DESERT SAND-DUNES 61 



it is increased. The latter effect appears to be 

 owing to the upper layers of loose sand being 

 unable to withstand the thrust to which they are 

 subjected owing to the sudden change in the 

 horizontal velocity of the air between the forward 

 current and the eddy, the strength of which in- 

 creases with the height of the ridge. 



The sand driven through the saddles is drawn 

 in by the eddies towards the peaks, and the fact 

 that the average ratio of length to height is the 

 same when the crests are level as when they are 

 unlevel suggests that the lateral action of the 

 eddies catches and retains all the extra sand driven 

 through the saddles. 



My next observations were of dunes about one 

 hundred times as high as those on the Helwan 

 sandbank, and composed of the coarser quartz sand 

 of the desert. They are situated east of the Suez 

 Canal on the route to El Arish. Leaving Kantara 

 on the canal one morning on camel-back, I 

 camped that night at Bir Nisf. Next day, starting 

 at six in the morning, I rode east as far as Abu 

 Ramie, and, turning back, camped for the second 

 night at El Ookha, and returned to Kantara on 

 the morning of the third day. 



For some miles east of Kantara we traversed 



4 



