262 Scientific Proceedings, Royal Dublin Society. 



and forwards from one furrow to another ; sometimes it would 

 stop on the intervening ridge, and so for the moment help to huild 

 it up ; at others it would fall over into the furrow towards which, 

 for the moment, it was being propelled ; but in no case did 

 it show any tendency to travel continuously in any particular 

 direction along the bottom." — {Proc. Royal 8oc., vol. xxxiv. 

 p. 3.) It will be observed that in this instance the observation was 

 made in very shallow water, at a point close behind the plunging 

 line of the wave, and that even here the primary and secondary 

 wave-currents were so evenly balanced, that though in the case of 

 light objects there was abundant horizontal alternating motion 

 on the bottom, I was unable to detect any sign of one wave-current 

 being more powerful than the other, still less that the incoming- 

 swells were in the least degree transformed into waves of transla- 

 tion. 



It may be observed that the fact that when the sea-bottom 

 slopes gently from the water-margin the sand is often covered with 

 symmetrical ripples, or ripj^les with their anterior and posterior 

 slopes equally inclined, from the vicinity of the plunging line to a 

 depth as great as the eye can distinguish them, is sufficient proof 

 that the currents that formed them were evenly balanced. 



As the question of the formation of ripple-mark is important, 

 as bearing on the subject of the present Paper, I take this oppor- 

 tunity of calling attention to the valuable Papers of Monsieur C. de 

 Candolle, Professor F. A. Forel, and of Dr. Gr. H. Darwin, published 

 respectively in the ^rc/M^es des Sciences, and in the Proceedings of the 

 Royal Society, all in the year 1883. But I would more especially refer 

 to two earlier notices of observations by Professor Eorel in the Bulletin 

 de la Societe Vaudoise des Sciences Naturelles for February and March, 

 1878. In the earliest of these notices Professor Forel points out how 

 the shape of sand- ripples formed by waves in the currentless waters 

 of lakes differs from that of ripples formed by running streams. 

 This was one of the points I endeavoured to establish in 

 1882, in ignorance of the prior published observations of Pro- 

 fessor Forel. 



The currents set up by a wave plunging on the shore are more 

 difficult to make out than those hitherto considered. Their sequence 

 seems to be as follows, viz. : — 



