PRINCIPLES OF GUNNERY. 
49 
The denser the grain the more powder there is in it for a given size, 
and consequently the time of combustion is longer. Other things being 
equal, the denser the powder the slower burning it is, and the less the 
pressure in the bore of the gun. 
The amount of moisture in the powder has the effect of modifying 
its time of explosion. The accompanying table shows the effect of 
moisture in powder : as the per-centage of moisture increases, the time 
of explosion is lengthened, and the pressure in the gun and the muzzle 
velocity of the projectile decreased. 
Table showing the Effect of Moisture in Powder on Muzzle Velocity and 
Pressure in the Gun. 
Sample of 
powder. 
Per-centage of 
moisture. 
Muzzle velocity 
in ft. secs. 
Maximum pressure 
in the bore (ft. tons). 
r 
•8 
1537 
21*38 
1-0 
1524 
20-18 
Pebble, fin....* 
1-2 
1512 
19-12 
1 
L 
1-4 
1502 
18-18 
The glazing and hardness of the grains retard the ignition of the 
powder, and consequently increase the time of explosion. 
The time of explosion of powder in a gun is further affected by the 
form of cartridge, diametpr of the powder-chamber, amount of air¬ 
space, and the point of ignition of the cartridge. ( Vide Chapter IV.) 
With large charges for heavy guns, it becomes especially important, 
in order to keep down the pressure within limits of safety, to increase 
the density and size of grain. The time of explosion is thus increased, 
and for a given charge both the muzzle velocity of projectile and the 
maximum pressure in the bore are decreased. 
Table showing Density and Size of Grain of Powders used with Rifled Ordnance. 
Description of powder. 
Size of grain. 
Mean 
density. 
R.L.G. 
4 to 8 meshes to an inch. 
1-68 
Pebble (P.)... 
f-in. cubes. 
1-80 
Cubical (P. 2 ) . 
1 ^-in. cubes. 
1-78 
Cubical powder (P. 2 ) was introduced for employment in the very large 
charges used with the 38 and 80-ton guns; since with ordinary pebble 
(P.) powder, owing to the effect of the heat and pressure developed in 
the partial explosion of such large charges, the time of explosion would 
be diminished, and the local pressure m the powder-chamber would 
exceed safe limits. 
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