CHAPTER XXVIII.

SOUND.

Q. _What is SOUND?_

A. The vibration of some sonorous substance produces motion in the air called SOUND WAVES, which strike upon the _drum of the ear_, and give the sensation of sound.

Q. _What are MUSICAL SOUNDS?_

A. Regular and uniform successions of vibrations, which are always pleasing to the ear.

Q. _How FAST does SOUND TRAVEL?_

A. About 13 miles in a minute, or 1142 feet in a second of time.

Q. _How fast does LIGHT TRAVEL?_

A. Light would go 8 times round the whole earth, while sound is going its 13 miles.

Q. _Why are SOME things SONOROUS, and others NOT?_

A. The sonorous quality of any substance depends upon its _hardness and elasticity_.

Q. _Why are COPPER and IRON SONOROUS, and not LEAD?_

A. Copper and iron are _hard and elastic_; but as lead is neither hard nor yet elastic, it is _not sonorous_.

Q. _Of what is BELL-METAL made?_

A. Of _copper and tin_ in the following proportions:--In every 5 pounds of bell-metal, there should be 1 lb. of tin, and 4 lbs. of copper.

Q. _Why is this mixture of tin and copper used for BELL-METAL?_

A. Because it is much _harder_ and more _elastic_ than either of the pure metals.

Q. _Why is the SOUND of a bell STOPPED by TOUCHING the bell with our finger?_

A. The weight of the finger _stops the vibrations_ of the bell; and as soon as the bell _ceases to vibrate_, it ceases to make sound-waves in the air.

Q. _Why does a SPLIT BELL make a HOARSE disagreeable sound?_

A. The _split_ of the bell causes a _double vibration_; and as the sound-waves _clash and jar_, they impede each other's motion, and produce discordant sounds.

Q. _Why does a FIDDLE-STRING give a musical sound?_

A. The bow drawn across the string _causes it to vibrate_, and this vibration of the string _sets in motion the sound-waves of the air_, and produces musical notes.

Q. _Why does a DRUM sound?_

A. The parchment head of the drum _vibrates_ from the blow of the drum-stick, and sets in motion the sound-waves of the air.

Q. _Why do MUSICAL GLASSES give sounds?_

A. Because the glasses _vibrate_ as soon as they are struck, and set in motion the sound-waves of the air.

Q. _Why do FLUTES, &c. produce musical sounds?_

A. The breath of the performer causes the _air in the flute to vibrate_, and sets in motion the sound-waves of the air.

Q. _Why do PIANO-FORTES produce musical sounds?_

A. The _keys of the piano_ (being struck with the finger) lift up a little hammer which _knocks against a string_; and the vibration thus produced, sets in motion the sound-waves of the air.

Q. _Why are SOME notes BASS and some TREBLE?_

A. _Slow_ vibrations produce _bass or deep sounds_; whereas, _quick_ vibrations produce _shrill or treble sounds_.

Q. _Why is an instrument FLAT when the STRINGS are UNSTRUNG?_

A. Because the vibrations are _too slow_; in consequence of which, the sounds produced are not _shrill_ or _sharp_ enough.

Q. _Why can persons living a mile or two from_

_a town HEAR the BELLS of the town-church SOME TIMES, and not at OTHERS?_

A. Fogs, rain, and snow, obstruct the passage of sound; but when the air is _cold and clear_, sound is propagated more easily.

Q. _Why can we NOT hear sounds (as distant church bells) in RAINY weather, so well as in FINE weather?_

A. Because the falling rain _interferes with the undulations of the sound-waves_, and breaks them up.

Q. _Why can we not hear sounds (as distant church bells) in SNOWY weather, so well as in FINE weather?_

A. Because the falling snow _interferes with the undulations of the sound-waves_, and stops their progress.

Q. _Why can we HEAR distant clocks MOST distinctly in CLEAR COLD weather?_

A. Because the air is most _uniform_ then: there are not _two currents of air_ (one up and one down) to interrupt the sound-waves.

Q. _Why can persons hear the VOICES of men in conversation for a MILE distant, near the POLES, in winter time?_

A. Because the air is very _cold and very clear_; in consequence of which, there are not _two currents of air_ (one up and one down) to interrupt the sound-waves.

Captain Ross heard the voices of his men in conversation, a mile and a half from the spot where they stood.

