1883.] 



On the Formation of Ripple-mark. 



33 



It mast be understood that these figures are drawn from the results 

 of long watching of the process. My attention was at one time 

 directed to one part of the phenomenon, and at another to a different 

 part, and the amplitudes were constantly varied. I do not pretend to 

 be able to see all these changes in a single half oscillation, lasting 

 barely half a second. It may appear that I am incorrect in some parts 

 of the construction, but I would ask any one who repeats the experi- 

 ments not to condemn me hastily, for the constructions which I have 

 given are the results of frequent trials and errors in the attempt to 

 represent the changes observed. 



I have not been able to determine exactly the mode of motion in the 

 initial stages of ripple-making, when the oscillation has large ampli- 

 tude, but when the ripple-marks are still in what I have described as 

 the curdling stage. 



If a current be passed over existing ripple-mark a vortex is esta- 

 blished on the lee of each ripple ; if the current be reversed the 

 vortex is on the other side. Thus intermittent opposite currents 

 will form ripple-mark, but probably without giving it a very regular 

 wave-length. 



If the intermittence is rapid, the vortex established on the lee-side, 

 when the current is in one direction, is not annulled when the current 

 is reversed, but it will be carried over the crest of the ripple-mark, 

 and will diminish in intensity, whilst the new vortex with opposite 

 rotation is established. 



The study of a very gentle oscillation over existing ripple-mark, by 

 means of a drop of ink placed on the ridge, enables us to observe 

 these vortices (see fig. 3). I think it depends on the amplitude of 

 oscillation whether both vortices are always in existence and simply 

 vary in intensity, or whether the vortex due to motion to the right is 

 quite annulled during the motion to the left, and vice versa. 



It may be suspected, therefore, that, in the early stages of ripple- 

 making, when the amplitude of oscillation is large, vortices are set up 

 in the lee of each aggregation of sand, in the same way as if the 

 current were permanent, and that when the current is reversed these 

 vortices are speedily annulled, and a new set on the other side of the 

 aggregations is established. When a drop of ink is put on an aggre- 

 gation, and the oscillation is started, the ink forms a layer of not more 

 than half a millimetre in thickness. It is easy to see that there takes 

 place some kind of rapid oscillation which is not simply harmonic. It 

 appears to present all the characters of the motion when gentle oscilla- 

 tion is established over ripple-mark of some height, and therefore it 

 is probable that the motion is of the same kind in both cases. 



When the aggregations are more pronounced, small correlated tree 

 vortices are set up. As above stated, it has seemed to me that fre- 

 quently only half of each tree vortex is set up at first. 



VOL. XXXVI. D 



