A Guide To The Scientific Knowledge Of Things Familiar Part 92

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 pa.s.sages; 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 pa.s.s _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 pa.s.sages, 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 pa.s.s_; 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 pa.s.s away_ before _another_ reaches the ear.

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

A. 1st--Gla.s.s is _sonorous_; and the air communicates its vibrations to the gla.s.s, 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.

CHAPTER XXIX.

MISCELLANEOUS.

Q. _Why do the BUBBLES in a CUP OF TEA range round the SIDES of the CUP?_

A. Because the cup _attracts them_.

Q. _Why do all the LITTLE BUBBLES tend towards the LARGE ones?_

A. Because the large bubbles (being the superior ma.s.ses) _attract them_.

Q. _Why do the BUBBLES of a CUP OF TEA FOLLOW a TEA-SPOON?_

A. Because the tea-spoon _attracts them_.

Q. _Why are the SIDES of a pond covered with LEAVES, while the MIDDLE of the pond is quite CLEAR?_

A. Because the sh.o.r.e _attracts_ the leaves to itself.