366 
PRINCIPLES OF GUNNERY, 
the pressure is applied on the head above the axis, the axis of the shot 
will deflect to the left; and finally, to complete the gyration, if the 
pressure is applied on the right front, the axis of the shot will rise. 
The pressure of the pencil-point corresponds with the direction of the 
resultant of the resistance of the air in the four quarters of gyration. 
Driftofan It will be seen that when the projectile is in motion and in the first 
headed and second quarters of gyration, it is driyen to the right , and in the 
projectile, third and fourth quarters of gyration the projectile is driyen to the 
left , by the resistance of the air; but as the time taken in the first 
and second quarters of gyration is longer than that taken in the third 
and fourth quarters,* the drift or derivation of the projectile is on the 
whole to the right of the plane of departure. 
If /^-handed rotation is impressed on the shot in the gyroscope, 
gyration under similar conditions will take place in the opposite 
direction, the axis of the shot first deflecting to the left. 
If a projectile has left-handed rotation impressed upon it, gyration 
takes place in the opposite direction, and the drift will be in the 
first and second quarters of gyration to the left of the plane of 
departure, and in the third and fourth to the right; but as the time 
taken in the first and second quarters of gyration is longer than in 
the third and fourth quarters of gyration, the drift on the whole will 
be to the left. 
In the above description of the spiral motion of an elongated ogival¬ 
headed projectile, the projectile has been supposed to be centred; 
but if this should not be the case gyration commences at once, and in 
an uncertain direction, and the axis of the projectile will be deflected 
at once, according to the position of the point on which the resultant of 
the resistance of the air acts on the projectile, and the projectile itself 
driven bodily in the same direction. Hence the shooting" of guns in 
which the centring of the projectile is not secured cannot be relied upon 
for such accuracy as those guns in which the projectile is centred, 
because the drift cannot be so accurately allowed for. 
Since the use of copper gas-checks in the 12*5-in. M.L. gun of 
38 tons, the projectile has been centred, and the mean drift to the 
right for a given range has increased, and the shooting has been more 
accurate. 
Drift of a The drift of flat-headed projectiles is not so marked as that of ogival- 
projectiie. headed projectiles, and owing to the higher velocity of rotation required 
to keep flat-headed projectiles steady in flight, there is a difficulty in 
obtaining reliable experiments with flat-headed projectiles with service 
guns. Colonel Owen, R.A., made some experiments at Shoeburyness, 
in 1864, with flat-headed and cylindro-conoidal-headed shot fired from 
the Armstrong B.L. gun and the conclusion drawn from them is 
* It may be noticed, also, that the resistance which drifts the projectile from the direction of 
motion is greater during the first and second quarters of gyration than during the third and fourth 
quarters, owing to the greater velocity which the projectile has at the first part of its trajectory. 
f Vide a paper on the “Derivation of Elongated Projectiles,” (“Proceedings, R.A, Institution,” 
Vol. IV. p. 180); also Owen’s “Modern Artillery,” p. 257. 
