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Page:Popular Science Monthly Volume 72.djvu/111

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107
A VISIT TO THE HANGCHOW BORE

currents in rivers, will enable the general reader to understand better the phenomenon we are about to consider, and to appreciate its proper place as compared with other more general and ordinary tidal phenomena. This must be our justification for presenting here much that can be found elsewhere and is already matter of common knowledge, but which needs to be correlated and reviewed in this present connection.

The discrepancies in the range of the tides at different places are due chiefly to the local conformations of coasts and sea-beds. Indeed, it seems, as Sir Robert Ball has pointed out, that if the whole earth were covered with a uniform and deep ocean of water, the tides would be excessively feeble, since barometric records give no very distinct evidence of tides in the atmosphere, which is a deep and vast ocean of air embracing the whole earth to a practically uniform depth.

Along the borders of land areas the range of the tide is found to vary from zero up to seventy feet. Few of us realize how small the range is in some places, where at first sight we should expect it to be considerable. In midocean, for instance, an island like St. Helena is washed by a tide only about three feet in range; an enclosed sea like the Caspian or the Black is subject to no appreciable tides whatever, and even the Mediterranean, notwithstanding its connection with the great Atlantic, is subject in general to inconsiderable tides, the range of water-level varying from eight inches at Brindisi to two feet four inches at Trieste. The Mediterranean tides are, however, more strongly developed in the Bay of Gibraltar (where the range is from five feet to six feet five inches), the upper Adriatic and the Gulf of Gabes.

In the deep wide reaches of the ocean, the tidal elevation progresses at the tremendous rate of about five hundred geographical miles an hour. But as this is merely the passing of an oscillation whereby the particles of water are gently moved through a cycle of positions, there can be no appreciable effect upon the distant ocean bottom, on an average of two or three thousand fathoms below. When, however, the tidal wave enters a shallow sea, the friction of the bottom becomes more and more effective in decreasing the speed while it increases the height and effective force of the wave. Again, when the tidal swelling is increased in height by the convergence of the shores between which it moves, it is no longer a mere oscillation or pulsation of the great ocean, but the water acquires a true motion of translation, and rushes past headlands and through narrow channels with tremendous force and speed—a phenomenon well known along the west coasts of Scotland and Scandinavia. In some cases the advancing tide on entering a narrow inlet or estuary gathers itself into one or more large waves, and rushes up between the converging shores. Thus, owing to the gradual