RELATION OF WIND TO TOPOGRAPHY 



559 



This has been shown by Sokoloff' and two of the experiments already- 

 described, those on days I and II, illustrate the same fact. Although 

 in the latter case the wind was swifter with nearly lo meters per 

 second the quantity of material moved as well as the percentage of the 

 coarser grades was only very little larger than in the former case, 

 when the separator was placed on a sloping surface. 



Any obstruction that comes in the path of the wind will greatly 

 reduce its force and consequently lessen the movements of the sand. 

 It is on this principle that some of the methods of arresting drift 

 sand are based. Planting rows of grasses or trees, or the making of 

 fences of sticks and other material on the dunes are means employed 

 for this purpose. Scanty rows of grasses act more effectively as wind- 

 breaks than as regular binders of the soil, and in planting such 

 ''sandstays" it is important to get the right distance between the 

 rows, which varies at different localities with the exposure to wind 

 and coarseness of the sand. If the local conditions have not been 

 studied and if the disposal of the windbreaks has not been done 

 properly, the results will be unsatisfactory. Too long distance 

 between the rows will not prevent drifting, and too close planting is 

 unnecessarily expensive. 



The experiments conducted for the purpose of ascertaining differ- 

 ence in velocity of wind on an even and a rough surface gave the results 

 shown below: 



The following compilation of these results will show the actual 

 difference of velocities as well as the proportion expressed in per 

 cent, of velocity on the even surface. 



I Die Diinen, 1894, p. 289. 



