DESERT SAND DUNES 



39 



viduals are very uniform in size and the crests 

 are quite level. It is very remarkable that in 

 the Helwan waves there should be the same 

 average steepness, although the ratio of height to 

 length in individuals varied from 8-9 to 60. 



The course of the wind as it blows over a series 

 of sand-waves in regular ridges, such as those in 

 one of the photographs, can be made out fairly 

 well by ordinary observation. It is, however, 

 much more difficult to observe in the case of the 



Fig. I. 



aeolian sand-ripples, which had been my first study, 

 and I had obtained my first clear notion of the 

 process from the accompanying figure, by which 

 the late Sir G. H. Darwin illustrated the motion 

 in a current of water made to flow over a series 

 of very small sand-ridges.' These lines of current 

 and eddy he had actually seen by means of a 

 special ink introduced into the water which 

 rendered its flow visible (Fig. i). I give the 



^ " On Ripple Mark," by G. H. Darwin, Proc. Roy Soc, 

 1883-4. 



