STUDIES FOR STUDENTS 813 



rent moves always in one direction we have intermittent friction 

 due to varying velocities. Otherwise we have oscillatory friction 

 due to alternating change of direction. Current ripples result 

 from the first type of friction, oscillation ripples from the second. 



Forel in his excellent essay on "Les rides de fond etudiees dans 

 le lac Leman" 1 sets forth the mature results of studies which had 

 been briefly mentioned by him in three communications of earlier 

 date. 2 Abandoning his first theory, that the formation of ripple 

 marks is dependent in part upon the vertical pressure of water 

 waves upon the bottom, 3 Forel reached the following important 

 conclusions as the result of many careful observations and experi- 

 ments: (1) Current ripples are asymmetrical and migrate with the 

 current like ordinary sand dunes, whereas oscillation ripples are 

 stationary and symmetrical. (2) Each oscillation ripple is really 

 a composite of two current ripples, resulting from the action of 

 two currents moving alternately in opposite directions, each cur- 

 rent attempting to form the ridge into a current ripple migrating 

 with it, but being defeated when the return current tries with 

 equal force to shape the ridge into a current ripple directed in the 

 opposite sense. (3) The length of the water body has no direct 

 effect on the spacing of the ripples. (4) Other things being equal, 

 the ripples are more closely spaced with increasing depth. (5) At 

 a given depth, and with other conditions uniform, the ripples are 

 more widely spaced with increase in coarseness of sand grains. 

 (6) Ripples once formed do not experience a change in spacing as 

 a result of diminishing amplitude of oscillation of the water, 

 although the original spacing does depend upon the amplitude of 

 oscillation, as pointed out by De Candolle. (7) For any given 

 coarseness of sand grains there is a certain mean velocity of the 

 oscillating currents which will produce ripples; lower velocities 



1 F. A. Forel, ' 'Les rides de fond etudiees dans le lac Leman," Archives des sciences 

 physiques et naturelles, 3 e Ser, X (1883), 39-72. 



2 F. A. Forel, "La formation des rides du Leman," Bulletin de la Societe Vaudoise 

 des sciences naturelles, X (1870), 518; "Les rides de fond," ibid., XV (1878), P.V. 66- 

 68; "Les rides de fond dans le golfe de Morgues," ibid., 76-77. 



3 F. A. Forel, "La formation des rides du Leman," Bulletin de la Societe Vaudois 

 des sciences naturelles, X (1870), 518; "Les rides de fond etudiees dans le lac Leman," 

 Archives des sciences physiques et naturelles, 3 e Ser., X (1S83), 40. 



