362 
PRINCIPLES OF GUNNERY. 
(3) On the position of the centre of gravity of the projectile. 
(4) On the length of the projectile in calibres. 
(5) On the distribution of the mass of the projectile. 
Effect of If the projectile is centred as it leaves the bore, there will be no 
tring. en " force tending to cause rotation round its shorter axis until gravity 
deflects the trajectory from the line of departure, and in this case the 
force is gradually applied, and in a constant direction, producing 
similar steady gyration; but if the projectile be not centred, there 
will be at once on leaving the bore a force causing rotation round its 
shorter axis, acting in an uncertain direction, and producing dissimilar 
unsteady gyration.* In order to keep the semi-amplitude of the gyra¬ 
tion small, it is necessary to give a higher velocity of rotation than 
would have been required if the projectile had been centred; so that 
the axis of the projectile may be kept as nearly as possible in the 
direction of motion of the centre of gravity of the projectile, or, in 
other words, tangential to the trajectory. If the velocity of rotation 
is not sufficient, the semi-amplitude of the gyration will increase, and 
the projectile becomes unsteady, and wabbles in flight. In extreme 
cases the projectile will turn over; as, for instance, in long shell when 
fired with a low velocity. 
Effect of The effect of the resistance of the air, and the point of application of 
feceofthe that resistance on projectiles of different shapes, will next be considered. 
air. 
Fig. 1. 
Fig. 1 represents a vertical projection of an ogival-headed projectile, 
G the position of its centre of gravity in its longer axis, GA.. The 
direction of motion of the projectile is represented by Gl\ being a 
tangent to the trajectory, which has dipped from the line of departure, 
which if the projectile is centred is coincident at the muzzle with the 
axis of the projectile, GA. 
* Gyration must be understood to mean tbe rotation of tbe axis of tbe projectile itself round the 
trajectory described by its centre of gravity. The semi-amplitude of the gyration is the angle 
which the axis of the projectile makes with the tangent to the trajectory at that point. 
