KiXAiiAX — Tiilal Cun'cnts and Wind-iracr-^. 443 



-XL. — The Deefiixg Po"s\-e:e or Tidal Cubeexts versus That of Wi>-d- 

 WATES. By G. H. Kes'ahax, IT. E. I. A., &c. 



[Read Xovember 30th, 18"o.] 



Ix might have iDcen supposed that the exhaiistire report on '\\'aTes, 

 by J. Scott Russell, F. R. S., &c.,''^ should have decided the relative 

 merits of the tidal currents f and wind- waves in regard to their drifting 

 powers. This, however, seems not to be the case, if we may judge 

 from the recent paper on the Chesil beach, Dorsetshire, read by Pro- 

 fessor Prestwich, before the Institution of Civil Engineers, Peb. 2nd, 

 1875, and the discussion that followed the reading of it. 



In the report above mentioned, Scott Russell divides waves into 

 f oui' orders. To the first of these, or the Wave of Translation, belongs 

 the gTeat tidal wave ; while wind-waves, according to that observer, 

 with a certain limitation, belong to the second order, the limitation 

 being, that those wind-waves that are in the act of breaking on a 

 beach change into waves of the first order. Indirectly, however, the 

 wind forms a different order of waves, for if water is piled up in a 

 narrow by the wind, the waves induced are "waves of translation." 

 Scott Russell also proves that a wave of the second order has little or 

 no carrying power ; consequently wind- waves can have little of this, 

 except when actually running up the beach, when they change to 

 "waves of translation ;" and even there their action is limited to a 

 quite narrow line. 



In a tideless sea, wind-waves breaking on the coast line form con- 

 siderable and pennanent banks, as in the iXIediteiranean, where the 

 detritus brought down by the Rhone is piled up during storms on the 

 neighbouring shores, forming banks and lagoons. Considerable wind- 

 wave action also will be found in freshwater lakes and in brackish- 

 water lagoons, if in the latter the cross tides counteract one another ; 

 but, as far as my experienca goes in the seas round Ireland, the wind- 

 waves do very little permanent work, if unaided by the tidal ciu-rents. If 

 wind-waves did effect permanent driftage, it ought to be apparent on 

 the coast lines, the direction of its movement corresponding with that 

 of the prevailing winds resolved along the trench of the coast. The 

 dii'ection of the prevailing winds is always registered by the lean of 

 the trees on a coast line, while the coui'se of the driftage is marked by 

 the sand ridges or banks forming the knee-shaped invers or mouths 

 to the streams, the inver being shifted laterally, in the dii-ection in 



* " Eepoi-t on "Waves," Brit. Assoc. Reports, Vol. xiii. 311, 18-i4. 



t Tidal CiuTents are due to the " Flow" and " Ebb " of the tides; these are 

 quite distinct from the " Rise " and " Fall " of the tides. Tliis subject i.s fully 

 explaincd in the tract on " The Tides and Tidal CuiTents of the Irish Sea and 

 English Channel," by the Rev. S. Haughton, F. T. C. D., ire., p. 3, et scq. 



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