CAUSES OF DRIFT. 243 
oblongs,” of 1873, “ Les explications qu’on a tenté d’en donner (du phéno- 
mene q“rv’on n'avait pas prévu, la dérivation) longtemps controverses sont 
awjoud’ hut acquises @ la science, et tl west pas permis aux hommes du 
métier de les ignorcr.”’ Captain Cooper has designed an apparatus which 
is before us on the table, and he has kindly promised to shew it in 
action at the end of the proceedings. The spinning projectile is here 
represented by a gyrostal enclosed in a box on wheels, running back- 
wards and forwards on the track representing a switch-back road, which 
guides the truck in a sort of inverted trajectory. When the gyrostal is 
not spinning the truck runs backwards and forwards as usual, but when 
we spin the gyrostal it swerves to one side or the other according as it_ 
is moving on the concave or the convex portion of the track. On the 
short straight part at the outset there is no deviation. We invite the 
company present to frame their theories @ priort as to the direction 
which the truck will take, and then we can put their theories to the 
test of experiment. However, the swerving effect being due to the 
curvature of the trajectory, it seems that it is this curvature which has 
an important influence on the drift, as is noticeable especially in high 
angle fire. J hope we shall have the benefit of the expression of his 
opinions from the Professor of Artillery, Major Curteis, whom I see in 
the room. We are engaged at present in bringing out a new edition of 
Major Mackinlay’s “Text Book of Gunnery,” so that General Owen’s 
paper is very opportune for us ; but in spite of the unfavourable opinion 
of General Owen of the brevity of the treatment of the subject by 
Major Mackinlay, I must confess that we are tempted to adhere to the 
treatment that is given by Major Mackinlay, for fear of being led too 
deep into a subject in which uncertainty seems still to reign to so great 
an extent. (Applause.) 
CaPpTraIn MAURICE B. LLOYD,R.A.—There are one or two points, sir, that 
struck me whilst considering the lecturer’s demonstrations, particularly 
in the first part of his mathematical treatment of it. One was why he 
should take the resistance of the air as acting midway between the two 
lines bounding the surface swept out. This surely would not be the 
case with an ogival projectile, though it may be so with the flat-headed 
projectile ; but in any case the theory upon which that resistance is 
taken is rather an exploded theory I think nowadays, as it only allows 
for the impact of the shell against the particles of air in front of it.* 
Now there is a very large force produced by the suction of the air upon 
the base of the shell, which was shewn by Professor Boys’ photographs 
of bullets in motion, where the wake of the bullet is marked by almost 
as strong a line as the bow wave, if I may so call it, of the compression 
of the air; but that force has not been taken into consideration at all 
in General Owen’s treatment of the matter. Of course it is very diffi- 
cult indeed to allow for a force of that sort which you have no means 
of measuring. 
Again, there is one point that I should like to draw attention to and 
that is about a golf ball. A golf ball exhibits the most extraordinary 
drift. (Langhter.) Everyone knows when you slice a golf ball it will 
go to the right, whereas if you toe it, it will go to the left. In either 
case this cannot be due to gyrostatic action because the golf ball is a 
sphere, and a sphere gives no gyrostatic action under any circumstances, 
* Taking this view of the resistance of the air assumes it to vary as the square of the 
velocity which is only true between certain restricted limits, _ 
