A. Because the _heat of the water_ (immediately below the frozen surface) _passes 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 mass 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 _mass 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 RETARD 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?_