“_Construction._—Let A B E be the arc of a circle: the chord A D E = 18 inches: the perpendicular B D = 7 inches: the width B C = 3 inches. Thus constructed, the centre of gravity will fall exactly on the edge of its concave surface at C. When used as an offensive weapon, it is usually thrown with the _convex_ side outwards; but when intended to return, it is held in the reverse position; although it will probably act in either direction, if properly managed. [Illustration] “For the latter purpose, however, it should be thrown from the hand at a considerable elevation (45°) with a sudden jerk, so as to combine with the projectile force a rapid rotation round its centre of gravity. The rotation acts constantly in opposition to its line of flight; so that if a similar rotation could be communicated without any projectile force, the instrument would move backwards: now, as the force with which it is thrown is constantly diminishing while the rotation continues, it must always arrive at a certain point where these opposite forces balance, or equalize each other. At that moment the weapon would fall towards the ground, were it not for its flat surface and rotatory motion; but in consequence of the centre of gravity being so placed that it will always present its broad surface to the air, it cannot descend perpendicularly, but slides down the inclined plane, up which it has been thrown, in consequence of the whirling motion continuing after the projectile force has ceased; so that if properly thrown, it will pass over the head of the thrower, and often to a considerable distance behind him. On the same principle, a hoop thrown from the hand with a spinning-motion _inwards_, will begin to return before it touches the ground; and also the curious, though not so familiar instance, of a ball fired from a musket, the barrel of which has been bent _to the left_, being carried at long distances considerably _to the right_ of the object aimed at, in consequence of the rotation of the ball on its axis, caused by the friction against the right side of the barrel overcoming the projectile force, and thus carrying it across the line of aim.