716. What is sound?

Sound is an impression produced upon the ear by vibrations of the air.717. What causes the air to vibrate and produce sounds?

The atoms of elastic bodies being caused to vibrate by the application of some kind of force, the vibrations of those atoms are imparted to the air, and sound is produced.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.

"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.

"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.727. What are the numbers of vibrations in a second that produce the various musical sounds?

C or Do, 480 vibrations in a second; B or Si, 450 vibrations; A or La, 400 vibrations; G or Sol, 360 vibrations; F or Fa, 320 vibrations; E or Mi, 300 vibrations; D or Re, 270 vibrations; C or Do, 240 vibrations. It is thus seen that the more rapid the vibrations, the higher the note, and vice versa.728. Why does the length of a wire or string determine the sound that it produces?

Because the shorter the string the more rapid are its vibrations when struck.729. Why does the tension of a wire or string affect its vibrations?

Because when the string or wire is tight, a touch communicates vibrations to all its particles; but when it is loose the vibrations are imperfectly communicated.730. Why are some notes low and solemn, and others high and quick?

Because the vibrations of musical strings vary from 32 vibrations in a second, which produces a soft and deep bass, to 15,000 vibrations in a second, which produces the sharpest treble note.731. Why can our voices be heard at a greater distance when we speak through tubes?

Because the vibrations are confined to the air within the tube, and are not interfered with by other vibrations or movements in the air; the tube itself is also a good conductor of sound.

"And I will cause the noise of thy songs to cease; and the sound of thy harps shall no more be heard."—Ezekiel xxvi.

732. Is air a good conductor of sound?

Air is a good conductor, but water is a better conductor than air; wood, metals, the earth, &c., are also good conductors.733. Why can we hear sounds at a greater distance on water than on land?

For various reasons: because the smooth surface of water is a good conductor; because there are fewer noises, or counter vibrations, to interfere with the transmission of sound; and because there are no elevated objects to impede the progress of the vibrations.734. Why do sea-shells give a murmuring noise when held to the ear?

Because what may be called expended vibrations always exist in air where various sounds are occurring. These tremblings of the air are received upon the thin covering of the shell, and thus being collected into a focus, are transmitted to the ear.735. Why can people in the arctic regions converse when more than a mile apart?

Because there the air, being cold and dense, is a very good conductor; and the smooth surface of the ice also favours the transmission of sound.736. Why do savages lay their heads upon the earth to hear the sounds of wild beasts, &c.?

Because the earth is a good conductor of sound. For this reason, also, persons working under ground in mines can hear each other digging at considerable distances.737. Why can church clocks be heard striking much more clearly at some times than at others?

Because the density of dry air improves the sound-conducting power of the atmosphere. The transmission of sounds is also assisted by the direction of the winds.

"The morning is come unto thee, O thou that dwellest in the land: the time is come, the day of trouble is near, and not the sounding again of the mountains."—Ezekiel vii.

738. Why may the scratching of a pin at one extremity of a long pole be heard by applying the ear to the opposite extremity?

Because wood is a good conductor of sound, and its atoms are susceptible of considerable vibration. It is, therefore, chosen in numerous instances for the construction of musical instruments.

Deaf persons have been known to derive pleasure from music by placing their hands upon the wood-work of musical instruments while being played upon.739. Why is the hearing of deaf persons assisted by ear-trumpets?

Because ear-trumpets collect the vibrations of the air into a focus, and make the sounds produced thereby more intense.740. Why are sounding-hoards used to improve the hearing of congregations?

Because, being suspended over, and a little behind, the speaker, they collect the vibrations of the air, and reflect them towards the congregation.741. What are echoes?

Echoes are sounds reflected by the objects on which they strike.742. Why do some echoes occur immediately after a sound?

Because the reflecting surface is very near; therefore the sound returns immediately.743. Why do some echoes occur a considerable time after a sound?

Because they are at a considerable distance, and the sound takes time to travel to it, and an equal time to return.744. Why do some echoes change the tone and quality of sound?

Because the reflecting surface, having vibratory qualities of its own, mingles its own vibrations with that of the sound.745. Why are there sometimes several echoes to one sound?

Because there are various reflecting surfaces, at different distances, each of which returns an echo.

"And God said, Let the waters under the heaven be gathered together onto one place, and let the dry land appear: and it was so."—Gen. i.

746. Are sounds reflected only by distant objects?

Sounds are doubtless reflected by walls and ceilings around us. But we do not perceive the echoes, because they are so near that they occur at the same moment with the sound. In lofty buildings, however, there is frequently a double sound, making the utterance of a speaker indistinct. This arises from the echo following very closely upon the sound.747. Why, when we are walking under an arch-way or a tunnel, do our voices appear louder?

Because the sounds of our voices are immediately reflected. And as a gas reflector increases the intensity of light, so a sound reflector will increase the apparent strength of our voices.

There are many places where remarkable echoes occur. On the banks of the Rhine, at Lurley, if the weather be favourable, the report of a rifle, or the sound of a trumpet, will be repeated at different periods, and with various degrees of strength, from crag to crag, on opposite sides of the river alternately. A similar effect is heard in the neighbourhood of some of the Lochs in Scotland. There is a place at Woodstock, in Gloucestershire, which is said to echo a sound fifty times. Near Rosneath, a few miles from Glasgow, there is a spot where, if a person plays a bar of music upon a bugle, the notes will be repeated by an echo, but a third lower; after a short pause, another echo is heard, again in a lower tone; then follows another pause, and a third repetition follows in a still lower key. The effect is very enchanting. The whispering galleries of St. Paul's, of the cathedral church of Gloucester, and of the Observatory of Paris, owe their curious effects to those laws of the reflection of sound, by which echoes are produced; but in these cases the effect is assisted by the elliptical form of the edifice, each person being in the focus of an ellipse.