DESERT SAND-DUNES 41 



direct wind, descending just above the node, drives 

 the sand of all the upper part of the ridges 

 forwards. Thus the upper part of each ridge is 

 continually being scoured away and the eddy space 

 to leeward is continually being filled up. By this 

 process the position of each ridge continually 

 advances to leeward, the core of eddying air 

 being thus pushed forward, the progression of each 

 successive core of eddying air permitting likewise 

 the motion of the passive sand-waves. 



A sand-grain of any given size and weight has 

 its own rate of subsidence through air. To raise 

 it needs an upward current of slightly greater 

 velocity. The scouring out of the troughs and the 

 increase in amplitude of the waves depends upon 

 the eddy being sufficiently strong. When the 

 wind is strong and the ridges small their growth 

 proceeds, the eddy gaining in strength as the ridge 

 grows. But if, after the ridge has reached its 

 maximum, the wind abates, the troughs begin to 

 silt up again, as I observed one day on the Helwan 

 sandbank . 



down the back part an inconvenience is caused by the fact 

 that the front of the cab has not been designed as a wind- 

 screen and is too high for the purpose, so that the return 

 current of air, instead of striking the back of the vehicle, 

 cuts in over it, blowing on the back of the passenger's head 

 and neck. 



