160 The Realm of Nature CHAP. 



the globe were arranged in strips from north to south, 

 cutting up the hydrosphere into a series of narrow com- 

 partments, there would be no appreciable tidal effect. By 

 the actual arrangement of land there is a free water ring 

 in the Southern Ocean only ; there is one long comparatively 

 narrow compartment, the Atlantic Ocean ; another wider 

 and shorter, the Indian Ocean ; while the rest of the hydro- 

 sphere forms the wide open surface of the Pacific extending 

 half-way round the globe at the equator. In the Pacific 

 and the adjacent Southern Ocean alone the tidal wave has 

 full room to form, and from them the wave passes westward, 

 being deflected northward into the other oceans. Co-tidal 

 lines on a map (Plate XIII.) show the places which the 

 same phase of the tidal wave reaches at the same hour. 

 Starting from 1 2 the position of the crest of the wave at 

 each successive hour is marked by i, 2, 3, up to 12. 

 The tidal wave travels most rapidly, and is longest and of 

 least amplitude in deep water ; in the central Pacific the 

 range between High Water and Low Water (the amplitude 

 of the tidal wave) is less than 2 feet, and no current is 

 produced. 



218. Tidal Currents. When the tidal wave enters 

 shallow water it becomes shorter and moves more slowly. 

 The under side of the wave becoming more retarded than 

 the top, the surface water is carried forward as a true 

 current, the energy of which is derived from the Earth's 

 rotation. In this way shoals or submarine peaks convert 

 the simple up and down movement of the tide in the open 

 ocean into rapid currents, usually for a very short distance 

 but sometimes extending to a great depth. These are 

 more definite along the shores. The usual tidal effects 

 observed on a broad gently-shelving shore are the gradual 

 rise of the level of the water, the submergence of the beach 

 and advance of the sea on the land ; then after the highest 

 point has been attained, the gradual lowering of level with 

 corresponding uncovering of the beach and retreat seaward 

 of the sea-margin. At New and Full Moon, when spring- 

 tides ( 114) occur, the rise and fall is at the greatest, and 

 then, at any one place, high water occurs at the same 



