CLASSIFICATION OF CURRENTS OF THE OCEAN By the term "oceanic circulation" is meant the whole of those progressive movements of the water generally called currents. But several causes tend to produce different aspects of these as well as other movements of the ocean. Thus constant currents, or stream currents—as they are sometimes called, from their analogy to " streams " on the land—are vir tually perennial "rivers in the ocean," regularly flowing in the same direction at all times ; while periodical, or variable currents, change their direction periodically, or at uncertain times, in obedience to similar changes in the direction of the prevailing winds. The distinction between deep-sea currents and drift currents is implied in the terms used—the latter being merely a " set " of a shallow layer of the surface water, while the former are constant currents of considerable depth.
Surface currents, also, generally overlie under-currents flowing in a different direction. Of two related currents, the principal stream is called the current, in contradistinction to the return stream or counter-current. The student must also bear in mind that marine currents are named differently to air cur rents. Winds are named according to the direction from which they blow—thus an "east wind" is a wind blowing from the east. Currents, however, are named according to the direction in which they are flowing. Thus a current flow ing to the north is called a " northerly current."