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) 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?