92 THE OCEAN. 



BOOK III.— THE TIDES. 



CHAPTER XI. 



OSCILLATIONS OF THE LEVEL OF THE SEAS,— THEORY OF THE TIDES. 



Another movement which keeps the waters of the sea in a constant 

 agitation is that of the tides. AVhile the currents carry the waves 

 from one pole to the other, and stir the very mass of the ocean, the 

 tides incessantly modify the level by the alternations of ebb and flow, 

 which they impart to its waters. They raise or depress without relax- 

 ation the mass of waves on all the shores of the globe ; the strand, 

 which by turns they invade and lay bare becomes debatable ground 

 between the two elements, and successively forms a part of the oceanic 

 basin, and the continental surface. Twice a day vast plains of sand 

 like those of Mount St Michael are invaded by the waves, deep bays 

 are formed far into the land, and barks glide with sails spread above 

 tlie path which the pedestrian has just quitted. Twice a day the 

 same tidal wave causes the waters brought to it from the continents 

 to return back again, transforms simple rivulets into large rivers, 

 clianges basins filled with mud into vast inland harbours, and carries 

 fleets of ships over sandbanks and hidden rocks. Six hours after- 

 wards all is changed. The tidal ports are strewn with ships stranded 

 and lying in the mud, the mouths of rivers allow their islands of 

 alluvium to emerge, and great bays are no more than plains of 

 sand. Thus the outline of continents incessantly changes in ap- 

 pearance; the girdle of estuaries and ports, beaches, rocks, and sand- 

 banks which surround their coasts, continually alter, and change 

 the geography of the shores in the same proportion. Besides, move- 

 ments so considerable cannot occur without being accompanied by 

 very powerful currents, flowing alternately from the open sea towards 

 the eoa^t, and from the coast to the open sea, and contributing greatly 



