A Guide To The Scientific Knowledge Of Things Familiar Part 79

Q. _Why is the water at the BOTTOM of a river NEVER FROZEN?_

A. Because when water is colder than 42 degrees, it instantly _ascends to the surface_; and (if it freezes) _floats there_ till it is melted.

(When a river is frozen, the water below the surface is never less than 42.)

Q. _Show the WISDOM of G.o.d in this wonderful exception to a general law._

A. If ice were _heavier than water_, it would _sink_; and a river would soon become a solid _block of ice_, which could never be dissolved.

Q. _Why does not the cold ICE on the SURFACE of a river CHILL the water BENEATH, and make it freeze?_

A. 1st--Water is a _very bad conductor_, and is heated or chilled by CONVECTION only:

2ndly--If the ice on the surface were to communicate its _coldness_ to the water beneath, the _water beneath_ must communicate its _heat to the ice_, and the ice would instantly _melt_: and

3rdly--The ice on the surface acts as a _s.h.i.+eld_ to _prevent the cold air from penetrating the river_ to freeze it below the mere crust.

Q. _Why does WATER FREEZE at the SURFACE first?_

A. Because the surface is in _contact with the air_, and the air _carries away its heat_.

Q. _Why does the coat of ice grow THICKER and THICKER, if the frost CONTINUES?_

A. Because the _heat of the water_ (immediately below the frozen surface) _pa.s.ses through the pores of the ice_ into the _cold air_.

Q. _Why then are not WHOLE RIVERS FROZEN (layer by layer) till they become solid ice?_

A. Because water is _so slow a conductor_, that our frosts never continue _long enough_ to convert a whole river into a solid ma.s.s of ice.

Q. _Why does not RUNNING water freeze so fast as STILL water?_

A. 1st--Because the motion of the current _dissolves the crystals_ as fast as they are formed; and

2ndly--The heat of the _under_ surface is more freely distributed to the _upper_ surface by the _rolling water_.

Q. _When RUNNING water is FROZEN, why is the ICE generally very ROUGH?_

A. Because little flakes of ice are first formed and _carried down the stream_, till they meet some _obstacle_ to _stop_ them; _other_ flakes of ice (_impinging against them_) are arrested in like manner; and the _edges_ of the different flakes _overlapping_ each other, _make the surface rough_.

Q. _Why do SOME parts of a RIVER FREEZE LESS than OTHERS?_

A. Because _springs_ issue from the bottom, and (as they bubble upwards) _thaw the ice_, or make it thin.

Q. _When persons FALL into a RIVER in winter time, why does the WATER feel remarkably WARM?_

A. Because the _frosty air_ is at least 10 or 12 degrees _colder_ than the water.

(The water below the surface is at least 42; but the air 32, or even less.)

Q. _Why is SHALLOW water FROZEN QUICKER than DEEP water?_

A. Because (as the _whole volume_ of water must be cooled to 42 degrees before the _surface can be frozen_) it will take a longer time to cool down a _deep_ bed of water than a _shallow_ one.

Q. _Why is SEA-WATER RARELY FROZEN?_

A. 1st--Because the _ma.s.s of water is so great_ that it requires a very long time to cool the whole volume down to 42 degrees:

2ndly--The _ebb and flow_ of the sea interfere with the cooling influence of the air: and

3rdly--_Salt_ never freezes till the surface is cooled down at least 25 degrees _below the freezing point_.

Q. _Why do some LAKES RARELY if ever FREEZE?_

A. 1st--Because they are _very deep_:

2ndly--Because their water is supplied by _springs_, which bubble from the bottom.

Q. _Why does the DEPTH of the water r.e.t.a.r.d its FREEZING?_

A. As the _whole volume of water_ must be reduced to 42 degrees before the _surface will freeze_, the _deeper_ the water, the _longer_ it will be before the whole volume is thus reduced.

Q. _Why do SPRINGS at the bottom of a lake PREVENT its FREEZING?_

A. Because they keep continually sending forth _fresh water_, which prevents the lake from being reduced to the necessary degree of coldness.

Q. _Why is it COLDER in a THAW than in a FROST?_

A. When frozen water is _thawed_, it absorbs _heat from the air and objects around_ to melt its ice, in consequence of which the cold is greatly increased.

Q. _Why is it WARMER in a FROST than in a THAW?_