RELATION OF WIND TO TOPOGRAPHY 557 



A wind with a velocity of 15.7 m. per second was thus able to lift 

 grains of 9.3 mm. diameter to a height of 6-8 cm., and experiments 

 for ascertaining the distance to which such grains were carried by the 

 same wind showed this to be 8 . 6 m. in 30 minutes. 



The few examples given are typical of a larger number (24) of 

 mechanical analyses of series of samples collected with the sand 

 separator during wind of different strength, but it is hardly necessary 

 to furnish additional data, as a discussion o'f the facts given will 

 bring out the points wanted for our present purpose. 



For the solution of the problem of the carrying capacity of the 

 wind, it is of primary importance to know whether the velocity is 

 changing on or near the surface. Among the observations on velocity 

 of wind in different heights, which have been recorded, we may men- 

 tion those by Stevenson, Montaigny, Ragona, and Sokoloff. All 

 these experiments show that the velocity increases considerably with 

 the height. 



A very significant feature of the above tables is that the bulk of 

 sand on the two lowest floors is of such uniform size. I take this to 

 indicate that the velocity of the lowest layers of air must be com- 

 paratively uniform, while the higher currents are of a more gusty 

 character and are able to pick up the smaller grades of grains and 

 lift them higher. It is an accepted fact that a current, which carries a 

 load, is retarded, and the retardation is greater the larger the particles 

 moved. Nearest to the ground there is a layer which on account of 

 the friction against the uneven surface is comparatively inert, and we 

 know that the velocity of the current in this layer increases only at 

 a very slow rate with an increase in the speed of the layers next above 

 it.^ These circumstances put together tend to support the above 

 theory that the movement of the lowest layer of the atmosphere is 

 more uniform than the higher. 



Although the bulk of sand on the lowest floor of the separator is 

 greater than on any of the higher, the difference between the quantity 

 of sand on A and B is not very remarkable. I understand this to 

 prove that the larger part of sand moved by the wind is lifted from 

 the ground, if only for a short distance. It seems at first sight as if all 

 the material on the lowest floor or on the same level as the ground, 



I J. A. Udden, The Mechanical Composition of Wind Deposits, 1898, p. 24. 



