Leaving their wanderings through the regions of air to the next chapter, I shall now describe the principal ocean currents, the simple, but powerful agencies by which they are set in motion, their importance in the economy of nature, and their influence on the climate of different countries. Even in the torrid zone, the waters of the ocean, like a false friend, are warm merely on the surface, and of an almost icy coldness at a considerable depth. This low temperature cannot be owing to any refrigerating influence at the bottom of the sea, as the internal warmth of the earth increases in proportion to its depth, and the waters of profound lakes, in a southern climate, never show the same degree of cold as those of the vast ocean. The phenomenon can thus only arise from a constant submarine current of cold water from the poles to the line, and strange as it may seem, its primary cause is to be sought for in the _warming_ rays of the _sun_, which, as we all know, distributes heat in a very unequal manner over the surface of the globe. Heat expands all liquid bodies, and renders them lighter; cold increases their weight by condensation. In consequence of this physical law, the waters of the tropical seas, rendered buoyant by the heat of a vertical sun, must necessarily rise and spread over the surface of the ocean to the north and south, whilst colder and heavier streams from the higher latitudes flow towards the equator along the bottom of the ocean, to replace them as they ascend. In this manner, the unequal action of the sun calls forth a general and constant movement of the waters from the poles to the equator, and from the equator to the poles; and this perpetual migration is one of the chief causes by which their purity is maintained. These opposite currents would necessarily flow direct to the north or south, were they not deflected from their course by the rotation of the earth, which gradually gives them a westerly or easterly direction. The unequal influence of the sun in different parts of the globe, and the rotation of the earth, are, however, not the only causes by which the course of ocean-currents is determined. Violent storms move the waters to a considerable depth, and retard the flow of rivers, and thus it is to be expected that continuous winds, even of moderate strength, must have a tendency to impel the waters in the same direction. The steady trade-winds of the tropical zone, and the prevailing westerly winds in higher latitudes, consequently unite their influence with that of the above mentioned causes, in driving the waters of the tropical seas to the west, and those of the temperate zones to the east. The tides also, which on the high seas generally move from east to west, promote the flow of the ocean in the same direction, and thus contribute to the westerly current of the tropical seas.