258 WAVES OF SAND AND SNOW 



by the alternating currents of the swell. If the 

 bottom of any vessel be covered with sand sub- 

 merged by water, similar ripple -marks are produced 

 almost at once when the vessel is rocked. The 

 late Sir George H. Darwin ascertained the motions 

 of the water during this process by observing the 

 convolutions and dispersion of a dark, heavy liquid, 

 a kind of ink, in fact, of which he introduced a 

 small quantity into the water. i In these experi- 

 ments the time of a complete oscillation of the 

 water was about one second, which is nearly that 

 of the smallest waves formed by wind, and less 

 than that by which the natural ripple -mark is 

 usually formed, the period of the swell of the sea 

 being usually from about three to twelve seconds. 

 In Sir George Darwin's experiments the ripples 

 under the action of rapidly alternating currents 

 could be seen to be built up from both sides by 

 eddies which acted in quick succession, first on one 

 side and then on the other, a very different action 

 from that of a simple current. The ripples formed 

 in a trough are symmetrical, as are those produced 



^ Mrs. H. Ayrton used, instead of ink, well-soaked ground 

 black pepper, from which the finer particles had been washed 

 away. This showed the eddying during violent motions which 

 would have dispersed the ink. See ** The Origin and Growth 

 of Ripple-mark," by H. Ayrton, Proceedings of the Royal Society, 

 Ser. A, 84 (1910). 



