32 G. H. KINAHAN ON THE CHESIL BEACH, DOESETSHIEE, 



like manner the offing tide on the nodal lines begins to ebb at 

 half-ebb on shore — which is the relation between the offing tide 

 and the onshore tide, known to sailors as " tide and half-tide "*. 



Driftage of Sea-beaches. 



Now as to the driftage of sea-beaches. In Prof. Prestwich's paper 

 on the Chesil Beach, and the discussion that followed thereon, certain 

 points were raised. Of these the first and principal one seemed to 

 be that wind-waves have greater driftage power than the tidal cur- 

 rent. On this point it would appear, from the report, that Prof. 

 Prestwich and nine of his critics believe in the greater driftage- 

 power of wind-waves, one of the latter in wind-waves and tidal 

 currents combined, while only two believed in the superior power 

 of the tidal current over wind-waves f . As to the second, the author 

 of the paper goes so far as to state that the currents of the flow 

 and ebb tides neutralize each other's effects. The third point brought 

 forward was that a current carried the smallest fragments furthest. 



First point. — Prom what has been stated it is evident that the fol- 

 lowing is at variance with the opinions of many eminent men. In 

 another place % I have attempted to show that on the Irish coast 

 the current due to the flow of the tide has greater drifting powers 

 than wind-waves, the driftage due to the latter, if contrary to 

 that of the incoming tide, being eventually carried back again, and 

 that the maximum driftage results from a combination of the 

 tidal current and of waves caused by a wind blowing in a similar 

 or nearly similar direction to the flow of the tide. 



Wind-waves make a great appearance ; but if we look for the 

 results, they seem to be very like the noisy fussy worker, who is 

 "all show but little work;" while the tidal current may be com- 

 pared to the quiet steady man who has most to show at the end of 

 his day's work. Wind- waves can throw up beaches ; but it would 

 seem that, unaided, they cannot cause them to travel. In the 

 delta of the Phone (Ansted " On the Lagoons and Marshes of the 

 Shores of the Mediterranean," Proc. Inst. C. E. vol. xxviii., read 

 February 16, 1869) the wind-waves throw up beach after beach. 

 So also on the north coast of Wales, where we find series of " storm- 

 beaches " consisting of beach after beach, each outside the preceding 



* In the English Channel and the Irish Sea, the surface tidal currents are 

 known from the investigations carried on by the officers of the Royal Navy ; 

 there are, however, tinder tidal currents that have never been investigated. 

 They are known to exist ; but their laws and relations one to another are 

 perfectly unknown. On the subject of an inquiry like the present,they must 

 exercise a vast influence ; but unfortunately we cannot refer to them. 



t Of the ten advocates for the wind-waves, all seem to have studied prin- 

 cipally on the south coast of England, where, as the prevalent wind-waves 

 and the "flow" of the tide are in the same direction, one force may easily 

 be mistaken for the other ; while of three who believe in the tidal current, 

 two have studied the wave on the south, and the other on the west coast of 

 England. One of the other commentators, who has studied on the east coast 

 of England, does not give any definite opinion. 



X " Tidal Currents versus Wind-waves," Proc. Roy. Irish Academy, ser. 2, vol. 

 ii. p. 443, and Geol. Mag. dec. ii. vol. iii. p. 83. 



