On cotidal maps

Abstract

N maps delineating the features of the earth, the sea is generally pictured as a vast plain undistinguished by any marks of whatever kind. Even when contour lines or soundings are shown in ocean areas it is to the land that they refer-to the land beneath the sea. The surface of the sea, however, is not a monotonous plane stretching from horizon to horizon. For altogether apart from the swells and waves which are brought about in the surface layers of the water by the play of wind and weather, the sea sustains periodic changes in response to the mighty pulse of the tide-producing forces. Twice daily, in rhythmic fashion, the tide-producing forces of sun and moon stir the sea to its depths and give rise to the phenomena which for short are called the tide. In so far as our knowledge of the tide is based on observed data, it may be said to be confined wholly to the coast line. The difficulties involved in measuring the rise and fall of the tide in the open sea are obvious, and as yet there are no such observations at hand. To be sure the mid-ocean islands interrupt, here and there, the wide expanse of tidally unknown seas; but these islands are few in number. Moreover, the tide rising and falling on the island shores is no longer the unaffected open ocean tide; for in breaking the uniformity of the ocean depths, the islands by the same token modify the tide. It may perhaps not be out of place here to emphasize the fact that the tide has its birth in the open ocean basins. It is the action of the tide-producing forces of sun and moon on the waters of these huge basins that makes possible the rise and fall along the coast. What, geographically, is the course of the tide across the oceans? How can the progress of the tide from place to place be pictured? It is these questions that cotidal maps attempt to answer.