The Sea and Its Work 



is not so much to abrade the land as to carry off 

 the results of its abrasion by other means, and to 

 distribute the finely suspended matter, far and 

 wide, over the floor of the ocean. 



In addition to the movements of the sea which 

 have been already noted in this chapter the 

 wind-waves, the surface-currents, and the general 

 circulation it must not be forgotten that the 

 ocean is subject to a grand rhythmical movement. 

 We saw, when standing on London Bridge, that 

 the water regularly ebbed and flowed, and, what 

 it does there, it does at every point along our 

 coast. Twice in every twenty-four hours the 

 margin of the sea rises, and twice it falls, so that 

 its level is constantly shifting up and down. 

 And yet it is a common practice to say that a 

 given elevation is so many feet above the sea- 

 level. Such a statement assumes that the stand- 

 ard taken is neither high-water mark nor low- 

 water mark, but the mean level between the two; 

 the water rising, at one time, as much above 

 our standard level as it falls, at another time, 

 below it. 



As the cause of the tides is to be found out- 

 side our earth, its explanation must be deferred 

 to a later portion of this work. It is sufficient to 

 remark, in this place, that the great tidal wave, 

 which travels round the earth, is an oscillatory 

 wave, and not a wave of translation; the water 

 simply rising and falling, but not moving onwards. 

 While, however, this is true of the tidal wave in 

 the ocean, it must be borne in mind that, in 

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