22 



Prof. G. H. Darwin. 



[Nov. 22, 



uniform current flattened the tops of the ripples, bnt made the lee- 

 side steeper. After being exposed to a prolonged current, the ripples 

 were not only not in course of obliteration, but became somewhat 

 more pronounced. 



The sand was then smoothed with the edge of a board ; after the 

 exposure of the sand to a current, marks made with the edge, which at 

 first were too faint to be seen, became by a course of development 

 well-defined ripples. The whole surface became gradually mottled 

 with irregular chains of ripples of which the weather-side was a 

 very gradual slope, and the lee-side was steep. The appearance was 

 strikingly like that of drifted snow. As it might be conjectured that 

 there would be eddies or vortices on the lee-side, I made some regular 

 ripple-marks by oscillation, and then exposed them to a current. I 

 shortly observed minute particles lying on the surface of the sand 

 climbing up the lee-slope of the ripples apparently against stream. 

 This proved conclusively the existence of the suspected vortices. 



If when the bath was at rest, a sudden motion was given in one 

 direction, the sand on the lee-side of each ripple was observed to be 

 churned up by a vortex. By giving a short and sudden motion, 1 

 was able to see the direct stream pile up the sand on the weather- 

 side and the vortex on the lee-side. Fig. 1 shows the effect of a 



Fig. 1. 



single short jerk. Two little parallel ridges of sand were formed, 

 namely (a) by the direct stream, and (b) on the lee-side by the 

 vortex, a little below the crest of the ripple-mark. 



It is thus clear that casual surface inequalities are accentuated by 

 the combined action of the direct stream and of the vortex. 



For the purpose of examining the vortices a glass tube was drawn 

 out to a fine point, and fitted at the other end with a short piece 

 of india-rubber tube. With this a drop of ink could be squirted out 

 at the bottom of the water. This method was adopted in all sub- 

 sequent observations, and it proved very valuable. It may be worth 

 mentioning that common ink, which is heavier than water, was better 

 than aniline dye ; and the addition of some sulphate of iron to the 

 ink improved its action. 



A drop of ink was placed in the furrow between two ripples ; as 

 soon as the continuous stream passed, the ink was parted into two 

 portions, one being sucked back apparently against stream up the 

 lee-side of the ripple-mark, and the other being carried by the direct 



