1883.] 



On the Formation of Ripple-mark. 



n 



stream towards the crest. By observing the limits of the transport 

 of the ink, I conclnde that the stream lines were as shown in fig. 2. 



Fig. 2. 



These points being settled, it remained to discover how the vortices 

 were arranged which undoubtedly must exist in the oscillatory for- 

 mation of regular ripples. The rapidity of the necessary oscillations 

 made this a task of some difficulty. 



§2. On the Formation of Bipple-marJc by Oscillation. 



The observations were made in two different ways. 



In the first of these, which also ultimately proved to be the most 

 successful, the ripple-mark was made in a glass trough about 1 foot 

 long, 5 inches wide, and 6 inches deep. In order to observe the 

 formation of the ripples absolutely in profile, a sheet of glass fitting 

 the trough was placed to stand on four short corks at the bottom. 

 The trough was then put to rock on two corks, one at each side, on 

 the line which bisects its length. Two other slightly shorter corks 

 were put at the ends ; these served as stops, and only allowed it to 

 rock through a very small angle. The trough was placed on a 

 window-sill with a strong light outside, and was gently rocked by 

 hand. 



When the trough is half filled with water, and sand is sprinkled 

 on the glass plate, it is easy to obtain admirable ripple-marks by 

 gently rocking the trough. 



When a very small quantity of sand is sprinkled in and the 

 rocking begins, the sand dances backwards and forwards on the 

 bottom, the grains rolling as they go. 



Very shortly the sand begins to aggregate into irregular little 

 flocculent masses, the appearance being something like that of 

 curdling milk. The position of the masses is, I believe, solely deter- 

 mined by the friction of the sand on the bottom, and as soon as a 

 grain sticks, it thereby increases the friction at that place. 



The aggregations gradually become elongated and rearrange them- 

 selves. As soon as the formation is definite enough to make the 

 measurement of the wave-length possible, it is found that the 

 wave-length is about half of what it becomes in the ultimate 

 formation. > 



