The strength of the explosive is proportional to the work performed in
reducing the height of the lead (or copper) plug, and to get an expression
for the work done it is necessary to find the number of foot-pounds (or
kilogrammetres) required to produce the different amounts of compression.
This is done by submitting exactly similar cylinders of lead to a crushing
under weights acting without initial velocity, and measuring the reduced
heights of the cylinders; from these results a table is constru