42 WAVES OF SAND AND SNOW 



The constant ratio of length to height in the 

 sand-waves shows that the air-eddy maintains the 

 same shape whether the ridges he large or small 

 and whether the wind be light or strong. The 

 characteristic shape of an eddy is ichthyomorphic — 

 i.e., fish-shaped, with blunt head and fine tail. 

 Numerous examples of this will appear in the 

 course of this record. The sand ridges have fine 

 lines aft and a blunt fore part — i.e., have a desk 

 shape, and advance as if the desk were being 

 pushed forward. But they are passive waves. 

 They are moulded upon, and move by virtue of, 

 the eddies of air, and each of these has a blunt 

 head and a fine tail. The eddies, however, are 

 annulled when the wind ceases, whereas the sand- 

 ridges remain, passive but permanent. 



So much of explanation for the present of the 

 growth and movement of aeolian sand-waves 

 already formed in transverse ridges. The explana- 

 tion of the process by which these waves originate 

 from a level surface of loose sand must be 

 reserved until later. 



On some parts of the Helwan sandbank the 

 waves, although in ridges of great lateral exten- 

 sion, had unlevel crests, each ridge consisting of 

 a series of peaks separated by saddles. The wind 

 sweeps through these depressions with increased 



