1882.] 



On the Formation of Ripplemark. 



Feeling satisfied that the bottom of Torbay in about 6 fathoms at 

 low water spring tides was rippled by the swells following the October 

 gales, I proceeded to construct a small tank, about 9 feet by 3 feet by 

 1 foot, in order to prove experimentally whether subaqueous ripple- 

 marks could be formed at will, and to what extent their dimensions 

 could be controlled. 



Working on this small scale I experienced no difficulty in forming- 

 good ripplemarks varying in size from ^-th of an inch to 4 inches 

 from crest to crest. The tank was commonly arranged as follows : — 

 The sand was so piled up at one end that the waves when generated 

 would quickly tear down what they wanted for a strand on which to 

 break, and from that strand outwards the amount of sand used was 

 regulated by the depth of water required for each experiment. The 

 further end of the tank where the waves were generated was kept free 

 from sand so as to have the greatest available depth of water, gene- 

 rally about 9 inches. The waves were generated by a vertical dis- 

 placement of the water, either by means of a Y-shaped trough worked 

 by hand or by means of a semi-cylindrical block of wood worked by a 

 small model steam-engine. 



The following five experiments will show how very rapidly ripple- 

 marks can be formed. 



(1.) Waves 60 per minute, height trough to crest about 1^- inches. 

 Result, 1^- inch ripples in water 2 and 3 inches in depth. 



(2.) Waves 115 per minute, height not measured. Result, £ inch 

 ripples well developed in 2 inch water, and discernible down to 3^ 

 inches deep. 



(3.) Waves 23 per minute, height not measured. Result, the small 

 ripplemarks now effaced and replaced by others 1J inch in size. 



In the above cases the experiments lasted exactly one minute each. 



(4.) Agitated the water at the centre of the tank, gradually getting 

 up an even swing of 13 to the minute. The time was taken after the 

 water was in full swing, and the experiment may have lasted one 

 minute and a half. Result, ripplemarks were now more or less 

 developed over the whole bottom, the largest being 3 inches in length. 



(5.) The beach was now removed and the sand levelled over the 

 whole tank. The water was disturbed with an even swing as much 

 as possible. It rebounded from end to end, and dashed over the two 

 ends, which in this experiment were 5 inches above the water-line. 

 The sand being completely stirred up was left a night to settle, and 

 the next day the water being still turbid, it was drawn off. Result, 

 the bottom proved to be strongly but unevenly rippled all over with 

 ripples varying in size from less than 1 inch to over 4 inches ; the 

 greatest depression being about J inch from trough to crest. In one 

 case a set of ripples had been formed exactly at right angles to a 

 larger set which was nearly obliterated by them. 



