THF DRIFT OF SERVICE PROJECTILES. 
471 
continuous precessions. Now the faster the spin of the shot the slower 
the precession. So far as I can make out from Captain Shortt, he came to the 
conclusion that the faster the spin of the shot the more rapid was the precession. 
I do not think that it is so at all; I have always understood that the faster the 
spin of the shot the slower is the precession. You can work it out analytically. 
It is analytically determined in the 1883 edition of Major MacKinlay’s “ Text¬ 
book of Gunnery 55 that the greater the spin the smaller the amount of movement 
for a given impressed force; so that if we have a slower spin the shot will throw 
itself at a greater angle across the trajectory. I do not enter into the details of 
the precession, because I think, as Professor Smith says, photography would be 
a good way of determining it. 
There is one other point that I do not understand. Major MacMahon took it 
up and General Owen said something about it: it is about the straight line trajectory. 
If there is a straight line trajectory, it seems to me that the point will never 
go out of it; if you have a straight line trajectory the only one I can understand 
is firing point blank in vacuo with no gravity. Supposing we do not fire in vacuo, 
but supposing we fire in the air eliminating gravity, then the resistance of the 
air will act through the centre of gravity and there will be no gyroscopic effect 
shown and therefore no precession. That is one point that I object to, if I may say 
so, on this precession point. There was a good deal of argument based on this 
straight line theory, and I do not understand the conditions; I do not understand 
how you can reason from an impossible condition like that. 
The reason why I mentioned the point about the rapidity of precession and the 
rapidity of spin was because of the remarks that Captain Shortt makes about a 
howitzer fired at high angles of elevation. 
I did not quite understand what Professor Greenhill meant about my vortex. 
The whole drift of my argument was that the shot must go to the left under the 
conditions that I named. I do not know whether he was under the impression 
that I meant that it must go to the right. 
Captain M. B. Lloyd —When I heard some weeks ago that Captain Shortt 
was going to give us a new theory on the subject of drift, I must own that I was 
rather sceptical about it, but when I saw the argument which Major Abdy was 
good enough to send me, I confess that I could not pick any holes in it at all, so 
I set to work then and made some diagrams of a 6" B.L. howitzer trajectory; 
I thought that, as I could not pick holes in the theory, I might try and put it to 
a practical test. In Pig. 1* I have plotted to scale the line A B ( describing the 
same), which is the trajectory of a 6" B.L. howitzer with a muzzle velocity of 
779 f.s., which is the highest given velocity, and an elevation of 35° 8'. I then 
calculated from the range-table the lateral acceleration due to drift by taking- 
second differences. That follows this line, C D, that I have shown along here. It 
keeps pretty nearly constant first and then towards the end of the trajectory at D it 
suddenly shoots up like that. Then from the range-table also I plotted the cur¬ 
vature of the trajectory; it has come out a little bit sinuous, but that is due to 
the way in which it was calculated. Well, you notice that (except this part of the 
end here, which I think is wrong) it shows fairly well that the acceleration due to 
drift varies as some power of the curvature of the trajectory, and so bears out 
entirely Captain Shortt’s argument. I then took the same howitzer, a 6" 
B.L. howitzer with its lowest velocity, which is 501 f.s. and at 60° 12' elevation. 
This (pointing to it) a h , Pig. 2*, is the trajectory ; this thin red line, c d , as 
plotted, is the plan of the trajectory with the drift exaggerated about 100 times; 
this black line, e f g h, is the lateral acceleration due to drift—that is very peculiar- 
in this case, as, you see, it comes a long way off the paper hereof, and comes 
back here , g\ this blue line is the curvature of the trajectory-—it is not so sinuous 
as the other, as I actually took it off the trajectory instead of calculating it from 
See plates, 
