DESERT SAND-DUNES 67 



in suspension, and again dropped. As rolling, 

 however, goes on at the same time, account has 

 to be taken of this concurrent circumstance. 



The condition for uniform drift of sand in sus- 

 pension is that the amount picked up should be 

 equal to the amount dropped. Then there occurs 

 neither scouring nor silting. If at any place the 

 air had less than its full charge of suspended sand, 

 it would scour the next section. If at any place it 

 became overcharged, it would silt-up the next 

 section. It is evident, therefore, that the continu- 

 ance of uniform sand-drift would require the main- 

 tenance of great steadiness in the velocity of the 

 air. But wind of sufficient strength to pick up 

 sand and carry it in suspension is not even approxi- 

 mately a steady current. On the contrary, its speed 

 always varies to the amount of about 50 per cent, 

 every few minutes, even in open places, and where 

 there are obstructions the variation is about 

 100 per cent. Thus, on December 16, 191 2, I 

 found that the self-registering anemometer of the 

 Meteorological Office in London showed a varia- 

 tion every few minutes of from 6 to 1 2 miles an 

 hour, on the 17th of 6 to 14 miles an hour, on the 

 2oth 5 to 8 miles an hour, on the 29th ^9 to 21 miles 

 an hour, and on January 4, 1 9 1 3, 5 to 10 miles an 

 hour. Let us suppose wind to be blowing over 



