rbola with center at F'.

These theorems may prove useful in preliminary calculations where the
pressure-curve is nearly straight; but, in the absence of any observable
law, the area of the pressure-curve must be read off by a planimeter, or
calculated by Simpson's rule, as an indicator diagram.

To measure the pressure experimentally in the bore of a gun, the
crusher-gauge is used as shown in fig. 6, nearly full size; it records the
maximum pressure by the compression of a copper cylinder in its interior;
it may be placed in the powder-chamber, or fastened in the base of the
shot.

In Sir Andrew Noble's researches a number of plugs were inserted in the
side of the experimental gun, reaching to the bore and carrying
crusher-gauges, and also chronographic appliances which registered the
passage of the shot in the same manner as the electric screens in
Bashforth's experiments; thence the velocity and energy of the shot was
inferred, to serve as an independent control of the crusher-gauge records
(figs. 4 and 5).

As a preliminary step to the determination of the pressure in the bore of a
gun, it is desirable to measure the pressure obtained by exploding a charge
of powder in a closed vessel, varying the weight of the charge and thereby
the density of the powder-gas.

The earliest experiments of this nature are due to Benjamin Robins in 1743
and Count Rumford in 1792; and their method has been revived by Dr Kellner,
War Department chemist, who employed the steel spheres of bicycle
ball-bearings as safety-valves, loaded to register the pressure at which
the powder-gas will blow off, and thereby check the indications of the
crusher-gauge (_Proc. R.S._, March 1895).

Chevalier d'Arcy, 1760. also experimented on the pressure of powder and the
velocity of the bullet in a musket barrel; this he accomplished by
shortening the barrel successively, and measuring the velocity obtained by
the ballistic pendulum; thus reversing Noble's procedure of gradually
lengthening the gun.

But the most modern results employed with gunpowder are based on the
experiments of Noble and Abel (_Phil. Trans._, 1875-1880-1892-1894 and
following years).

[Illustration: FIG. 6.]

A charge of powder, or other explosive, of varying weight P lb, is fired in
an explosion-chamber (fig. 7, scale about 1/5) of which the volume C, cub.
in., is known accurately, and the pressure p, tons per sq. in., was
recorded by a crusher-gauge (fig. 6).

[v.03 p.