Ok SURFACE WAVES PRODUCED BY SLEDGES. 155 



continued, became more and more unsymmetrical in profile, 

 until their form resembled more that of drift waves of sand or 

 snow than spontaneous cahots, which are generally symmetrical. 



I decided to change my "sledge," and obtained a small iron 

 mould, about i foot long, used for casting lead, which was very 

 heavy, weighing 14 pounds, and was suitably shaped, having 

 somewhat the form of a barge. 



With this I began again, drawing it along slowly and 

 without the employment of an initial inequality. The small 

 heavy sledge sank somewhat in the loose, dry, sandy, or shingly 

 surface of the higher parts of the beach, driving before it a 

 mound of loose particles, as in the former experiments. The 

 furrow quickly deepened as the sledge was drawn backwards and 

 forwards over the same track, and suddenly (my back being 

 turned in hauling) I noticed a change in the pull of the cord 

 almost simultaneously with a change in the sound made by the 

 sledge. The rattling over dry gravel gave place to a more 

 muffled note, and the prow of the sledge was felt to rise and 

 fall. On turning to watch the sledge, the cause of this was 

 immediately apparent. The level of slightly damp gravelly sand 

 had been reached, and the travelling mound or wave of detritus 

 in front of the sledge was soon welded into a coherent mass by 

 its pressure. The sledge then rode over this obstruction, which 

 after its passage was found to be perfectly incorporated with the 

 road-bed, forming a gentle convexity of the surface. The sledge 

 at once began to accumulate more material in front of it, which 

 soon cohered, and the sledge again overrode the obstruction, 

 which in its turn formed a second wave -crest. 



These undulations increased very rapidly in amplitude, not 

 only when the sledge was drawn backwards and forwards, but 

 also when drawn only in one direction. The undulations were 

 symmetrical and similar in appearance and steepness to the 

 cahots of the Quarry Track. No appreciable inequality was 

 present to originate them, and the speed of the sledge was 

 throughout only that of a very slow walk, say two and a-half to 

 three miles per hour, and the pull was as steady as possible. The 



