Now, under the continuous blazing sun of the torrid zone the sea-water near the surface gets fairly warm,—having an average temperature of about 85° along the equator,—while in the polar regions the ocean is always chilled by permanent or floating ice until it is nearly cold enough to freeze; but these masses of warm and cold water cannot remain separate in the universal ocean. The hot tropical flood, continually rising, _must_ flow away somewhere to find its level; and it can flow nowhere except toward the poles, for there the ever-sinking volume of chilled and therefore heavier water sucks it in to take its place, while it, in turn, creeps underneath toward the equator, there to fill the gap which the escaping warm water leaves behind. So we know there is constantly going on an interchange of water—a constant flowing _away_ from the equator northward and southward on the surface, and a flowing in _toward_ the equator along the bottom; an endless springing up in the torrid zone and a steady settling down in the polar seas. One out of many proofs of this fact is that the thalassal abysses below the depth of a mile or so are known to be ice-cold. This could not happen unless they were constantly filled and refilled with new water from the great coolers at the poles; for if the water at those depths should remain unchanged, it would soon become very warm from the heat of the interior of the earth, whence it does constantly extract some heat. But while this invisible _vertical circulation_ is going on, another more visible and interesting set of movements is in progress on the surface, forming what are known as _ocean currents_. These are vast rivers in the ocean flowing across its face in certain directions and to a certain depth, as rivers make their way along the land. They begin and are kept going mainly by a union of the two causes already explained—heat and wind. [Illustration: A STEAMER BORNE ASHORE BY AN EARTHQUAKE-WAVE.]