718. _How do we know that sounds are produced by the vibrations of the air, induced by the vibrations of the atoms of bodies?_ 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_. [Verse: "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?_ They travel at a rate of rather more than _a quarter of a mile in a second_, or _twelve miles and three-fourths in a minute_. 722. _Do all sounds travel at the same rate?_ All sounds, whether strong or weak, high or low, musical or discordant, _travel with the same velocity_. 723. _Why are bells and glasses stopped from ringing by touching them with the finger?_ Because the contact of the finger _stops the vibration_ of the atoms of the metal and glass, which therefore _cease to impart vibrations to the air_. 724. _Why does a cracked bell give discordant sounds?_ Because the _connection_ between the atoms of the bell being _broken,_ their vibrations are not uniform: some of the atoms vibrate _more intensely_ than the others; the vibrations imparted to the air are therefore _jarring_ and _discordant_. 725. _Why, when we see a gun fired at a distance, do we see the flash and smoke, before we hear the report?_ Because _light,_ which enables us to _see_, travels at the velocity of 192,000 miles in a _second_; while _sound_, by which we _hear_, travels only at the rate of a quarter of a mile in a _second_. [Verse: "My heart maketh a noise in me: I cannot hold my peace, because thou hast heard, O my soul, the sound of the trumpet, the alarm of war."--JER. IV.] 726. _Why does the tread of soldiers, when marching in long ranks, appear to be irregular?_ Because the sounds proceeding from _different distances_, reach our ears in _varying periods of time_.