THE ROYAL ARTILLERY INSTITUTION. 
891 
however, the initial pressure in the bore were doubled or trebled, there 
would also be a maximum pressure double or treble the amount cal¬ 
culated.* The twist increasing from 0 to 1 in 40 would have the effect 
of giving a greater pressure towards the muzzle of the gun than one 
from 1 in 100 to 1 in 40 ; but it would have the advantage of having 
no maximum pressure at the commencement of motion, when the initial 
pressure on the base of the shot was doubled or trebled. The practical 
result, therefore, of a spiral increasing from 0 is that with a good uniform 
powder the maximum pressure is about § that with an uniform twist; 
but with a maximum pressure of twice or thrice the legitimate pressure 
in the powder-chamber, it will be reduced to J or f of what an uniform 
twist would give under similar circumstances. 
In order, therefore, to provide against all possible contingencies, an 
uniform twist ought to have four times the strength of studs or ribs to 
be as safe as one increasing from 0 to the same amount of twist with 
guns of the service length. If, therefore, an uniform twist has four times 
the bearing surface of an increasing twist, it is practically as safe. 
Comparing Captain Scott's system with the Woolwich on this subject, 
it will be seen that it is quite as safe, and indeed very much safer, for 
it has much more than four times the bearing surface. 
The shunt system has nearly—perhaps not quite—four times the bear¬ 
ing surface, and no doubt would be quite strong enough. The Whitworth 
gun has much more than four times the bearing surface. The pressure 
which produces rotation, however, by no means acts at the extremity 
of the radius of the projectile, but at between \ and I that distance, as 
will be observed by the accompanying woodcut, where the pressure 
at A —the driving edge of the groove—acting normally to the surfaces, 
has a direction AD , which practically makes BD less than one-half the 
radius BC. 
* Captain Noble takes no account of these very high pressures in his calculation. They ought 
not, however, to be omitted in the consideration of the proper rifling of our heaviest guns, in which 
they are found to a very great extent. It is because they exist that the increasing twist from 0 to 1 
in 40 is superior to one from 1 in 100 to 1 in 40; and no doubt it is for this reason that the latter 
twist has been abandoned in favour of the former. The 10-in. gun has a twist from 1 in 100 to 
1 in 40, and it will be observed that projectiles fired from this gun have the studs very much worn. 
It is worthy of remark, that both the 10-in. and 35-ton guns shoot remarkably well; the latter gun 
has a twist commencing from 0. 
