Volume 3, Part 1, Slice 2 Part 32 (2/2)

In Sir Andrew n.o.ble'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 pa.s.sage 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 n.o.ble's procedure of gradually lengthening the gun.

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

[Ill.u.s.tration: 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.0277] The result is plotted in figs. 8 and 9, in a curve showing the relation between p and D the _gravimetric density_, which is the specific gravity of the P lb of powder when filling the volume C, cub. in., in a state of gas; or between p and v, the reciprocal of D, which may be called the _gravimetric volume_ (G.V.), being the ratio of the volume of the gas to the volume of an equal weight of water.

[Ill.u.s.tration: FIG. 7. Explosion Vessel.]

The results are also embodied in the following Table;--

TABLE 1.

Pressure in Tons per sq. in.

G.D. G.V. Pebble Powder. Cordite.

0.05 20.00 0.855 3.00 6 16.66 1.00 3.80 8 12.50 1.36 5.40 0.10 10.00 1.76 7.10 12 8.33 2.06 8.70 14 7.14 2.53 10.50 15 6.66 2.73 11.36 16 6.25 2.96 12.30 18 5.55 3.33 14.20 20 5.00 3.77 16.00 22 4.54 4.26 17.90 24 4.17 4.66 19.80 25 4.00 4.88 20.63 26 3.84 5.10 21.75 30 3.33 6.07 26.00 35 2.85 7.35 31.00 40 2.50 8.73 36.53 45 2.22 10.23 42.20 50 2.00 11.25 48.66 55 1.81 13.62 55.86 60 1.66 15.55 63.33

The term _gravimetric density_ (G.D.) is peculiar to artillerists; it is required to distinguish between the specific gravity (S.G.) of the powder filling a given volume in a state of gas, and the specific gravity of the separate solid grain or cord of powder.

Thus, for instance, a lump of solid lead of given S.G., when formed into a charge of lead shot composed of equal spherules closely packed, will have a G.D. such that

(4) G.D. of charge of lead shot --------------------------- = 1/6 [pi] [sqrt]2 = 0.7403; S.G. of lump of solid lead

while in the case of a bundle of cylindrical sticks of cordite,

(5) G.D. of charge of cordite -------------------------- = 1/6 [pi] [sqrt]3 = 0.9067.

S.G. of stick of cordite

At the standard temperature of 62 F. the volume of the gallon of 10 lb of water is 277.3 cub. in.; or otherwise, 1 cub. ft. or 1728 cub. in. of water at this temperature weighs 62.35 lb, and therefore 1 lb of water bulks 1728 62.35 = 27.73 cub. in.

[Ill.u.s.tration: FIG. 8.]

Thus if a charge of P lb of powder is placed in a chamber of volume C cub.

in., the

(6) G.D.= 27.73P/C, G.V. = C/27.73 P.

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