“Its elasticity: a ball of damp clay, which does not possess this property, will produce no other sound, when struck, but that which arises from the condensation of the small portion of air between the clay and the hammer which strikes it. A hollow ball of brass will produce more sound, because it is elastic; but still very little effect will arise from this, since a ball is the worst shape for admitting of vibration, on account of its forming an arch or dome, in every direction, so that one part stiffens and sustains the other; but if such a ball be divided, and the edge of one half of it struck, a loud, clear, and distinct tone will be produced; because a hemisphere will admit of the exertion of elasticity, or of momentary change of figure, which is conducive to the perfection of sound; and accordingly the bells used for clocks, and for musical purposes, have generally such a figure.” “I see clearly,” said Louisa, “that it is the vibration of a sonorous body that communicates the necessary motions to the air; and I suppose that a body vibrates in proportion to its elasticity.” “Certainly it does: but to render this subject still more intelligible, I have prepared a diagram.” Mr. Seymour then exhibited a figure, of which the annexed is a copy, and proceeded to explain it in the following manner:-- [Illustration: Diagram of a string vibrating.] “You are well aware that an elastic body, after having been struck, not only returns to its former situation, but having acquired momentum by its velocity, like the pendulum or swing,[54] springs out on the opposite side. If, then, I draw the string A B, which is made fast at both ends, to C, it will not only return to its original position, but proceed onwards to D. This is the first vibration, at the end of which it will retain sufficient velocity to bring it to E, and back again to F, which constitutes its second vibration; the third vibration will carry it only to G and H, and so on, till the resistance of the air destroys its motion.” “That is exactly like the swing or pendulum,” said Tom. “As you are struck with the resemblance, take care and preserve the remembrance of it; for I shall, hereafter, have occasion to revert to it.” “As I now understand how sound is produced and carried to a distance, I should much like to learn the cause of different tones,” said Louisa.