Deviation of Oblong Projectiles

—The cause of the deviation of an oblong rifle projectile is quite different from one of spherical form. An oblong projectile moving in the air is acted upon by two rotary forces, viz.: one which gives it its normal rotary motion around its axis of progre ion, and another the resistance of the air, which, in consequence of the deflection of the axis of progre ion from the tangent to the trajectory by the action of gravity, does not pa through the centre of inertia, but above or below it; depending on the shape of the projectile. From a law of mechanics, a body thus circumstanced will not yield fully to either of the forces that thus act upon it, but its apex will move off with a slow uniform motion to the right or left of the vertical plane, depending on the relative direction of the two rotary forces. If the action of these forces be continued sufficiently long, it will be seen that the axis of the projectile before referred to describes a cone around a line pa ing through the centre of inertia and parallel to the direction of the resistance of the air. Owing to the short duration of the flight of an ordinary projectile, it is only nece ary to consider the first part of this conical motion. If the projectile rotates in the direction of the hands of a watch to the eye of the marksman, and the resultant of the resistance of the air pa above the centre of inertia, as it does in the service bullet with a conoidal point, then the point of the projectile will move to the right, which brings the left side of the projectile obliquely in contact with the current of the air. The effect of this position with reference to the air will be to generate a component force that will urge the projectile to the right of the plane of fire. This peculiar deviation was called by the French officers that first observed it, “ derivation ,” or “ drift .”