Were the whole Earth covered by a single unbroken sheet of water, these drift currents might circle round and round the globe for ever, undisturbed in their working. But the Earth has lands as well as oceans; and when a current strikes a coast its course is altered, part at least being turned in a fresh direction. Much discussion has taken place about the causes which bring these great Drift Currents into being. Disturbing elements, many in number, may have had a hand in the matter. So soon as one part of the sea-surface becomes warmer than another part, movements are set going; the heavy cold water sinks, the light warm water rises, and streams are started from the one place to the other. There are also countless rivers pouring into the ocean, each helping to upset its equilibrium. Heavy downpours of rain raise the level of the sea here or there, inducing more currents. But these are lesser causes. It is now recognised, as a fact beyond question, that the main power in starting and sustaining ocean-currents is that of Wind. CHAPTER VII. OF WIND AND WATER “He commandeth and raiseth the stormy wind, which lifteth up the waves thereof.”--PSALM cvii. 20. “Measure me a measure of wind.”--2 ESDRAS. Over the great Deep of blue water lies the greater Deep of blue air. And between these two--the Ocean of Air and the Ocean of Water, both dragged earthward by the perpetual pull of gravitation--endless intercourse takes place. Each finds its way in small fragments into the other. There is always water present in the air. There is always air present in the water. Perhaps the latter fact is not so widely recognised as the former; yet it is equally true. Sea-water contains large supplies of dissolved gases, absorbed by the ocean-surface from the atmosphere, and passed on into lower depths, for the use of creatures living there. But the presence of water in air is known to us all. When we casually remark, “What a damp day it is!” we mean, “What an amount of water to-day is in the atmosphere!” Water-vapour, drawn up from Ocean’s broad bosom, is carried far and wide “on the wings of the wind,” to fall as rain where needed; and drying winds in their turn pass over regions where much rain has fallen, to bear away superfluous moisture. A good deal was said in the last chapter about oceanic currents; and mention was made of Winds as their chief cause. That the power of wind over water is great, anybody may know who has watched the lashing of the ocean into fury by a gale. But the degree of that power was scarcely grasped, until within the last few years. Rivers and streams in the sea were long ascribed to any cause, except the most weighty of all. No one supposed _the_ immense current-producing force to be that of Wind--not of mere local breezes or gales here and there, but of strong constant winds, which prevail month after month over wide ocean-districts.