24 DRIFTING SAM). 



distance above the ground, but the dune sand is moved 

 only by the very lowest layer in the atmosphere. !Xo\v it 

 is known that the velocity of the current in this lower- 

 most layer is increased at a very slow rate with an 

 increase in the speed of the layers next above it. The 

 velocity in the layer next to the surface of the ground 

 probably never reaches three miles per hour. It is this 

 comparatively^ inert la3^er, which alone comes in contact 

 with the resting sand and first causes it to stir. As veloc- 

 ities much lower than this will not move sand at all, the 

 range of variation of the velocity of the currents which 

 impel dune sand, is most likely quite limited. Another 

 circumstance aids in bringing about the same result. Any 

 load which is picked up, has the effect of retarding the 

 current in which it is carried and the greater the particle 

 which is moved, the greater the retardation will be. In an 

 element of such lightness as the air this retardation must 

 be considerable. 



Another significant feature in the analyses of the dune 

 sand is the more rapid decrease in the percentages of the 

 finer grades than in the percentages of the coarser grades, 

 in the opposite direction. Evidently the law which gov- 

 erns the separation of the fine admixtures from the dune 

 sand is different from the law which determines the 

 separation of the latter from the coarse admixtures. A 

 little reflection makes this clear. Materials finer than 

 dune sand are wholly lifted up into swifter currents, which 

 promptly remove them. The dune sand itseh", on the 

 other hand, is partly lifted and also partly rolled, just as 

 the grains of the nearest larger sizes. Working in this 

 last manner the transporting power of the wind varies 



