On Wind Waves and Tidal Curre7its. 85 



In illustration of these points the author exhibited a series of 

 diagrams to show the way in which, when two headlands project 

 about equal distances from the general coast-line, the secondary cur- 

 rents branching from the main tidal current decrease in power from 

 the headland first touched, while it is not unusual for a half-tide 

 counter-current to set backwards along the shore from the second 

 headland. Also that when one headland projects much further than 

 the other, if the coast-line is formed of soft materials, off-shore 

 shoals usually are originated, and the secondary currents increase 

 in power from the first headland, if it be the longest. 



If, however, the longest headland is the second one, and a strait 

 occurs at the end of it, as in the English Channel between Portland 

 Bill and Cape La Hague,.^ the secondary currents increase in intensity 

 towards the strait, — a condition which may account for the peculiar 

 sorting of the pebbles on the Chesil Bank, from Bridport to Portland. 



Eaces formed by the meeting of counter-currents with the main 

 tidal current are always connected with the tail end of an off-shore 

 bank ; but whether the bank is due to these causes, or the currents 

 to the bank, it is hard to determine. 



An island off a headland seems always to have secondary currents 

 passing it on each side, between which a bank grows from the main- 

 land, the counter-currents much resembling those which would have 

 occurred were the island connected with the shore. 



The flow of a large river generally forms a bank or shoal off its 

 mouth at the junction of the river current with that of the tidal 

 current. About this bank the currents run in various complicated 

 directions ; at different heights of the spring- and neap- tides — high- 

 water spring-tide currents often running right across it. 



In a muddy estuary the affluents of the main channel are nearly 

 invariably deflected up stream, showing that the arrangement of the 

 detritus is influenced more by the incoming tidal current than by 

 the efflux. Such tidal accumulations, however, are greatly modified 

 by the river floods, as a strong freshet will effect considerable 

 denudation while the tide is out. Denudation may also be effected 

 by artificial means, as in the estuary of the Boyne, where, by 

 judicious arrangements, the tidal waters have been made to assume 

 the functions of river freshets. 



Scott Eussell has shown that at the centre a wave of translation 

 is higher, stronger, and swifter, than at its margins. Somewhat in 

 the same way the driftage of the incoming tidal wave is usually 

 much stronger offshore than it is inshore. This is well known to 

 fishermen, who often neglect outlying fishing grounds on account of 

 the additional labour connected with them arising from the augmented 

 velocity of the tides. 



On a steeply sloping beach the driftage solely due to the tidal 

 wave current is conspicuous when there are no waves formed by the 

 wind. Under these conditions some particles of the beach may be 



1 The effect here is also augmented by the increased current in the sea between 

 these two headlands due to the " nodal point" of the tide in the English Channel 

 at Swanasre. 



