Kin AH AN — On Sea-heaches. 19-5 



increase in size, and at the south, end of both strands the beaches are 

 principally shingle.'' Prom tower '^o. 4 the beach in Killiney Bay 

 travels northward with the current ; but in the vicinity of Dalkey 

 Island there is, a little before high water, a "half counter-tide" 

 south-westward. 



The "ebb-tide" waves flowing out of the lagoons and estuaries 

 deepen the channels and cut away the beaches margining the 

 narrows; but elsewhere on this coast line they do not affect the 

 beaches, except in the vicinity of Courtown, where the average rise of 

 tide is about three feet. Even here the travelling of the beach, which 

 is solely due to the ebb-tide current, is very small, as it only continues 

 for an hour or so during spring-tides. 



II. The Effects of the Wind Waves. 



The waves of this class that act on this coast are of two kinds, 

 "viz., " ground swells," or waves generated by storms in the Atlantic 

 or the Channel, and the waves directly due to the winds blowing on 

 the coast. Their effects are either to pile up and fill the beaches, or 

 to cut them out. 



The ordinary wind waves assist the "flow-tide" current if both 

 waves are going in the same direction, or if the wind waves strike the 

 beach at an acute angle ; if they strike the beach at a right angle they 

 fill it up, forming "fulls" and "storm beaches," while if they are 

 running in a more or less opposite direction to the flow-tide they cut 

 out the beach. As an example : Let the flow-tide current be from 

 south to north on an east coast. If the wind is blowing from any point 

 between S. and E., E.S.E. by E. the wind waves assist the tidal current ;. 

 but if it is blowing any point between E.S.E by E. and E.N.E by E, 

 strong, the wind waves will stop the travelling of the beach, and pile 

 it up, forming "full" or " storm beaches ; " while if the wind is 

 coming from any point between E.N.E. by E. and W. a " cutting out 

 tide " is the result. This cutting out is due to the " dancing waves," 

 generated by the meeting of the tidal current and the wind waves, 

 which toss and churn up the sand and other detritus, thus causing it 

 to be carried out by the back-wash into deep water. A continuous 

 heavy wind in the same direction as the flow-tide will accelerate the 

 carriage of a beach to such a degree that every particle of sand, gravel, 

 and shingle may be carried with the tide, thus leaving the up-stream 

 portion of a beach empty. These seem to be the general effects; but 

 near Courtown, where the difference between low and high- water is 



'' In a Paper " On the Drifting power of tidal cun-ents versus that of wind 

 waves" (Proc. Eoyal Irish Academy, 2nd ser., vol. ii. p. 448), I have demon- 

 strated that the reason why the larger pebbles occur down stream on some beaches 

 is because the "back-M^ash" carries down and out to sea the small gravel and 

 sand, leaving behind it on the slope of the beach the larger stones, which are pushed 

 higher and carried farther along the beach by each successive wave. 



R. I. A. PKOC, SER. II., VOL. III. — SCIENCE. Q 



