TJIE DRIFT OF SERVICE PROJECTILES. 
404 
left, because the radius vector is shortening from 0 B to 0 D (Fig. 4). 
After 0 B it lengthens out again. 
Now, as regards amounts in a very low velocity howitzer, the point 
has just enough velocity and precession to get round at all; in some of 
the very high elevations for experimental purposes which have been 
given occasionally, as much as 60° to 70° it has not enough velocity of 
precession, which means that the trajectory is changing direction faster 
than the point can follow it down. The result is that it gets left 
behind and generally, with a howitzer of low velocity, you may take it 
that it has only just enough precession to get round at all, simply 
because, as I say, with a smaller charge and a higher elevation, it is 
found that the velocity of precession is not enough and the shot turns 
broadside. In a high velocity gun, on the other hand, the spin of rifling 
is faster and therefore the precession is faster and the trajectory does 
not change its direction so fast. The result is that the precession will 
have much less difficulty in passing the first quarter and the curve will 
more nearly approximate to a circle, that is to say there will be less 
difference between the size of the curve on the right and that on the 
left. The preponderance, however, will always be on the right, but, 
as I say, the lower the velocity and, therefore, the greater the curva¬ 
ture, the greater preponderance there will be on the right in point of 
time and the wider will it go, because this radius vector lengthens out 
more; so that you have two things. First of all you have that the 
time is very much greater on the right than on the left and then you 
have the radius vector lengthening out on the right very much more 
than it can on the left. 
Well, gentlemen, that is the whole of the theory. The effect of it 
on drift is a matter of opinion, of course, but it seems (which is true 
in practice) that with high velocity guns you get very little drift; 
there will be a differential effect between the curve on the right and 
that on the left, which will be small. If the trajectory were perfectly 
straight, as I say, there would be no drift at all, because it would lie 
evenly on each side. But another effect noticeable in ordinary shoot¬ 
ing is the periodic sound that the shot makes in flight and that, I 
believe, is almost always noticed, at all events I think I have always 
noticed it myself. That I put down to the change of angle that the 
axis makes with the trajectory : if the point is wide from the trajec¬ 
tory you get a much louder noise in flight than if the point is nearly, 
in the trajectory; the wider the point is away from the trajectory I 
think you must get a louder noise, so that periodic sound, loud and 
soft, in flight would seem to imply a period in which the point is first 
wide and then not so wide and then wide again. This periodic sound 
corresponds with the theory of the shot being wider on one side than 
the other exactly and, as far as I remember, in the 12 Pr. with which I 
was serving, the time of one “ period ” would be about one-half second. 
In a howitzer, on the other hand, of low velocity I believe it has been 
noticed that the shell goes with its point very wide to the right and if it 
has not got enough spin it turns broadside shortly after the top of the 
trajectory. If it has enough spin to overcome that it will go down and 
