364 
PRINCIPLES OP GUNNERY. 
service conditions. If, on tlie contrary, tlie centre of gravity is towards 
the head, this distance is smaller, and a lower velocity of rotation 
suffices to keep the projectile steady in flight ; for instance, in the 
Snider bullet the centre of gravity is more towards the head than it 
would be if the bullet were solid, and the very low velocity of rotation 
imparted to it ensures tolerable accuracy of fire. 
length °f Generally speaking, the longer a projectile is in calibres for a given 
jectSe?' weight, the greater will be the velocity of rotation required to keep its 
axis close to the trajectory; because the resultant of the resistance 
usually acts with a greater leverage— i.e., the moment of the couple 
tending to produce rotation is increased as the length of the projectile 
is increased. But it is quite possible, with some shapes of head and 
positions of centre of gravity, for the moment of the couple to be less 
with a longer projectile than with a shorter one of the same weight. 
In that case a greater velocity of rotation may be dispensed with. 
The principle to be remembered in considering the velocity of rotation 
to be given to an elongated projectile in order to keep it steady in 
flight is this—that the velocity of rotation must be increased propor¬ 
tionately to the moment of the resultant resistance about the centre of gravity 
tending to rotate the projectile round its shorter axis. 
Distribute The rate of loss of velocity of rotation owing to the resistance of the 
mass°of the air depends on the distribution of the mass of the projectile. The 
projectile. ve l 0 city of rotation is maintained longer in the case of an elongated 
shell than in that of an elongated shot of given weight. Hence it is 
found in practice that an elongated shell is steadier in flight than an 
elongated shot of the same weight; the reason being that the radius 
of gyration is longer in the shell, and for a given velocity of rotation 
the energy of rotation is greater. (Vide p. 40.) 
motionThe direction of the rotation given to all service elongated pro- 
an > eion- jectiles is “ right-handed 99 — i.e., the upper part turns from left to 
fectiie? 10 ” p ight, with reference to an observer placed behind the gun. If the 
projectile is accurately centred, there will be no tendency to gyrate 
until gravity deflects the trajectory from the line of departure, and the 
direction of motion of the projectile ceases to be coincident with its 
axis. When this is the case, the resultant of the resistance of the air 
acts as in Fig. 1, tending (as explained before) to raise the point of 
the projectile, and to give it rotation round the shorter axis through 
the centre of gravity.* The combination of these rotations, in 
obedience to a well-known law in Mechanics, causes the axis of the 
projectile to gyrate in a right-handed direction; and the resistance of 
the air acting on the left side of the projectile drives the projectile 
itself bodily towards the right. 
* Vide a paper by Major Morgan, R.A., on the “ Rifling for Heavy Guns,” read at tlie United 
Service Institution on May 19th, 1873,- also a paper by General Mayevski in the “Revue de 
Technologic Militaire,” Tome V. p. 13. 
