CAUSES OF DRIFT. 221 
an elongated projectile, and as the effect of the pressure differs 
according to the shape of the head, both a conoidal point anda flat head are 
given. 
i It is evident that immediately after leaving the bore of a gun, the 
shot commences to descend, and the centre of gravity must follow a 
downward course ; consequently the resultant of the air’s resistance 
must act below its point if conoidal, or the centre of the head if a flat- 
headed. 
In Figs. 1 and 2, R acts below a, and is half-way between the dotted 
lines, which inclyde between them a space representing that occupied 
by the opposing current of air. 
Ione. IL. 
P represents the force of projection. 
99 59 gravity. 
RR -- resultant of the air’s resistance, which must be parallel 
to the trajectory AB described by G, the centre of 
gravity of the shot. 
The effect of R in Fig. 1, is to give the shot a rotation round its 
shorter axis, the point being turned up, as shown by the dotted lines. 
In fact, a pressure exerted anywhere, and at any angle, between a and 3, 
that is, before and below the centre of gravity G, will raise the point ; 
and a force exerted behind and below G, between 6 and ¢, will depress 
the point, no matter whether ¢ is placed in the middle, or nearer either 
end of the shot. 
Now, in Fig. 2, the pressure R will not raise, but depress the head, as 
shown by the dotted lines; and if R acts anywhere between a and 8, 
the same effect will be produced, but if R acts between b and c, the 
head will be raised, as with the cylindro-conoidal shot in Fig. 1. 
The way in which the resultant of the air’s resistance acts upon the 
shot, may be further explained thus—Resolve 2 into two forces, O and 
S, Fig. 3, the latter acting in the direction of a tangent to the curve, 
and representing the friction between the air and the surface of the 
