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. 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 masses) _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 shore _attracts_ the leaves to itself. Q. _Why do all fruits, &c. (when severed from the tree) FALL to the EARTH?_