STUDIES FOR STUDENTS 815 



then proceeded to study the vortices by watching the movements 

 of a drop of ink released from the end of a fine glass tube at that 

 point in the water where the action was to be observed. In this 

 manner the vortices associated with the alternating currents which 

 produce oscillation ripples were analyzed with a high degree of 

 precision, and much light was thrown upon the method of ripple 

 growth. Darwin concluded that "the formation of irregular 

 ripple marks or dunes [current ripples] by a current is due to the 

 vortex which exists on the lee side of any superficial inequality 

 of the bottom; the direct current carries the sand up the weather 

 slope and the vortex up the lee slope. Thus any existing inequali- 

 ties are increased, and the surface of sand becomes mottled over 

 with irregular dunes." The intermittent friction which De 

 Candolle adduced is not essential in this explanation of current 

 ripples. Oscillation ripples of regular pattern are changed by 

 a continuous current into regularly spaced current ripples; but a 

 uniform current cannot of itself initiate regularly spaced ripple 

 marks. "Regular ripple mark [oscillation ripples] is formed by 

 water which oscillates relatively to the bottom. A pair of vortices, 

 or in some cases four vortices, are established in the water; each 

 set of vortices corresponds to a single ripple crest." Forel's con- 

 ception of an oscillation ripple as a composite of two current 

 ripples formed alternately by oscillating currents is regarded as 

 correct; but his law for the relation of ripple spacing to amplitude 

 of oscillation is believed to require some modification. 



Further studies of ripple-forming vortices were made by 

 Mrs. Hertha Ayrton, the results of which were not published until 

 1910. 1 With the aid of well-soaked grains of ground black pepper, 

 or of particles of potassium permanganate dissolving and coloring 

 the water while the latter was in oscillation, she observed the 

 formation of vortices and endeavored to explain the mechanics 

 of their growth. Although she expressed disagreement with the 

 conclusions of Darwin and others on certain points, most of her 

 results afford essential confirmation of their main contentions. 

 Some doubt must attach to certain of her deductions, such as 



1 H. Ayrton, "The Origin and Growth of Ripple-mark," Proc. Roy. Soc. London, 

 Ser. A., LXXXIV (1910), 285-310. 



