When an elastic body is struck, that body, or some part of it, is made to vibrate. This is evident to sense in the string of a violin or harpsichord, for we may perceive by the eye, or feel by the hand, the trembling of the strings, when by striking they are made to sound. If a bell be struck by a clapper on the inside, the bell is made to vibrate. The base, of the bell, is a circle, but it has been found that by striking any part of this circle on the inside, that part flies out, so that the diameter which passes through this part of the base will be longer than the other diameter. The base, by the stroke, is changed into an ellipse or oval, whose longer axis passes through the part against which the clapper is struck. The elasticity of the bell restores the figure of the base, and makes that part which was forced out of its place, return back to its former situation, from which the same principle throws it out again; so that the circular figure of the bell will be again changed to an ellipse, only now the shorter axis will pass through the part which was first struck. The same stroke, which makes the bell vibrate, occasions the sound, and as the vibrations decay, the sound grows weaker. We may be convinced by our senses that the parts of the bell are in a vibratory motion while it sounds. If we lay the hand gently on it, we shall easily feel this tremulous motion, and even be able to stop it, or if small pieces of paper be put upon the bell, its vibrations will put them in motion. These vibrations in the sounding body will cause undulations or waves in the air; and, as the motions of one fluid may often be illustrated by those of another, the invisible motions of the air have been properly enough compared to the visible waves of water produced by throwing a stone therein. These waves spread themselves in all directions in concentric circles, whose common centre is the spot where the stone fell, and when they strike against a bank or other obstacle, they return in the contrary direction to the place from whence they proceeded. Sound in like manner expands in every direction, and the extent of its progress is in proportion to the impulse on the vibrating chord or bell. Such is the yielding nature of fluids, that when other waves are generated near the first waves, and others again near these, they will perform their vibrations among each other without interruption; those that are coming back will pass by those that are going forwards, or even through them, without interruption: for instance, if we throw a stone into a pond, and immediately after that, another, and then a third, we shall perceive that their respective circles will proceed without interruption, and strike the banks in regular succession.