A VISIT TO THE HANGCHOW BOTH-: 107 



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 phe- 

 nomena. This must be our justification for presenting here much 

 that can be found elsewhere and is already matter of common knowl- 

 edge, 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 Eobert 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 evi- 

 dence 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 in- 

 creased 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 



