254 Scientific Proceedings, Royal Dublin Society. 



Eussell are accompanied by antecedent hollows, though the depres- 

 sion may be small, and very extended, in proportion to the height 

 of the wave. The fact can be easily proved in the case of artificial 

 oscillating waves made to roll over water of gradually diminishing 

 depth. If under these circumstances a wire be stretched across the 

 water at the level of repose, it will be found that on the approach of 

 a wave the water will sink away, and leave the wire suspended in 

 mid-air, thus showing that each wave is preceded by a depression. 



If oscillating waves, on approaching the shore, were transformed 

 into waves of translation, raised entirely above the level of repose 

 of the water, it would naturally follow that such waves would 

 plunge on the shore landwards of the margin of repose for the time 

 being. 



This point, as affecting the erosion of the land by sea- waves, is 

 of some importance to geologists. Professor J. D. Dana seems to 

 refer to it in the following passage : " Since a wave is a body of 

 water rising above the general surface, and when thus elevated 

 makes its plunge on the shore, it follows that the upper line of 

 wearing action may be considerably above high-tide level. Again, 

 the lower limit of erosion is above loio-tide level ; . . . . there is there- 

 fore, a level of greatest wear, .... and another of no wear, which is 

 just above low tide." — [Man. of Geol., 3rd ed., p. 676.) 



Now, this statement, though strictly in accordance with Mr. 

 Russell's theory, has, so far as I can ascertain, no foundation in fact. 

 An attentive study of heavy waves breaking on a beach will satisfy 

 the observer that all waves derived from oscillating waves break 

 far below, or seawards of, the margin of repose of the water. As 

 it is sometimes difficult on the sea-shore to detect the line of mean 

 level for the time being, it will, perhaps, be better in the present 

 inquiry to let observation be preceded by experiment. 



On the 6th October, 1882, I banked up the sand at one end of 

 my experimental tank, and caused waves to attack this artifical cliff 

 for about three quarter of an hour. My object was to ascertain the 

 general action of waves in tearing down and distributing beach 

 material, and I find no special entry in my note-book as to the re- 

 lation of the line of plunging with the water margin ; but a series 

 of careful measurements recorded at the time supplies the informa- 

 tion required. 



At the close of the experiment, the breadth of the strand from 



