SNOW-WAVES AND SNOW-RIPPLES 179 



driven below by their suction. This current corre- 

 sponds to the " tongue " of the barchan, the right 

 and left-handed swirls to its cusps, and the convex 

 boundary sternwards to the convex boundary which 

 the mound of sand or snow presents to the wind. 

 It must be remembered that as the disturbance 

 from the oar travels astern, so also is the motion 

 of the barchan relatively to the wind in the direc- 

 tion of its convex front, although relatively to the 

 ground it travels in the opposite direction. 



In foaming waves, nearly breaking, there are 

 numerous double eddies with a forward current 

 between, and as the wave passes on it leaves the 

 foam in the form of a squat figure of 8 — i.e., a 

 double ellipse with a bridge of foam common to 

 both along the line of advance of the direct current. 

 This line of foam corresponds to the longitudinal 

 tongue sometimes formed on the lee of barchans 

 of sand and snow. 



It is of interest to consider the profile of the 

 eddy-spaces left unfilled between the transverse 

 ridges of a group of travelling waves in sand and 

 snow. These, it must be remembered, can only 

 have their truly typical form if there be a deposit 

 of granular material below the troughs, for, if a 

 hard floor be exposed, it necessarily interrupts the 

 vertical sinuosity of the aerial current. The direct 



