cylinder and against
the plug, and so effectually prevents any escape. In the firing plug F is
a conical hole closed by a cone fitting with great exactness, which, when
the vessel is prepared for firing, is covered with fine tissue paper to
act as an insulator. The two firing wires GG, one in the insulated cone,
the other in the firing plug, are connected by a very fine platinum wire
passing through a glass tube filled with meal powder. The wire becomes
red-hot when connection is made with a Leclanche battery, and the charge
which has previously been inserted into the vessel is fired. The crusher
plug is fitted with a crusher gauge H for determining the pressure of the
gases at the moment of explosion, and in addition there is frequently a
second crusher gauge apparatus screwed into the cylinder. When it is
desired to allow the gases to escape for examination, the screw J is
slightly withdrawn. The gases then pass into the passage I, and can be led
to suitable apparatus in which their volume can be measured, or in which
they can be sealed for subsequent chemical analysis.

The greatest care must be exercised in carrying out experiments with this
apparatus; it is particularly necessary to be sure that all the joints are
perfectly tight before exploding the charge. Should this not be the case,
the gases upon their generation will cut their way out, or completely blow
out the part improperly secured, in either case destroying the apparatus.
The effect produced upon the apparatus when the gas has escaped by cutting
a passage for itself is very curious. The surface of the metal where the
escape occurred presents the appearance of having been washed away in a
state of fusion by the rush of the highly heated products.

~The Pressure Gauge.~--The pressure is found by the use of a little
instrument known as the pressure gauge which consists of a small chamber
formed of steel, inside of which is a copper cylinder, and the entrance
being closed by a screw gland, in which a piston, having a definite
sectional area, works. There is a gas check E (Fig. 60) placed in the
gland, and over the piston, which prevents the admission of gas to the
chamber. When it is desired to find the pressure in the chamber of a gun,
one or more of these crushers are made up with or inserted at the extreme
rear end of the cartridge, in order to avoid their being blown out of the
gun when fired. This, however, often takes place, in which case the gauges
ar