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?