54 WAVES OF SAND AND SNOW 



drove a reed into the sand at the crest of a wave. 

 In the course of forty-eight hours it advanced 

 29!^ inches, which is at the rate of 061 inches 

 per hour or 150 yards per annum. 



I will now describe the process which I believe 

 originated the aeolian sand-waves upon the Helwan 

 sandbank. My views developed gradually in the 

 course of several years' observations of phenomena 

 of this class, and their full justification will become 

 more evident to the reader later on when more 

 of the phenomena have been described. Mean- 

 while it will be convenient to have the explanation 

 now, even though much of the evidence on which 

 it is based be delayed. 



In the first place it must be stated that the mere 

 rolling of sand-grains along the surface does not 

 produce waves. On the contrary, when there is 

 only this mode of motion the tendency is for wind 

 to obliterate transverse inequalities, the miniature 

 rolling stones accumulating in the hollows where 

 the forward velocity of the wind parallel to the 

 surface is least and where the resistance to forward 

 rolling is greatest. 



The formation of waves in drifting sand is, on 

 the contrary, dependent upon another mode of 

 action of the wind, viz., that by which the sand- 

 grains are picked up, carried for some distance 



