Sea-water does not freeze so quickly as fresh water. On account of the salt which it contains, it does not become solid until it is four degrees colder than fresh water needs to be. The freezing-point of fresh water is 32° F.; that of salt water is 28° F. As ocean-water freezes, most of the salt is separated from it; and the ice formed is practically fresh, though often it holds unfrozen salt water in tiny hollows. CHAPTER X. RECEIVING--TO GIVE AGAIN “O end to which our currents tend, Inevitable Sea!”--A. H. CLOUGH. “All the rivers run into the sea; yet the sea is not full.” So wrote the wisest of men, long centuries ago; and the words are true in a world-wide sense, which the writer with all his wisdom could not then have fully grasped. “All the rivers” meant to him a great many streams, large and small, in southern Asia, and in southern Europe, with some in northern Africa. “All the rivers” of the world _now_---- Let us see what it means. The chief work of a river is to drain the land; to bear superfluous waters into the great deep. Water lying stagnant on land becomes harmful. Immense supplies are perpetually needed; but it must be fresh water, clear water, running water, water newly fallen from the skies, or newly received from ever fresh because ever renewed springs. Never stagnant water. So Earth’s rivers--we are talking of land-rivers now--gathering to themselves the multitude of lesser streams and tributaries, which in their turn have been earlier fed by an infinite number of burns and runnels in country and town, hurry downward to the Ocean, with rich presents from the Land. But all the while, Land expects “to receive as much again.” When we stand beside a broad river, watching the steady flow, hearing the little murmurs of sound and the soft suck and “swish” against the banks, we do not often realise the greatness of the task which that river may have in hand. Suppose we are on the banks of the Seine, the mother-stream of gay Paris. Many a century has Paris lived; but the Seine existed countless centuries before Paris was ever heard of. Year by year the Seine drains away surplus water from some twenty-three thousand square miles of French territory. Year by year the Seine carries down to the Ocean a gift of more than five cubic miles of water. Five hundred cubic miles in the course of a century! Or suppose we turn to the Rhine, that river of castles and legends, which starts with the meltings of over one hundred and fifty glaciers, and is said to be fed in its course by something like twelve thousand lesser streams. The Rhine drains, at the very least, between thirty and forty thousand square miles of country; and it pays to the Ocean an annual tribute of more than ten cubic miles of water. More than a thousand cubic miles each hundred years.