CHAPTER XXXIV.
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_.
[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_.
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_.
[Verse: "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.
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_.
[Verse: "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.]
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.
[Verse: "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.