2ndly--Because the _centre_ of a river _flows faster_ than its _sides_.

Q. _Why do the SIDES of a river flow more TARDILY than its CENTRE?_

A. Because they _rub against the banks_, and are delayed in their current thereby.

Q. _Why does SOAPY water BUBBLE?_

A. Because the soap _makes the water tenacious_, and prevents the bubbles from _bursting_ as soon as they are formed.

Q. _Why will not water bubble WITHOUT SOAP?_

A. Because it is not tenacious enough _to hold together the bubbles_ that are formed.

Q. _When SOAP BUBBLES are blown from a pipe, why do they ASCEND?_

A. Because they are _filled with warm breath_, which is lighter than air.

CHAPTER XXVI.

ICE.

Q. _What is ICE?_

A. FROZEN WATER. When the air is reduced to 32 degrees of heat, water will no longer remain in a _fluid state_.

Q. _Why is SOLID ICE LIGHTER than WATER?_

A. Because water _expands by freezing_; and as the _bulk is increased_, the _gravity_ must be _less_.

Nine cubic inches of water become ten when frozen.

Q. _Why do EWERS BREAK in a FROSTY NIGHT?_

A. Because the water in them _freezes_; and as the _water is expanded by frost_, it bursts the ewers to make room for its increased volume.

Q. _Why does it not expand UPWARDS (like boiling water), and RUN OVER?_

A. Because the _surface_ is first frozen, and the frozen surface acts as a _plug_, which is more difficult to burst than the earthen ewer itself.

Q. _Why do TILES, STONES, and ROCKS often SPLIT in winter?_

A. Because the moisture (which they imbibed) _freezes_, and by its expansion _splits the solid ma.s.s_.

Q. _In winter time, FOOT-MARKS and WHEEL-RUTS are often covered with an icy NET-WORK, through the interstices of which the soil is clearly seen,--WHY does the water freeze in NET-WORK?_

A. The water in these hollows froze first at the _sides_ of the foot-prints: other crystals gradually shot across the water, and would have _covered the whole surface_, had not the earth _absorbed_ the water before it had time to freeze.

Q. _In winter time these FOOT-MARKS and WHEEL-RUTS are sometimes covered with a perfect SHEET of ice, and not an icy net-work,--Why is THIS?_

A. The _air being colder_ and the _earth harder_ (than in the former case), the _entire surface_ of the foot-print is frozen over, before the earth can _draw the water in_.

Q. _Why is not the ice SOLID in these ruts?--WHY is there only a very thin FILM or NET-WORK of ice?_

A. Because the earth _absorbs the water_, and leaves the icy _film behind_.

Q. _Does not water expand by HEAT as well as COLD?_

A. Yes; it expands as soon as it is more than 42 degrees _till it boils_, and then it flies off in steam.

[Ill.u.s.tration: Here A B measures the bulk of a portion of water at 42 degrees.

It goes on increasing in bulk to C D, when it boils. It also goes on increasing in bulk to E F, when it freezes.]

Q. _Why do WATER-PIPES frequently BURST in FROSTY weather?_

A. Because the water in them _freezes_; and as the water _expands by frost_, it bursts the pipes to make room for its increased volume.

Q. _When does WATER begin to EXPAND from cold?_

A. Water (which is wisely ordained by G.o.d to be an _exception_ to a very general rule) _contracts_ till it is reduced to 42 degrees, and then it _expands till it freezes_.

(Water freezes at 32.)

Q. _Why does water expand when it freezes?_

A. Because it is converted into _solid crystals_, which _do not fit close_, like the particles of water.