If we take a tuning fork, and hold it to the ear, we hear no sound. If we move it rapidly through the air, or if we blow upon it, it produces no sound; but if we strike it, a sound immediately occurs; the vibration of the fork may be seen, and felt by the hand that holds it; and as those vibrations cease, the sound dies away.
719. How do we know that without air there would be no sound?
Because if a tuning fork were to be struck in a vacuum (as under the receiver of an air pump) no sound would be heard, although the vibrations of the fork could be distinctly seen.
"And even things without life giving sound, whether pipe or harp, except they give a distinction in the sounds, how shall it be known what is piped or harped."—Corinth. xiv.
720. How are the vibrations of sonorous bodies imparted to the air?
When a bell is struck, the force of the blow gives an instant agitation to all its particles. The air around the bell is driven back by the impulse of the force, and thus a vibration of compression is imparted to the air; but the air returns to the bell, by its own natural elasticity, thus producing a vibration of expansion—when it is again struck, and thus successive vibrations of compression and expansion are transmitted through the air.
721. How rapidly are these vibrations transmitted through the air?