TIDES. 6 1 



Apart from the fact of tidal water being instrumental in 

 maintaining the depth in river-channels, only vessels of the 

 smallest class could reach such ports as London, Hull, Bristol, 

 Cardiff, etc., if tidal range did not exist. It is, further, eminently 

 useful from a sanitary point of view. These considerations, 

 however, apply chiefly to river-works ; and, excepting in the case 

 of small breakwaters or piers constructed above low- water mark, 

 range of tide is decidedly prejudicial to breakwater construction. 



Where the tidal range is considerable, advantage is sometimes 

 taken of it to lift and float large blocks of concrete into position, 

 they having been previously constructed upon a suitable plat- 

 form. This system was adopted and, I believe, initiated by Mr. 

 B. B. Stoney, in works which he carried out in the River Liffey. 

 Large blocks (about 70 tons in weight) were also transported 

 and placed in position in this manner, under Sir John Coode's 

 direction, in the Jersey harbour works. 



There are many interesting phenomena connected with the 

 tides in rivers and estuaries, but a consideration of them here 

 would carry us into a field of inquiry which lies outside the 

 limits of this work. 1 I will, however, before closing this chapter, 

 say a few words in reference to the curious phenomenon which 

 is known by the name of " bore." 



When an estuary contracts shoreward, and when its depth 

 decreases so as to form flat low-lying foreshores, if the rise of 

 tide is considerable and rapid, the tidal wave advances over this 

 flat foreshore in the form of a " breaker," or " bore." This is due 

 to the tidal water rushing on with great rapidity, and the wave 

 or it may be only its wings being raised and made to break 

 as it traverses the shore on either side of the channel. 



Bores may vary in height from only a few inches to many 

 feet, according to the conditions under which they are formed. 

 In the Severn they attain a height of from 5 to 6 feet at spring 

 tides, the wave being of less height in mid-stream, where it does 

 not break. In the Amazon their height is said to be more than 

 double this. Bores also occur in the Hoogley, Orinoco, Solway 

 Frith (above Silloth), Morecambe Bay, the Seine, the Gulf of 

 Fundy, and other places. 



In a bore, as in a wave breaking upon the sea-shore, the 

 velocity of the wave and of the water which transmits it are 

 nearly equal, the wave being pre-eminently one of translation. 

 1 The subject has been fully treated in " Tidal Rivers," Wheeler. Longmans. 



