40 WAVES OF SAND AND SNOW 



figure, but amplify the information it contains 

 from my own observations of the action of wind 

 on waves in sand and snow. On the lee side of 

 each cliff an eddy or vortex is active, a body of 

 air which whirls vertically round a horizontal axis. 

 At the crest the surface currents converge. Both 

 currents are loaded with sand, which deposits on 

 the lee side of the crest. At a position on the 

 long, gently sloping weather side of the next wave 

 the surface currents are, on the contrary, divergent, 

 that at the tail end of the elongated vortex drawing 

 sand back i from the nodal position, whilst the 



^ Since I began the study of sand-waves and the eddies 

 which make them the use of motor vehicles has made famihar 

 to most of us the upward deflection of wind by an obstacle, 

 and the consequent return draught on a lower level. On 

 the front seat of the motor omnibuses now running in London 

 one can look over the top of the wind-screen without feeling 

 the wind in the eyes, the direct current of air being thrown 

 slightly upwards, as can be proved by raising the hand a few 

 inches above the head, where the wind is felt. If the hand be 

 held close to the floor a return draught can be felt, and this 

 brings the dust and dirt which accumulates at the foot of the 

 wind-screen. This effect is the same as that which occurs 

 when the wind blows outwards from the area of sand-dunes 

 over the neighbouring stony plain. Those sand-grains which 

 drop near the dunes are swept back by the return draught 

 which blows along the ground, and are deposited against the 

 cliff of the dune, the neighbouring part of the stony plain being 

 thus kept clear of sand. When a taxi-cab is opened by letting 



