Conduction of Sound.
How does sound come to our ears?
Sound is carried to our ears by the air.
Is the air a conductor of sound?
Yes; the air is a conductor of sound.
Is there any sound where there is no air?
No; there is not any sound where there is no air.
How do we know this?
When a bell is rung in a vacuum, no sound is heard.
Can sounds be distinctly heard when the air is rare?
No; the rarer the air the more poorly it conducts sound.
What proof have we of this?
On the top of a high mountain the human voice is heard only at a distance of a few yards; and a pistol when exploded sounds as faintly as the breaking of a stick.
Can sounds be heard better when the air is dense?
Yes; the denser the air, the better it conducts sound.
What proof have we of this?
The human voice sounds so distinctly in the dense air of deep mines, that conversation may be carried on in a whisper.
Why are sounds heard better at night than in the day?
Because the air at night is condensed by cold, and becomes a better conductor of sound; also, because the nights are stiller, and fewer sounds take our attention.
Why is moist or damp air a better conductor of sound than dry air?
Because the particles of water in the damp air increase its power of conduction.
Why is sound heard farther when over the water?
Because the damp air over the water is a better conductor, and because it contains fewer objects to obstruct the waves of sound.
How far has the human voice been heard over the water?
The words “all’s well” have been heard across the Strait of Gibraltar, a distance of ten miles.
How far has the human voice been heard over the land?
It has been heard at the distance of four miles.
How does the wind affect sound?
The wind carries the sound the way in which it is blowing, so that a noise may be heard much farther with the wind than against it.
Why does water conduct sound faster than air?
Because the particles of water are closer together than the particles of air.
How can we prove that water is a conductor of sound?
When a bell is rung under water, the sound may be heard by any person whose head is beneath the surface of the water.
Does a solid substance conduct sound more rapidly than air?
It does, much more rapidly.
How can we prove this?
When the ear is placed at one end of a log of wood and the other end is struck with a hammer, two sounds will be heard; first, the one coming through the log, and, afterwards, the one coming through the air.
What use is made of this fact?
The tramping of horses or the rumbling of cars and wagons, when at a distance, may be heard by placing the ear near the ground.
Do all solid substances produce the same amount of sound when struck?
No; soft substances, such as lead and wood, produce but little sound; while hard substances, such as copper, bell-metal, and glass, produce much more sound.
What are hard bodies, producing much sound, called?
They are called sonorous bodies.
Is a bell a very sonorous body?
Yes; a bell is one of the most sonorous bodies that we have.
Why does a bell ring when struck?
Because the stroke causes the particles of the bell to vibrate.
When we touch a bell while ringing, why does the sound stop?
Because we stop its vibrations when we touch it.
Why does a crack in a bell check its ringing?
Because the crack stops the vibrations in the metal.
What other substances ring when struck?
All vessels made of glass, earth, stone, iron, etc., ring when struck.
How may we detect a crack in any such vessel?
By striking the vessel; if it is cracked there will be no ringing sound.