1883.] 



On the Formation of Ripple-mark. 



'61 



The first of these is by Mr. A. R. Hunt.* In it he makes an exten- 

 sive collection of observations on the natural history of ripple-mark. 

 As, however, he does not touch at any length on the mode of forma- 

 tion, I have but little to say on his work. He remarks that regular 

 ripples are due to alternating currents, and that the irregular marks 

 due to currents ought to be distinguished by another name from the 

 regular marks formed by oscillating water. M. Forel, whose paper 

 is referred to below, takes the same view, and describes these irregular 

 marks as dunes. My own observations seem to accord well with the 

 facts collected by Mr. Hunt. 



The second paper is by M. Casimir de Candolle.f His experiments 

 have led him to enounce (p. 245) the following general law : — 



" When a viscous material in contact with a less viscous liquid 

 experiences an oscillatory or intermittent friction, arising from the 

 relative motion of the liquid layer, 1st, the surface of the viscous 

 material is rippled perpendicularly to the direction of motion ; and 

 2nd, the wave-length is directly proportional to the amplitude of the 

 oscillation." 



The word viscous cannot here have its usual meaning, for sand 

 cannot be called viscous. The epithet seems to denote that the 

 constituent parts of the material are mobile, and that there is a 

 considerable amount of internal friction. 



When oscillations are set up in a vessel containing two fluids of 

 very unequal viscosity, such as tar and water, ripples are formed on 

 the more viscous fluid. But if the two fluids do not differ widely in 

 viscosity, as mercury and water, water and turpentine, essence of 

 cinnamon and water, ripple-mark is not generated. If, however, a 

 layer of powder be introduced at the surface of separation, ripple- 

 mark is easily formed. 



Ripples were made in sand with a variety of fluids, but with olive 

 od it was found impossible. According to the views maintained in 

 the present paper, the viscosity of oil is too great to permit the 

 generation of the ripple-making vortices. 

 At p. 257, M. de Candolle writes— 



"Chaque ride se termine a la partie superieure par une crete 

 composee des particules les plus lege res. Tant que dure le balance- 

 ment du liquide, les particules sont animees d'un mouvement pendu- 

 laire qui les transporte alternativement de part et d'autre de la crete. 

 Aussi longtemps que l'amplitude de ce balancement est egale a celle 

 qui a donne" naissance aux rides, les particules mobiles parcourent a 



* " On the Formation of Kipple-mark." Proc. Eoy. Soc, April 20, 1882 

 vol. 34, p. 1. ....... 



' f "Eidesformees," &c. Archives des Sciences Physiques et Naturelles. Geneve 

 JSo. 3, vol. ix, 15th March, 1883. 



