ER OSION PERFORMED B Y THE A TMOSPHERE. 323 



Average Behavior of the particles when thrown 



diameter of -^^^ ^j^^ ^^y_ 



particles. 



.75 mm. Described a path diverging about 10° from a vertical line. 



.37 mm. Described a path diverging about 45° from a vertical line. 



.18 mm. Described a path diverging but a few degrees from a horizontal 

 line, were blown upward by eddies. 



.08 mm. Could scarcely be noticed to settle in transport. 



.04 mm. Apparently completely borne up by the wind. 

 .007 mm. Completely borne up by the wind, 

 .001 mm. Completely borne up by the wind. 



It is hardly necessary to add that the average size of the largest 

 particles carried varies greatly with the velocity of the wind. 

 Sand grains will occasionally be found to have been thus carried, 

 which have a diameter many times larger than the average maxi- 

 mum here stated. The presence of such large grains can readily 

 be accounted for by the chances for becoming entangled in spe- 

 cifically lighter objects, such as fragments of leaves and other 

 vegetation, and thus to be carried by them. It will be under- 

 stood, also, that the statement made above does not apply to 

 that phase of wind-transportation which takes place on the sur- 

 face of a sand-dune, where the sand is as if rolled forwards, nor 

 to that in the very lowest part of the atmosphere generally, 

 where materials are thrown forwards short distances at a time by 

 eddies due to the contact of the atmosphere with the more or 

 less irregular surface of the land. 



The capacity of the atmosphere for tra7isporting particles of quartz 

 below the size of .1 mm. i7i diameter, is very great. 



Disregarding the occasional transference of matter by vol- 

 canic forces and by living organisms, there are only three prin- 

 cipal agents known to be at work removing materials from place 

 to place on the surface of the globe. These are water, ice, and 

 air. It is believed that, with the above limitation as to the fine- 

 ness of the material, the transporting power of the atmosphere, 

 as compared with that of water and ice, is very great. The trans- 

 porting capacity of the water in our continental rivers is better 

 known than that of glaciers or of ice fields, and it makes our best 



