236 



KNOWLEDGE. 



[October 1, 189G. 



ripple marks. To aee them in perfection one must look 

 down tliroiigh the clear water on a calm day. A pair of 

 long wading boots enables one to examine in comfort the 

 ridges nearest to the beach. In order to sec those further 

 out I sometimes float just beyond the brealcors in one of 

 tl cse llat-bottomed coracles which are still used on the 

 ^\ye. It draws scarcely any water, and is so light that 

 one can carry it on one's back and launch it unaided. 

 Sometimes, however, it capsizes. 



In the clear waters of Lake Geneva, according to M. 

 Forel, the ripple pattern can be seen at a depth of thirty 

 feet. On our own coasts the ripple mark ot ' 

 Torbay is celebrated, and has been well described 

 by Mr. Hunt in the I'l-in-epdhhix of tlif Hdi/hI 

 Snrieti/, Vol. XXXVI., 1883-1. 



The phenomenon has two aspects, according 

 as we have regard to the motions of the water 

 or to the tactics of the sand. The formtr have 

 been studied in exquisite detail by Prof. G. H. 

 Darwin, and stand recorded in the volume 

 of the Royal Society's ri-oci'edin<is to which 

 reference has been made. The tactics of sand 

 under rippling action were dealt with in two 

 papers read at the recent meeting of the British 

 Association by the present writer, and this 

 article is confined to the fluid motions. If a 

 trough or basin partly tilled with water be 

 gently rocked, a wave travels backwards and 

 forwards from end to end of the trougli, and 

 faud placed upon the bottom of the trough soon 

 beconifs lippled. The experiment is readily 

 tried, always succeeds, and is extremely pretty 

 and insfrucfive. Rippling, indeed, is so readily 

 produced that it may commonly be observed 

 when a little sediment has settled in a basin ; 

 the slightest disturbance of the wafer sets up 

 an oscillation, and this almost inevitably ripples 

 the sediment. The fine deposit which settles 

 from hard water after heating readily ripples : 

 the pattern may be seen at the bottom of a 

 jug of shaving water. A glass trough with 

 vertical sides is best for the experiments upon 

 ripple marks, as they can then be seen in 

 section, and the eye can be placed close enough 

 to watch the movement of the sand grains. 

 Having made some regular ripple marks by 

 oscillation, Prof. Darwin tried the effect of 

 exposing them to a current. He then observed 

 that small particles lying on the surface of the 

 sand climbed up the hv slope of the ripples, apparently 

 against stream. This showed the existence of an eddy 

 or vortex on the lee side of the ridge. By giving a 

 sudden motion to the water he was able to see the 

 sand piled up on the weather side by the direct 



Fig. 1. — Actiou of Current upon Ripple Marls. 



current, and on the lee side by the eddy or vortex. 

 In order better to observe the action of the vortices, 

 a drop of common ink (not aniline ink), with a little 

 sulphate of iron added to it, was squirted to the bottom 

 of the water in the furrow between two ridges. On 



causing the current again to pass, the ink was seen to 

 divide into two portions, one being sucked back up the 

 lee side of the ripple mark, and the other being carried 

 by the direct stream towards the next crest. Fig. 1 ia 

 Prof. Darwin's representation of the streamlines during 

 this action. 



By the aid of a drop of the heavy ink it was found 

 possible to watch the more complicated action of the 

 vortices during oscillation of the water. Rippling is 

 started by sand grains sticJcing, and thus causing little 

 vortices or eddies on their lee side. If the agitation is so 



Fig 2. — Action of the Vortices in Ripple Making. 



violent that the sand does not stick, but is simply swept 



along, no ripple mark is formed. A steady current, 



however, seems to be incapable of producing nyulur ripple 



mark, such as that of the seashore, which was found to 



be due to the periodic strengthening and weakening 



of vortices on either side of each ridga as 



the direction of oscillation changed. The 



drops of ink recorded almost perfectly the effect 



of each oscillation, for a pumping action, 



due to the upward motion of the vortex, 



separated the convolutions of the thread of ink, 



which formed a sort of tree. Fig. 2 shows the 



successive stages of the phenomenon for half 



an oscillation, the water swinging from right 



to left. The oscillation of the sand crest is 



shown, and the arrows at the base of ilie ink-spiral show 



the motion of the vortices which build up the ridges and 



scour out the troughs. "The only difficulty," says 



Prof. Darwin, " is in Stage IV., where the root of the 



(ink) tree is in the state of transference from one crest to 



