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, 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. While at the same time a rotation will be given to it in the direction indicated by the arrows. 2nd CAUSE, ROTATION. ~Rotation without translation.~ 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.~ ~Rotation and translation combined.~ By a motion of rotation, a body without changing its place, turns round on an axis passing through its centre of gravity. 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.~