SNOW-MUSHROOMS AND CAHOTS 241 



above the level of the lake. It displaced the loose 

 material in a wave before its bows, but the track 

 which it made was flat. 



I then tried the effect of drawing it backwards 

 and forwards over the same track, thinking that 

 in this way I might produce some undulation ; the 

 track, however, remained flat. I was careful 

 not to draw the sledge over my own foot- 

 prints. 



Friends having come to my assistance, a higher 

 speed was tried, the sledge being dragged at about 

 8 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 quite flat. 



I had been trying whether undulations would 

 form upon a level surface, because my experience 

 with waves of sand and snow had shown me that 

 the formation of surface-waves does not require 

 the pre-existence of appreciable inequalities. How- 

 ever, 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 



