58 WAVES OF SAND AND SNOW 



the Helwan sandbank as it blew in London on 

 December 17, 191 2. It would be described as 

 a wind of 10 miles an hour, but as a matter of 

 fact it is a jerky current of 6 to 14 miles an hour. 

 Further, this particular range of speeds is not itself 

 steadily persistent. If we consider the velocity at 

 successive minutes above any fixed place on the 

 ground, it is not constant ; and if we follow in 

 imagination any particular particle or parcel of air 

 in its onward course, its velocity will likewise vary 

 greatly. It is evident, therefore, that in one place 

 the air will become suddenly surcharged with sand 

 owing to loss of speed, and will drop the excess, 

 forming a mound, and at the same moment in 

 another place it will scour out a hollow. Thus 

 vertical inequalities inevitably originate. I have 

 already explained how, when once formed, they are 

 increased, owing to a confluence of currents at 

 the crest and a separation of currents at an inter- 

 mediate position, and to the effect of this arrange- 

 ment upon sand carried in suspension. Transverse 

 ridges are thus soon formed. They increase in 

 wave length, and maintain level crests until, 

 apparently, a particular height is reached, when 

 peaks and saddles are developed. 



When the ridges are small an incipient depres- 

 sion is quickly mended, but when they are large 



