ROTATION OP PROJECTILES. 
I'll 
General Conclusions. 
From the formula obtained for the value of n we derive the following 
conclusions; bearing, however, in mind that the article deals only 
with stability of rotation, not with other matters affecting the shooting 
of a gun :— 
1. The twist required to be given to the rifling of a gun is, for 
similar projectiles of the same material, independent of the calibre of 
the gun when the twist is given in the usual form—viz., one turn in 
% calibres. 
In other words: if a given gun has twist enough to give stability of 
rotation to a given projectile, any other gun firing similar projectiles 
of the same material will also give sufficient stability to its projectiles, 
provided the twist of the rifling of the guns, reckoned in calibres, be 
the same. 
We thus see that the usual form of expressing the twist of rifling as 
one turn in so many calibres is a truly mathematical one. 
2. It matters not whether the rifling have an uniform or an in¬ 
creasing twist, as far as giving stability of rotation to the projectile is 
concerned. Obviously the projectile leaves the bore of the gun with 
the angular velocity due to the rifling at the muzzle; and in this 
respect it is indifferent whether the rifling has the twist at the muzzle 
all through the bore, or whether the twist gradually increases towards 
the muzzle. Hence, also, in this respect the length of the gun is 
immaterial. 
3. Differences of muzzle velocity make no difference in the twist 
required to give stability of rotation, on the hypotheses of incom¬ 
pressibility and absence of friction in the medium. If a gun with 
muzzle velocity V gives sufficient spin to its projectile, then that 
gun will give sufficient spin to that projectile when the latter has a 
muzzle velocity either less or greater than V. It is true that an 
increased velocity of translation requires a greater velocity of rotation, 
but the additional velocity of rotation required increases in precisely 
the same ratio as the velocity of translation increases. Obviously the 
angular velocity varies directly as the linear velocity; thus the two 
go, as it were, hand-in-hand, whatever be the value of V. 
An error has crept in here. The effect of increase of spin is not to Page 589 
diminish but to increase drift. There are considerations which render 
it desirable for ordnance fired at a high angle to have a quick twist; 
but as far as stability of rotation of the projectile is concerned, the 
quicker twist in howitzers is not necessary. 
The density (10*9) here given is that of the Martini-Henry bullet, Page 59 i 
which contains 12 parts of lead and 1 part of tin by weight. 
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