Q. _Why are not SOUNDS (such as distant church bells) heard so distinctly on a HOT DAY as in FROSTY weather?_

A. Because there are _two currents of air_; the current of _hot_ air _ascending from the earth_, and the current of _colder_ air _falling towards_ the earth; and these two currents _break up the sound-waves_.

Q. _Why can we not hear SOUNDS (such as distant clocks) so distinctly in a thick MIST or HAZE, as in a CLEAR night?_

A. Because the mist _diminishes the velocity_ of the sound-waves, and (by overburdening them with vapour) _limits their length_.

Q. _Why do we hear SOUNDS better by NIGHT than by DAY?_

A. 1st--Night air is _more uniform_, because the ascending currents of air (raised by the action of the sun's rays) _cease_ as the evening advances; and

2ndly--Night is more _still_ from the suspension of business, and the cessation of the hum of men.

Q. _How should PARTITION WALLS be made to PREVENT the voices in adjoining rooms from being HEARD?_

A. The space between the laths (or canvass) should be filled with _shavings or saw-dust_; and then no sound would ever pass from one room to another.

Q. _Why would SHAVINGS or saw-dust PREVENT the transmission of sound from room to room?_

A. Because there would be _several different media_ for the sound to pass through: 1st--the air;

2ndly--the laths and paper;

3rdly--the saw-dust or shavings;

4thly--the air again: and every _variety_ diminishes the _strength of the sound-waves_.

Q. _Why can DEAF people hear through an EAR TRUMPET?_

A. The ear trumpet restrains _the spread of the voice_, and limits the _diameter of the sound-waves_; in consequence of which, their _strength_ is increased.

Q. _Why are MOUNTAINS so NOISELESS and quiet?_

A. Because the air of mountains is _very rarefied_; and as the air becomes _rarefied_, sound becomes less _intense_.

Q. _How do you know that the RARETY of air DIMINISHES the intensity of SOUND?_

A. If a bell be rung in the receiver of an air-pump, the sound becomes _fainter and fainter_ as the air is exhausted, till at last it is quite _inaudible_.

Q. _What is the cause of ECHO?_

A. Whenever a sound-wave strikes against any _obstacle_ (such as a wall or hill), _it is reflected_ (or thrown back); and this _reflected sound_ is called an ECHO.

The same laws govern echo as light. (_See p. 370._)

Q. _What places are most FAMOUS for ECHOES?_

A. Caverns, grottoes, and ruined abbeys; the areas of antique halls; the windings of long passages; the aisles of cathedral churches; mountains, and ice-bergs.

Q. _Why are caverns, grottoes, and ruined abbeys FAMOUS for ECHOES?_

A. 1st--Because the sound-waves cannot pass _beyond_ the cavern or grotto, and _must flow back_:

2ndly--The _return waves_ (being entangled by the cavern) are _detained_ for a short time, and come _deliberately_ to the ear.

Q. _Why are antique halls, winding passages, and cathedral aisles FAMOUS for ECHOES?_

A. Because the sound-waves _cannot flow freely forward_, but strike against the winding walls perpetually, and are beaten _back_.

Q. _Why are MOUNTAINS and ice-bergs FAMOUS for ECHOES?_

A. Because they present a _barrier_ to the sound-waves _which they cannot pass_; and are sufficiently elastic to _throw them back_.

Q. _Why do not the walls of a ROOM or church produce ECHO?_

A. Because sound travels with such _velocity_, that the echo is _blended with the original sound_, and produce but _one impression_ on the ear.

Sound travels 13 miles in a minute.

Q. _Why do very LARGE buildings (as cathedrals), often REVERBERATE the voice of the speaker?_

A. Because the walls are _so far off from the speaker_, that the echo does not _get back in time_ to blend with the original sound; and, therefore, _each_ is heard separately.

Q. _Why do SOME echoes repeat only ONE syllable?_

A. The _further_ the echoing body is _distant_, the _more sound_ it will _reflect_. If, therefore, the echoing body be _near_, it will repeat but one syllable.

Q. _Why does an ECHO sometimes repeat TWO or more syllables?_

A. Because the echoing body is _far off_; and, therefore, there is time for one reflection _to pass away_ before _another_ reaches the ear.

Q. _Why do WINDOWS RATTLE when CARTS pass by a house?_

A. 1st--Glass is _sonorous_; and the air communicates its vibrations to the glass, which echoes the same sound: and

2ndly--The _window-frame is shaken_ by the sound-waves _impinging against the window_, and contributes to the noise.