CHAPTER IX 

 TIDAL PHENOMENA AND ICE 



Section 119. Certain Peculiarities of the Tides 



Tidal phenomena are expressed in periodic variable movements of water particles in a 

 vertical direction (variation in water level) and in a horizontal direction (ebb and flow currents). 



As observations have shown, the tidal force of the moon and sun create tidal phenomena 

 even in small and restricted sea basins, but basically these phenomena are generated in the belt 

 of the World Ocean embracing the globe around the high parallels of the southern hemisphere. 

 Here the tidal wave, continually stimulated and running from east to west, passes the southern ends 

 of America, Australia and Africa. After rounding these extremeties it spreads out to the north 

 as a free wave. The tidal wave enters the Atlantic Ocean and here spreads out along the conti- 

 nental slope of Eurasia right to the North American archipelago. On its way this wave sends out 

 branches into the individual surrounding seas. 



The spreading of the ocean tidal wave into shallow seas discloses some curious peculiarities: 



1. The tide-forming force of the moon and sun acts upon water particles practically inde- 

 pendent of their depth below the ocean's surface. Therefore the entire water mass of the World 

 Ocean takes part in the tidal phenomena. Since the tidal wave, as it spreads out, must pass some- 

 times over greater and sometimes over lesser depths, it develops that wherever the depth de- 

 creases from greater to smaller, the energy of the larger water masses is transferred to the 

 smaller masses and the amplitude of the tide is correspondingly increased. Contrarily, with 

 transfer from lesser to greater depth the amplitude is decreased. 



The depth of the ocean changes most sharply on the continental slope. Here the tidal wave 

 passes, in a short space, from the ocean depths to the lesser depths ofthe continental shallows. 

 Obviously the amplitude of the tide is thereby sharply increased and the speed of the tidal current 

 is proportionately increased. The same occurs with the further progress of the tidal wave along 

 the continental shallows. This is diagrammatically illustrated in figure 113. It is natural that the 

 larger amplitudes which are created at the borders of the ledges are slightly decreased in the sub- 

 sequent level deep places. 



2 . The increase in amplitude of tide upon approaching the shores is attributable not only to 

 the decrease in depth but also the decrease in width of the gulfs. Still another contributing factor 

 is the interference created as a result of the reflection of the tidal wave from the shores and its 

 addition to the waves arriving from other regions. 



The explanation of high tides by local conditions, i.e. , the configuration of the bottom and 

 the shores, is well demonstrated by the example of the Bay of Fundy, where the width and depth 

 decrease gradually. Correspondingly, the amplitude of the tide, which is equal to 4 m at the 

 entrance to the bay, gradually increases to 16 m as it progresses into the body of the bay. 



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