390 
MINUTES OF PROCEEDINGS OE 
Then, in a cylinder of r — radius and M — mass, for one turn in 
n calibres, 
6 
x— nr-, 
7r 
But 
and 
therefore 
and 
Thus, with an uniform twist of 1 turn in 40 calibres, the force pro¬ 
ducing rotation at the surface of the shot is 4 per cent, of the total 
pressure of the gas at any moment on the base of the shot. The useful 
effect of the force of translation absorbed is, however, the resolved part 
of this along the bore, or about T3 of 4 per cent.— i.e., J per cent, 
nearly. The friction, however, has to be added to this, which, sup¬ 
posing it to be | of the pressure, gives 1 per cent, in addition; thus 
using altogether 1J per cent of the force of translation. Thus, with a 
60,0-lb. shot in a 12-in. bore, with 20 tons per square inch of maximum 
pressure of powder, the pressure producing rotation would be 90 tons ; 
with a 15-in. gun and 1200-lb. shot and 25 tons pressure, it would be 
180 tons. If we suppose excessive pressures, such as often occur in the 
bores, of twice or thrice the amount supposed, these pressures will be 
doubled or trebled. 
The effect of the increasing twist is to make the amount of pressure 
depend, not on the amount of maximum pressure of the powder, but 
on the amount of rotation to be produced, distributing the pressure 
more uniformly along the whole bore of the gun, and making it less 
as the length of the gun is increased. With the length in use with our 
service guns, and with the twist used in the 10-in. gun—from 1 turn in 
100 to 1 in 40 calibres—it has been shown by Captain Noble,* late R.A., 
now of Elswick, that the pressure is uniformly distributed, with the 
pressures found by the Committee of Explosives to exist in that gun, and 
does not exceed half of what it would be if the twist were uniform. If, 
d 2 x ^ nr d^0 
~dfi ~ 
dW __ pr __ 2p 
W ” ~ Mr* 
<Px _ P 
clfi ~ M’ 
F nr 2p 
M~w X Mr’ 
P = 
_ 2 np 
= 25^?, if n — 40. 
* “ On the Pressure required to give Rotation to Rifled Projectiles,” by Captain Noble, F.R.S., &c., 
late R.A. “ Proceedings, R.A. Institution,” Vol. VIII. p. 359. 
