THE EOYAL ARTILLERY INSTITUTION. 381 
produce a second rotation round an axis perpendicular to the first. If 
the amount of original rotation be low, the shot will begin to gyrate with 
a smaller deflection of axis than if the rotation be great. A low rotation, 
therefore, is to be preferred to a high rotation, as keeping the point closer 
to the trajectory. There are some causes, however, which interfere 
with the possibility of giving a very low rotation. In the case of the 
top, we have seen that, when the rotation is low, the gyrations become 
larger and larger. The same is to be observed in firing rifled service 
shot with too low rotations. They seem to go steadily at first, but 
afterwards begin to wabble as they proceed. In the case of the 
top, we saw that the friction of the peg on the ground tended to 
diminish the size of the gyrations, but in the case both of cylindrical* 
and service shot, the friction of the air tends to increase their size; for, 
according to the principles investigated by Robins and Magnus, the 
rotating surface carries with it a current of air which meets the re¬ 
sistance of the air in different directions, and the side of the head which 
leads the gyration and is rotating inwards, meets a greater resistance 
than the opposite side which is rotating outwards. In order, therefore, 
that the gyrations may not become too large before the end of the 
trajectory is reached, a sufficient amount of rotation must be given. 
The exact amount required is a matter well worthy of consideration, 
and varies in different guns, not only according to the length and 
diameter of the projectile, but also according to the amount of “cen¬ 
tring 99 the shot has when it leaves the gun. With equal masses, the 
relative stabilities of shot varying in length may be expressed inversely 
as the twist in calibres; but the longer projectile allows the resistance of 
the air to act with a greater leverage,f and increased rotation must be 
given to compensate for this. After the necessary allowance has been 
made, the main cause which prevents a very low rotation being 
given is, that a shot rarely ever leaves the gun exactly centred. If 
the shot come out not centred, but having an initial deflection of the 
axis, it at once commences to gyrate, and that also at the time when the 
resistance of the air is the greatest; so that when a properly centred 
projectile may be only at its first gyration, a projectile not properly 
centred may have reached its second or third, under the adverse cir¬ 
cumstances of increased resistance of the air which the other would 
have escaped. In order, therefore, to prevent the large gyrations being 
attained too soon, an increased rotation has to be given. 
* “Proceedings O.S.C.,” 19th September, 1862, Minute 7381; and 8th May, 1863, Minute 9015. 
The greater unsteadiness of cylindrical shot compared with service shot is probably caused by the 
fact that the flat head causes greater resistance of the air, and the friction acts at a greater leverage. 
This is not inconsistent with the supposition that cylindrical flat-headed shot deflect to the left; 
because, though the resultant action of the resistance of the air might act behind the centre of 
gravity, the action of the friction would probably still be felt most at the head, where the air was 
most, though uniformly^ condensed all round; The true deflection of flat-headed projectiles cannot 
as yet be said to be satisfactorily determined; 
f This greater leverage is, however, compensated by the smaller surface of the head which is 
exposed to the resistance of the air. It is probable, however, that the leverage increases more 
rapidly than the length of the shot. "With the Snider bullet the resultant action of the resistance 
of the air must have acted very nearly through the centre of gravity, otherwise the low spin would 
not have been sufficient to ensure the tolerable accuracy of fire which has been obtained from it. 
