82 WAVES OF SAND AND SNOW 



If the ratio of length to height in the last ten 

 ripple measurements hold for the first two, of which 

 only the length and the diameter of the sand -grains 

 were measured, the sand is already regularly rippled 

 when the trough is one grain lower and the crest 

 one grain higher than the mean surface. 



The movement of the ripples was measured on 

 several occasions. When the wind was blowing 

 as a strong breeze—/.^., about 27 miles an hour — 

 the following measurements were taken in different 

 positions not far from each other, but in which 

 ripples varied in size on account of the difference 

 in their exposure : — 



Rate of Advance of Ripples of Nine Inches Wave-Length. 

 I'oo inch in i'5 minutes = 0'67 inch per minute 

 1*50 „ 3'o „ = 0-50 „ 

 2-25 „ 5-0 „ = 0-45 „ „ 



3'25 n 7*5 ., = o'43 n V 



4'25 „ 9"5 „ = 0-447 „ „ 



the average of the five determinations being 

 0-50 inch per minute. 



Rate of Advance of Ripples of Four Inches Wave-Length. 



I'o inch in 2'0 minutes = 0*50 inch per minute 



i'5 n 3-5 „ = o"43 n 



2-0 „ 5-0 „ = 0-40 „ „ 



2-5 ,.. 6-25 „ = 0-40 „ „ 



275 » 7*00 „ = 0-39 „ „ 



