38 



Prof. G. H. Darwin. 



[Nov. 22, 



chaque demi-oscillation* toute la distance qui separe Tune de l'autre 

 deux cretes consecutives. Ce va-et-vient des particules s'etend 

 jusqu'a une certaine distance au-dessous du sommet de chaque crete, 

 mais son amplitude va en diminuant de haut en bas, en raison du 

 poids plus considerable des particules inferieures. II en resulte que 

 chacune de ces cretes mobiles a l'apparence d'une lamelle qui oscille 

 sur le sommet de la ride qu'elle termine et s'etire en meme temps dans 

 le sens du mouvement de l'eau, ce qui donne tout a fait l'apparence 

 d'un corps visqueux. 



"Lorsque l'amplitude du balancement du liquide diminue, il en est 

 naturellement de meme des excursions de ces lamelles, et si Ton vient 

 a arreter subitement le balancement, les particules composant les 

 cretes mobiles peuvent se deposer entre les rides ou. elles forment un 

 systeme de rides secondaires plus minces, intercalees entre celles qui 

 correspondent au maximum d'araplitude du balancement." 



In this passage the dance of the particles is in the first place 

 described as being from one side to the other of the ridge, and this, I 

 believe, is the fact. This statement is, however, apparently contra- 

 dicted by what follows, viz., that the dance is from crest to crest. I 

 have very rarely seen the intercalated ripple-marks to which 

 M. de Candolle refers, but I venture to think that his explanation is 

 not sound, and that they are formed by the particles of sand which, 

 in violent oscillation, have been caught up by the secondary or tree 

 vortices, carried quite round and dropped at the root of the tree, 

 when the oscillations of the water are dying out. 



M. de Candolle arrives at the interesting conclusion that the wave- 

 length of ripple-mark is independent of the nature of the oscillating 

 fluid. His suggestion that cirrus clouds are ripple-marks between two 

 aerial currents will be referred to below. The whole paper forms a 

 valuable contribution, and should be read by those who are interested 

 in the subject. 



The last paper to which I shall refer is by M. Forel.f He has 

 made extensive observations on ripple-mark, formed both naturally 

 and artificially. He distinguishes between dunes formed by continu- 

 ous currents either of air or water and ripple-marks formed by 

 oscillation. His view accords with the experiments in § 1 above, but 

 he has not apprehended the importance of the vortex in the lee of the 



* In a letter M. de Candolle tells me that "demi-oscillation" should read 

 " quart d'oscillation ; " but I still do_not see how the ambiguity pointed out below 

 is removed by this correction. 



f " Les Eides de Fond." Archives des Sciences Physiques et Naturelles. 

 Geneve, 15 Juill^t, 18bS. M. Forel quotes an article of mine in " Nature " as 

 attributing the formation of ripple-mark to the action of currents in the sea. My 

 statement was intended merely to imply that shallowness of water is favourable to 

 the formation of ripple-mark. I had already made a great part of these experiments 

 when that article was written in 1882. 



