DEVIATIONS OF PROJECTILES FROM SMOOTH-BORED GUNS.
Causes of deviation of shot.
Very great irregularities occur in the paths described by projectiles fired from smooth-bored guns. It is a fact well known to all practical artillerists, that if a number of solid shot or any other projectile be fired from the same gun, with equal charges and elevations, and with gunpowder of the same quality, the gun carriage resting on a platform, and the piece being laid with the greatest care before each round, very few of the shot will range to the same distance; and moreover, the greater part will be found to deflect considerably (unless the range be very short) to the right or left of the line in which the gun is pointed.
Four causes of deviation.
The causes of these deviations may be stated as follows:—1st, Windage; 2nd, Rotation; 3rd, Wind; 4th, from Rotation of the Earth.
1st CAUSE, WINDAGE.
Action from windage.
Windage causes irregularity in the flight of a projectile, from the fact of the elastic gas acting in the first instance on its upper portion, and driving it against the bottom of the bore; the shot re-acts at the same time that it is impelled forward by the charge, and strikes the upper surface of the bore some distance down, and so on by a succession of rebounds, False direction.until it leaves the bore in an accidental direction, and with a rotatory motion, depending chiefly on the position of the last impact against the bore. Thus should the last impact of a (concentric) shot when fired from a gun be upon the right hand side of the bore, as represented, ([plate 23], fig. 5); the shot will have a tendency to deflect to the left in the direction. Gives rotation.While at the same time a rotation will be given to it in the direction indicated by the arrows.
2nd CAUSE, ROTATION.
Every body may have a twofold motion, one by which it is carried forward, and the other by which it may turn round on an axis passing through its centre, called a motion of rotation.
When a body has only a motion of translation all the particles of which it is composed move with equal swiftness, and also in parallel directions; and by the first law of motion, every particle put in such motion will constantly move with the same velocity in the same direction, unless it be prevented by some external cause.
Rotation.
By a motion of rotation, a body without changing its place, turns round on an axis passing through its centre of gravity. Rotation and translation combined.A body may have at the same time both a progressive and rotatory motion, without either disturbing the other, and one may suffer a change from the action of some external force, while the other continues the same as before.
Force through centre of gravity, causes progressive motion only.
If the direction of the force be through the centre of gravity, it causes a progressive motion only, that is, if the body was at rest before, it will move forward in the direction of the impressed force.
Effect of force on a body in motion.
If a body had a progressive motion before, then impressed force will cause it to move faster or slower, or to change its direction, according as the direction of this second force conspires with or opposes its former motion, or acts obliquely on its direction.
Rotation not disturbed by second force in direction of centre of gravity.
If a body, besides its progressive motion had a motion of rotation also, this last will not be changed by the action of a new force passing through the centre of gravity.
Rotation of force does not pass through the centre of gravity.
If the direction of the force does not pass through the centre of gravity, the progressive motion will be altered, and the body will then also acquire a rotatory motion round an axis passing through the centre of gravity, and perpendicular to a plane passing through the direction of the force and this centre.