The rapidity of currents is very different, but always important enough to be taken into account by navigators. The well-informed seaman makes use of them to traverse wide spaces with greater rapidity, and, after an apparently circuitous course, arrives sooner and more safely at his journey's end than the ignorant steersman, who vainly endeavours to strive against their power. [Illustration: Pavonia lactuca, with Polypes in Natural Position.] [Illustration: LIGHTHOUSE AND WATER-SPOUTS.] LIGHTHOUSE AND WATER-SPOUTS. A Lighthouse on a rocky shore is represented as just lighted, the twilight having become darkened by a sudden storm, during which the phenomena of "water-spouts" occur, which are represented to the left of the Lighthouse. CHAP. VI. THE AËRIAL AND TERRESTRIAL MIGRATIONS OF THE WATERS. Movements of the Waters through Evaporation.--Origin of Winds.--Trade-Winds.--Calms.--Monsoons.--Typhoons.--Tornadoes.-- Water-Spouts.--The Formation of Atmospherical Precipitations.--Dew.-- Its Origin.--Fog.--Clouds.--Rain.--Snow.--Hail Sources.--The Quantities of Water which the Rivers pour into the Ocean.--Glaciers and their Progress.--Icebergs.--Erratic Blocks.--Influence of Forests on the Formation and Retention of Atmospherical Precipitations.--Consequences of their excessive Destruction.--The Power of Man over Climate.--How has it been used as yet? Neither storms nor ocean-currents, nor ebb and flood, however great their influence, cause such considerable movements of the waters, or force them to wander so restlessly from place to place as the silent and imperceptible action of the warming sunbeam. In every zone evaporation is constantly active in impregnating the atmosphere with moisture, but the chief seat of its power is evidently in the equatorial regions, where the vertical rays of the great parent of light and heat plunge, day after day, into the bosom of ocean, and perpetually saturate the burning air with aqueous vapours. In this chapter I intend following these invisible agents of fertility and life, as they lightly ascend from the tropical seas, and accompanying them in their various transformations, until they once more return to the bosom of their great parent. A cursory view of the benefits they confer on the vegetable and animal world, as they wander over the surface of the land, will, I hope, agreeably occupy the reader, and serve to increase his admiration for that deep and dark blue ocean without which all organic life would soon be extinct upon earth. I begin with a few words on the winged carriers of marine exhalations, the _winds_, which, although now and then detrimental or fatal to individuals by their violence, largely compensate for these local injuries, by the constant and inestimable benefits they confer on the whole body of mankind.