PRINCIPLES OP GUNNERY. 57 
charge of 60 lbs. E.L.G. powder, the work done by the powder is 
increased 5 per cent, when tbe weight of the projectile is doubled; 
with a charge of 70 lbs. P. powder, the work done by the powder is 
increased 84 per cent, when the weight of the projectile is doubled. 
In a 3-in. M.L. gun, with a charge of 2 lbs. E.L.G. powder, the work 
done by the powder was increased 14 per cent, when the weight of 
the projectile was doubled. 
It is evident that when the weight of the projectile is increased, it 
takes a slightly longer time to move through the bore; so that more 
time is given for the pressure to act, and consequently a greater useful 
effect is obtained from the charge, and the factor of effect is increased. 
Approximately correct muzzle velocities may be calculated on this 
principle for projectiles differing slightly in weight when fired from 
similar rifled guns with the same charge. Thus, if the weight ( W) 
and muzzle velocity [V) of a projectile be known with a particular 
gun, then the muzzle velocity (V) of another projectile of known 
weight ( W) fired from a similar gun with the same charge may be 
approximately found by the proportion 
It will be seen from formula (2) that if K (the work done per 
pound of powder) were constant, 
.< 5 > 
or the muzzle velocity of a gun would vary directly as the square root 
of the weight of its charge, and inversely as the square root of the 
weight of its projectile. But it has been shown that the work done 
per pound of powder depends on the number of volumes of expansion 
of the powder charge contained in the bore of the gun; so that this 
formula is of no practical use for the calculation of muzzle velocity 
when the weights of charges vary in the same gun, because the 
smaller charge has more expansions than the larger when fired in the 
same gun. 
A more practical way of approximating to the muzzle velocity of a 
gun, when the weight of charge and projectile are known, is as 
follows, by means of Noble's table (p. 55) :— 
Example.— Find approximately the muzzle velocity of the 38-ton 
gun of 12*5 ins., when firing an 800-lb. projectile with 110 lbs. of 
P. powder. (Length of bore 198 ins.) 
The number of volumes of expansion of charge 
= * X (12- 5p X 198 
4 x 110 x 27*7 
then by the table (p. 55) the maximum work capable of being per¬ 
formed by the charge is 
110 x 99*52 = 10047-2 ft. tons. 
Approxi¬ 
mation to 
the muzzle 
velocity 
of a gun, 
by means 
of Noble’s 
tables. 
8 
