274 WAVES OF SAND AND SNOW 



formed beneath the stationary water-waves did not 

 move, although the sand-grains could be seen 

 to travel pretty freely over their corrugated 

 surface. Where these things happened, with a 

 current of 1-5 feet per second, the water was 

 quite clear. 



On some days it happened that the stream 

 acquired in its lower reaches a higher velocity, 

 and churned up the sand so as to form a turbid 

 current. Whenever this happened a train of water- 

 waves, each with an underlying sand -wave, imme- 

 diately arose. Unlike the waves last described, 

 their fronts quickly broadened until they extended 

 across the whole width of the stream. These waves 

 were formed when the velocity of the current was 

 2-2 feet per second. Their length on different 

 days was from 6 to 9 inches. The following is 

 the most complete measurement which I obtained 

 of one of the sand-waves and its superimposed 

 water-wave. The length of both was 9 inches. 

 The height of the sand-wave from trough to crest 

 was f inch, which is ^^ ^^ the length. The 

 height of the water-wave from' trough to crest was 

 I inch. The depth of water was i^ inches on 

 the crest of the sand -wave and i inch over the 

 trough. The trough of the water waiS therefore 

 ^ inch higher than the crest of the sand. It is. 



