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Currents in the ocean arise from various causes. They may be produced by long-continued gales of wind, by the melting of polar ice, or by any cause that may give rise to onward movements of limited portions of the great mass of waters. Other currents, and of these only is it necessary to speak in this connection, are permanent. The most remarkable of these are the polar currents and the equatorial currents.

Polar Currents are produced by the perpetual movement of the waters from the poles to the equator. In accordance with the laws of mechanics, an accumulation of the waters takes place on that part of the globe which has the greatest velocity of motion; and as the earth in turning on its axis moves with far greater velocity at the equator, the waters continually flow toward that line from the poles.

Equatorial Currents. This accumulation of the waters at the equator tends to produce the equatorial currents, which consist of the continuous progression of the tropical seas in a westerly direction. When the wave brought by the polar currents arrives—coming as it does from regions where it naturally has less velocity—it does not at once acquire the velocity of the earth’s motion at the equator; and since the rotation of the earth is from west to east, this portion of the water lagging behind forms a stream or current which has an apparent motion from east to west, that is to say, apparent as regards the earth, but real in relation to the adjacent land and water. The trade winds, which in this zone blow constantly in the same direction, lend their aid in maintaining the equatorial current.