DESERT SAND-DUNES 35 



the weather side, whilst the lee side consisted of a 

 straight cliff standing at the maximum angle 

 allowed by the looseness of the material — that is 

 to say, about 33^° The rise is i vertically in 

 1 1 horizontally. The angle of the weather slope 

 is 3^°, the rise being i vertically in 1 6| horizon- 

 tally, the length of the mound being thus eighteen 

 times as great as the height. The weather side 

 was covered with minute waves or ripples of a 

 few inches from crest to crest, with which I shall 

 deal in the next chapter. The slipping lee face 

 was unrippled. A photograph taken looking 

 towards the Nile (Plate II) shows a remarkably 

 regular series of aeolian sand-waves. When look- 

 ing up wind the series of waves stands out boldly 

 owing to the marked difference in the light reflected 

 from the steep lee slopes and from the adjacent 

 upper part of the weather slopes, which is nearly 

 horizontal. Looking down wind the steep lee slope 

 is out of sight, and there is so little contrast between 

 the weather slopes of succeeding waves that it is 

 not easy to realize how much the surface undu- 

 lates. I paid a second visit to the sandbank in 

 the afternoon, when the wind had increased in force 

 to about the strength of a " fresh breeze," which 

 has a velocity of 2 1 miles an hour. The ridges 

 or waves were higher than in the morning — that 



