i8o PHYSIOGRAPHY. [chap. 



be forgotten that the ocean is subject to that grand 

 rhythmical movement which was referred to in the first 

 chapter. 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 four-and-twenty 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 standard 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. The 

 Ordnance Survey has fixed its datum line, or standard from 

 which all heights are measured, as the mean tide-level at 

 Liverpool. The level of high water at London Bridge, 

 which is sometimes taken as a standard, is called, from 

 the Trinity House, " Trinity High-water mark." 



As the cause of the tides is to be found outside 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 oscilla- 

 tory wave, and not a wave of translation ; the water simply 

 rising and falling, but not moving onwards. While, how- 

 ever, this is true of the tidal Avave in the ocean, it must be 

 borne in mind that, in narrow seas, it becomes converted 

 into an actual wave of translation. Where the channel is 

 contracted, as in a narrow strait, the tide may produce a 

 rapid rush of water, or a race. If, again, the tidal wave 

 rolls into a narrow estuary, the water becomes heaped up, 

 and produces a sudden rush into the channel of the river : 

 such a wave is called a bore, and is well seen in the Bristol 

 Channel, at the mouth of the Severn, where at certain 



