How to obtain knowledge of the bullet’s rotation.
If, however, we would wish to obtain some idea of the rotatory motion of a bullet in its path through the air, By the arrow.let us consider the action of the arrow, and see how it is constructed, so that the resistance of the air should not act in an unfavourable manner. First, nearly all its weight is concentrated at the point, so that its centre of gravity is close to it. Use of feathers on arrows.At the opposite end feathers are placed, the heaviest of which does not affect the centre of gravity, but gives rise to an amount of resistance in rear of the projectile, and which prevents its ever taking a motion of rotation perpendicular to its longer axis, and keeps it in the direction of its projection. This difficulty which the arrow finds in changing its direction must concur in preventing its descending so rapidly as it would do were it only to obey the law of gravity, and must therefore render its trajectory more uniform.
Similar effects on bullet with grooves.
Let us, however, now come back to the grooves of Mons. Tamisier, and we shall find that they concur in giving to the bullet the two actions of the resistance of the air, which we have demonstrated with respect to the arrow.
Suppose that such a bullet describes the trajectory M, and A.B. be the position of its axis, it will be seen that the lower part of the bullet re-establishes the air compressed, whilst the upper part finds itself in the rarified air. That, consequently the lower parts of the cannelures are submitted to the direct action of the air’s resistance, whilst their upper parts totally escape this action. ([Plate 21], fig. 2). Effect of grooves.The resultant of the air’s resistance evidently tends to bring back the point of the moving body, according to the trajectory; but as this action is produced by the pressure of an elastic fluid, it results that the point B, after having been an instant upon the trajectory, will fall below, in virtue of the velocity acquired; but then the upper grooves finding themselves acted on by the action of the air’s resistance, this action, joined to its weight, will force the point of the projectile upwards, which will descend to come up again, so that the projectile will have throughout its flight a vertical swing, which is seen distinctly enough in arrows.
Union of Robins and Tamisier.
Let us connect the suggestion of Robins, with the experiments of Captain Tamisier, to cause the posterior end to act as a rudder to guide the projectile in its true path, as undoubtedly during the descent of a bullet there is a tendency for the centre of gravity to fall first, as the ball of the shuttlecock. In the first Prussian balls, and in those used in the Tige, the centre of gravity being nearer the base, the rear end of these balls have a tendency to fall before the foremost, but this is most undoubtedly counteracted by grooves, while it would be impossible to fire an elongated projectile with its centre of gravity backwards, with any accuracy out of a smooth-bored gun.
Cannelures improved shooting.
Captain Jervis says that these grooves have the effect of improving the accuracy of firing when the bullets are not perfectly homogeneous, is certain, Why none in English bullet.but the British Committee on small arms justly considered that owing to the careful way in which the bullets are made in England by compression, these grooves might be dispensed with.
Variety of forms.