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. [Illustration: 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 God 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. 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 GOD 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 _shield_ 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?_