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 than the water in situ there and is therefore denser; it sinks below the surface and continues to flow along the bottom either back to the polar regions or towards the equator.

This main scheme is complicated in various ways: (1) by the rotation of the earth, which continually deflects currents of water or air to the right in the northern or to the left in the southern hemisphere; (2) by the conformation of the land masses (as in the case of the equatorial stream which is banked up in the Gulf of Mexico and flows out through the Straits of Florida); (3) by the varying depth of the ocean, for currents tend to flow more readily through deep than in shallow waters (as in the case of the main Atlantic drift, which flows most strongly through the deep channel between Shetland and the Faroe Is.); and (4) by the driving force of the winds acting on the surface of the sea (thus the drift of water from the equator is not N.E., as one might expect, but from E. to W., because of the impelling force of the N.E. and S.E. trade-winds).

Long-range Periodicities in Oceanographical Changes.—More and more the science seeks to discover periodicities and to correlate these with others. In these attempts new methods are elaborated and in their criticism contributory phenomena are discovered. An interesting example is the discussion, by Otto Pettersson, of the effects of long-range fluctuations in the tide-generating force: this memoir was published about 1914, but has only recently become available to English readers.