196 WAVES OF SAND AND SNOW 



there was a strip of road -dust composed of sand- 

 sized particles at a distance of a foot or two from 

 the wall. The reason of this is that the wind, 

 ascending when it reaches the wall, drops the heavy, 

 leaves there, their motion ceasing where the hori- 

 zontal current ceases, but much of the finer material 

 is dropped outside the eddy. 



The wearing of roads made with flints produces 

 a grit of the size of coarse sand. This collects 

 in two strips on pavement under a wall, a narrow 

 strip against the wall and a broad strip commencing 

 about a foot away from the wall. The former 

 lies in the dead air between the inner curve of the 

 eddy and the angle of wall and pavement., The 

 latter, I presume, lies outside the space where the 

 eddy-current sweeps the pavement. 



I have not had an opportunity of observing 

 where the finest kind of dust would deposit under 

 such circumstances, but I think it would lie between 

 these positions, fo,r it would remain entangled in 

 the eddy as long as the wind blew and then would 

 drop out, forming a strip below the line where the 

 eddy had been. 



In dealing with the intricate phenomena of the 

 simultaneous transport of shingle, sand, and mud 

 by the sea I find it best to analyse the motion of 

 the water into its horizontal and vertical com- 



