476 
THE Dill FT OF SERVICE PROJECTILES. 
borne out in tlie case of the 8" howitzers of 46 cwt. and 70 cwt., taken from the 
text-book of gunnery:— 
Howitzer 
Weight 
of 
Projectile 
Time 
Elevation 
of 
Plight 
Drift 
f 46 cwt. 
8"] 
(.70 „ . 
13° 5' 
185 lbs. 
9-6 
27-6 
13° 14' 
180 „ 
9-0 
18-0 
The 46 cwt. gun lias a twist of 1 in 16, the 70 cwt. gun 1 in 35, and the drift is 
very much greater in the case of the one with sharper twist; but, in this case, the 
rifling is the only factor in which the two guns differ, and the result of the differ¬ 
ence of twist is directly shown. Generally, however, other forces come into play, 
and from a formula Captain Mansell quoted, the time of a complete precession 
X directly as the angular velocity due to rifling; 
X inversely as g , which in this case is the retardation to the projectile’s 
flight, and is not, therefore, a constant force; 
and X directly as A , which is a moment of inertia, and increases largely with 
the size of the projectile. 
Each of these three factors directly influence drift therefore, and it will be seen 
that, while the angular velocity of rifling varies pretty much with the muzzle 
velocity (except in the extreme case mentioned above), the second factor—the 
retardation—varies as the (velocity) 3 , so that with projectiles of the same weight 
the drift will generally diminish with higher muzzle velocities, and the precession 
be faster. 
The third factor shows why drift is so much smaller with small projectiles, as 
A will increase roughly as the 5th power of the calibre, the drift of a rifle being 
practically nil. 
The Chairman —Professor Smith was asking to-night about photographs of 
projectiles. It is true that we have photographs of projectiles in the air, but I 
do not think we have any satisfactory results and it seems a great pity that we 
should not, because a great many howitzer shells are fired at low velocity at 
Shoeburyness and elsewhere and it only requires the camera to get a photograph of 
them. I do not know who is the proper person to start the thing; perhaps the 
Committee of the Institution might take it up. 
Nothing now remains for me, I think, but to thank Captain Shortt for the very 
interesting lecture that he has given us and for the very interesting discussion 
which it has led to (loud applause). 
