24 



Prof. G. H. Darwin. 



[Nov. 22, 



Some of the elongated patches disappear, and others fuse together 

 and form ridges, the ridges then become straighter, and finally a 

 regular ripple-mark is formed with the wave-length double that in 

 the initial stage. 



When a drop of ink is put on the glass without any sand, it simply 

 slides to and fro, with perhaps a faint tendency to curdle, but it 

 cannot be caused to form ripple-mark. This shows that the initial 

 stage when the sand is beginning to curdle is due simply to friction. 



When the ink is put upon a flocculent mass it betrays some kind 

 of dance in the water, but the layer of water disturbed is so thin that 

 it would hardly have been possible to detect the law of the motion 

 from this case alone. When, however, the nature of that motion, as 

 described below, has been discovered, the same kind of motion may 

 be recognised in the dance of the ink over these flocculent aggre- 

 gations. 



I found in the later experiments that it was advantageous to have 

 a very regular ripple-mark. I therefore sprinkled sand on the sheet 

 of glass, and, before beginning the rocking, I traced regular furrows 

 in it with the point of my finger. A few oscillations of the trough 

 soon effaced all signs of the artificial origin, and the ripple crests 

 with the bare glass in the furrows, were absolutely indistinguishable, 

 except by perfect regularity, from those produced naturally. Most of 

 the observations were, however, made with the natural ripples, and it 

 was only towards the end that I adopted this plan in order to save 

 time and to obtain perfect regularity. 



In the rocking trough, the water moves whilst the bottom of the 

 vessel is still, save for the small rocking motion. A second arrange- 

 ment was, however, made in which the converse is true. A sheet of 

 plate glass is caused to oscillate in the bottom of a trough with glass 

 sides. The oscillator is moved by a connecting rod and crank driven 

 by a small water-motor, the throw of the crank is small, and the 

 rapidity of oscillation can be varied within considerable limits. 



When sand is sprinkled on the oscillating sheet of glass, phenomena 

 such as described above are again observed, and good regular ripple- 

 mark is formed in the sand. Although much was learned from this 

 instrument, still the rocking trough was on the whole more useful. 



It appeared to be certain from the first set of experiments that 

 ripple-mark was due to eddies or vortices, and the question remained 

 as to how the vortices were arranged in oscillatory motion. It 

 required some practice, and many hours of watching, to establish the 

 conclusions explained below, indeed the phenomena next described 

 were only detected long after that which follows them in this paper. 



If a very gentle oscillation be started, the layer of ink on the crest 

 of a ripple-mark becomes thicker and thinner alternately, swaying 

 backwards and forwards; then a little tail of ink rises from the 



