G. H. Kinahan — On Wind and Currents. 83 



Fifth, Pentecost Island. — On this island the marks are not so clear, 

 when seen from seaward, but on landing they are noticeable ; it has 

 also old coral rock at a great elevation. 



The absence of barrier reefs round these islands is a remarkable 

 characteristic ; is it due to the gradual upheaval ? 



The islands have generally a short fringe reef surrounding them. 

 If the above theory is correct, these will, in course of time, form 

 another step of the elevation. The black volcanic rock that shows 

 out from the limestone cliffs at various heights seems to uphold this 

 idea, as this rook must form the base upon which the coral lias 

 ffrown.. 



n^OTXOIES OIF ^VEIEinyrOIlK^S. 



The Drifting Power of Tidal Currents versus that of Wind- 

 waves. By G. H. Kinahan, M.E.I. A., etc. 



[Abstract of a paper read before the Britisb. Association, Bristol, August, 1875, 

 and before tbe Eoyal Irish Academy, Dec. 1875.] 



IT might have been supposed that the exhaustive Eeport on Waves 

 by J. Scott Eussell, F.E.S., etc.,^ should have decided the rela- 

 tive effectiveness of the tidal currents and wind-waves with regard 

 to their drifting powers. This, however, seems not to be the case, 

 if we ma}^ j^^dge from the recent paper on The Chesil Beach, Dorset- 

 shire, read by Professor Prestwich before the Institution of Civil 

 Engineers, Feb. 2nd, 1875, and the discussion which followed. 



In the Eeport above mentioned, Scott Eussell divides waves into 

 four orders : — To the first of these or waves of translation belongs 

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

 (with a certain limitation) belong to the second order ; this limita- 

 tion being that those wind-waves 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 Eussell 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 running up the beach, they become waves of 

 translation, and even then their action is limited to quite a narrow 

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

 considerable and permanent banks, as in the Mediterranean, where 

 the detritus brought down by the Ehone is piled during storms on 

 the neighbouring shores, forming banks and lagoons. Considerable 

 wind- wave action also will be found in fresh-water lakes and in 

 brackish-water lagoons, if in the latter the cross-tides counteract one 

 another ; but as far as my experience goes in the seas round Ireland, 

 the wind-waves do very little permanent work if unaided by the 

 tidal currents. If wind-waves did effect permanent driftage, it 

 ought to be apparent on the coast- lines ; the direction of its move- 

 ment corresponding with that of the prevailing winds resolved along 



^ Eeport on Waves, Brit. Assoc. Report, vol. xiii. 1844, p. 311. 



