The most obvious characteristic of the sea is its state of constant motion. To this property, sooner or later, must be referred all questions relating to the medium itself, or to its inhabitants. Horizontal movements, or currents, especially in the upper sea layers, are, generally speaking, of the first importance biologically. Their measurement in the North Sea, besides leading to the production of the chart illustrated on this page, has afforded results which have proved of value in the elucidation of biological problems in connection with fisheries.