Appendix VIIIa
The Rifled Projectile
In the flight of a projectile fired from a rifled gun two influences are at work of an entirely different kind tending to modify the form of the trajectory[1]; these are gyroscopic action and the aerodynamic action dealt with in Vol. I. § 30, as an effect of discontinuity in the fluid motion on the flight of a ball in rotation.
Before the days of rifling the second of these influences alone took effect on the flight of the spherical bullets then in use, and as the accidental rotation that the ball acquired in the barrel of the gun was an unknown quantity, so was the resulting aerodynamic reaction, and the accuracy of the weapon was little better than that of the crossbow it superseded. About the middle of the 18th century Robins showed that a bend in the barrel would produce a rotation of the ball owing to the friction due to its centrifugal force, and that this rotation, after a comparatively short flight, would more than compensate for the direct effect of the bend; he further advocated rifling (a well-known expedient even then) as a remedy for the defects of the musket of that time.
When a spherical bullet is fired from a rifled gun the axis of rotation remains sensibly constant in direction, for the aerodynamic forces are initially symmetrical; the result of this is that at first the flight path is a simple trajectory, and thus the bullet is very soon travelling at an angle to its axis of
- ↑ In addition of course to the direct aerial resistance.
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