RELATION OF WIND TO TOPOGRAPHY 551 



than that of coarser texture, because of its greater capillarity, it is 

 evident that, if some means are furnished which increase the amount 

 of moisture, or rather lessen the evaporation, the action of wind will 

 be in a measure counterbalanced. This is accomplished by a 

 vegetative covering, which fixes the sand and thus protects it against 

 the influence of the air currents. The climatic conditions, especially 

 a larger or smaller degree of moisture, is therefore a great factor in 

 the development of eolian sand formations. 



It is a matter of everyday observation that the velocity of the 

 wind changes rapidly and varies considerably. A wind which appears 

 to be very uniform is, when subjected to close observation, only a 

 series of gusts following each other with intervals of lesser velocity, 

 and even of complete calm. The carrying capacity of the wind is, 

 therefore, also very variable. The wind that one moment carries 

 and drops pebbles, a few minutes later carries only sand for deposi- 

 tion, and finer sand follows coarser. 



A series of observations made by the writer in this connection will 

 here be referred to. 



Methods of observation. — The experiments were conducted on the 

 sandy beach north of Fremantle in Western Australia during Septem- 

 ber and October, IQ02, and they were intended to ascertain the carry- 

 ing capacity of wind of certain velocities, as well as the effect of these 

 winds on the movements of the sand. 



For measuring the velocity of wind I used anemometers of Crova 

 type, purchased from Negretti and Zambra in London. The in- 

 strument was placed on a support steadily secured in the sand, and 

 elevated 5 cm. above the ground. It was not practicable to lower 

 the apparatus more, because nearer the surface the amount of sand 

 particles carried by the air current was still greater than at the eleva- 

 tion chosen, and I found that the results were somewhat influenced 

 by the density of the sand-shower. 



Samples of sand of different grain sizes were secured by sifting 

 through sieves with meshes of known diameter. Three sizes were 

 used, 0.2 mm. (fine sand), 0.3 (medium sand), and 0.6 (coarse sand). 



Of each kind of sand a quantity of about 5 cu. dm. was dyed with 

 different colors, black, bright blue, and orange being considered the 

 most suitable and best distinguishable from the natural sand. The 



