e usually found a few yards in front of the muzzle. The copper cylinders
which register the pressure are made 0.5 inch long from specially selected
copper, the diameters being regulated to give a sectional area of either
1/12 or 1/24 square inch.

[Illustration: FIG. 60.--CRUSHER GAUGE. _E_, GAS CHECK.]

Hollow copper cylinders are manufactured with reduced sectional areas for
measuring very small pressures. It has been found that these copper
cylinders are compressed to definite lengths for certain pressures with
remarkable uniformity. Thus a copper cylinder having a sectional area of
1/12 square inch, and originally 1/2 inch long, is crushed to a length of
0.42 inch by a pressure of 10 tons per square inch. By subsequently
applying a pressure of 12 tons per square inch the cylinder is reduced to
a length of 0.393 inch. Before using the cylinders, whether for
experimenting with closed vessels or with guns, it is advisable to first
crush them by a pressure a little under that expected in the experiment.
Captain Sir A. Noble used in his experiments a modification of Rodman's
gauge. (Ordnance Dept., U.S.A., 1861.)

~By Calculation.~--To calculate the pressure developed by the explosion of
dynamite in a bore-hole 3 centimetres in diameter, charged with 1
kilogramme of 75 per cent. dynamite, Messrs Vieille and Sarrau employ the
following formula:--

P = V_{o}(1 + Q/273._c_)/(V - _v_).

Where V_{o} = the volume (reduced to 0 deg. and 760 mm.) of the gases produced
by a unit of weight of the explosive; Q the number of calories disengaged
by a unit of weight of the explosive; _c_ equals the specific heat at
constant volume of the gases; V the volume in cubic centimetres of a unit
of weight of the explosive; _v_ the volume occupied by the inert
materials of the explosive. The volume of gas produced by the explosion of
1 kilogramme of nitro-glycerine (at 0 deg. and 760 mm.) is 467 litres.

V_{o} will therefore equal 0.75 x 467 = 350.25.

The specific heat _c_ is, according to Sarrau, .220 (_c_); and according
to Bunsen, 1 kilogramme of dynamite No. 1 disengages 1,290 (Q) calories.
The density of dynamite is equal to 1.5, therefore

V = 1/1.5 = .666.

If we take the volume of the kieselguhr as .1, we find from above formula
that

P = 350(1 + 1290/(273 x .222))/(.600 - .1) = 13,900 atmospheres,

which is equal to 14,317 kilogrammes per square centimetre. The pressure
developed by 1 kilogramme of pure nitro-glycerine eq