“All the rivers run into the sea; yet the sea is not full.” It does not overflow its limits. And long ago as King Solomon wrote these words, he was able to give the reason. “Unto the place from whence the rivers come, thither they return again.”
The ocean receives these generous gifts from the land, only to be generous in return. That which the ocean freely accepts she gives again, lavishly and royally. Enormous supplies of water are needed on land, and it is the work of the ocean to meet that need.
Throughout the world, and especially within the tropics, a constant drying up of the sea-surface is going on. This passing away of visible water out of sight is, like countless other everyday events, very extraordinary; though we think nothing of it because of its familiarity.
A tumbler of water is spilt upon a boarded floor. “Oh, never mind,” says a careless voice. “It will soon dry up.” Yes, of course it will; and who troubles himself to think what the “drying up” means? Particle by particle the spilt water will creep softly away, not as a liquid but as a vapour, into the atmosphere, there to float, hidden from sight, and to be carried hither and thither wherever the air may be moving.
Had such a thing never been seen before, it would arouse the wonderment of every thoughtful mind in Europe. But miracles of daily occurrence cease to be miracles.
Some few solids, such as camphor, have the same property of passing slowly out of sight into the air, though it is a much slower process with camphor than with water. Snow and ice also, when there is no thaw, dry off gradually. I have known a whole slight fall of snow disappear thus, without any thaw.
Vapour is always present in the air; sometimes a large amount, as much as the atmosphere is able at that particular temperature to hold; sometimes a less amount, not so much as the air could contain. In the latter case “drying” goes on briskly; in the former case it languishes, and wet clothes, wet pavements, remain long damp.
Warm air can hide away a much more abundant supply of vapour than cold air; so the hotter and drier the air, the quicker is the evaporation or drying-up of water-surfaces. This is remarkably seen in the Red Sea. An amount of water passes away from that sea in a single year, sufficient to bring down its level some fifteen or twenty feet,—which it would do, but for a powerful current always flowing in from the Indian Ocean.
When sea-water thus passes off in vapour, it leaves the salt behind. Were this amount of evaporation to go on steadily in the Red Sea, without any fresh supplies of water being received, about two thousand years would suffice to dry up the whole Sea, nothing but a great mass of salt remaining in its bed.
If the Red Sea alone loses in one year a slice of water, fifteen or twenty feet thick, imagine what the enormous quantity must be which is raised, year by year, from the surfaces of all tropical oceans, and in a less degree from the seas in colder regions. The weight of the whole could only be told in hundreds of millions of millions of tons. And this vast mass is drawn up, gently and mildly, particle by particle; to be wafted by breezes to regions where water is urgently required, and there to be poured down as rain upon the thirsty ground. Thus the great exchange is carried on, with steady sequence, between Land and Ocean, Ocean and Land.