AEOLIAN SAND-RIPPLES 83 



the average of the five determinations being 

 ©•424 inch per minute. 



Rate of Advance of Ripples of Three Inches Wave-Length 

 (in a Sheltered Spot). 



0-25 inch in 8 minutes = 0-03 inch per minute 

 0-50 „ 16 „ = 0-03 „ „ 



The movement of the ripples even in a strong 

 breeze is achieved almost entirely during the gusts. 

 Thus during a gust lasting only 45 seconds I have 

 seen a small ripple in an exposed position advance 

 nearly i inch. 



When a patch of blown sand, drifted from a 

 distance, is deposited in a fairly exposed position 

 the surface is smooth and the texture of the sand 

 uniform at first, but almost as soon as rippling 

 commences the sand on the crests is seen to be 

 coarser than elsewhere. This difference becomes 

 more marked as the ripples grow in size. Even 

 during my first observation of the occurrence I 

 thought I could detect two modes of motion of 

 sand-grains, and the two modes of motion appeared 

 to be connected with this sorting of the sand. The 

 large grains which accumulated at the crests rolled 

 upon the surface, but the finer grains seemed to 

 be caught up in the air and whisked away. I shall 

 describe presently how I verified this by experi- 



