154 N SURFACE WAVES PRODUCED BY SLEDGES. 



smooth. Care was, of course, taken that the sledge was not 

 drawn over one's foot-prints. 



Another friend having come to our assistance, " high speeds " 

 were tried, the " sledge" being dragged at about eight miles 

 an hour. I thought an undulation might arise from the 

 tendency of small inequalities to make a sledge jump when 

 moving rapidly, but in spite of a good deal of hard work the 

 track of the sledge remained as smooth as ever. In parenthesis 

 I may here remark that when I hit upon the right method the 

 experiment " did itself," so to speak ; as so often happens, 

 everything was easy and simple, including the explanation of the 

 process, when once the natural method was adopted by the 

 experimenter. 



I had been trying whether the undulating surface would arise 

 spontaneously without the pre-existence of appreciable inequal- 

 ities, because my experience with sand waves (sub-aqueous and 

 seolian) and drifting snow waves had shown me that, in spite of 

 popular opinion to the contrary, the formation of such waves 

 does not require the pre-existence of appreciable inequalities. 

 However, as I had so far failed with the cahots, I made a heap 

 of loose sand, and the sledge was run quickly over it again and 

 again, always in one direction. In this way an undulating 

 surface of regular wave-length was speedily produced by the 

 jumping and bumping of the sledge. First a depression was 

 scooped out to " leeward " of the initial inequality, then a 

 mound was thrown up to leeward of the depression, and another 

 trough was scoured out to leeward of this, and so on, wave by 

 wave, the group of waves continually extending, much as a 

 group of waves forms to leeward of any considerable obstruction 

 which may be submerged in a fairly rapid stream. So far I 

 had got the effect similar to that due to an initial snowdrift, 

 causing the sledge to bump. I could not rest satisfied with 

 this result, however, without further modifications of the experi- 

 ment. There was, indeed, one important difference between 

 the experimental cahois and those usually produced in a 

 spontaneous manner. The former, as the experiment was 



