GEOLOGICAL WORK OF THE ATMOSPHERE 89 



THE MECHANICAL WORK OF THE ATMOSPHERE 



The mechanical work of the atmosphere is accomplished chiefly 

 through its movements. A feeble breeze is competent to move 

 particles of dust, a wind of moderate velocity to shift dry sand, 

 and exceptionally strong winds sometimes move small pebbles. 



The principal movement of the wind is horizontal; but every 

 obstacle against which it blows deflects a part of it, and some of 

 these deflections are upward. Furthermore, there are exceptional 

 winds, in which the vertical element predominates. Particles of 

 dust are often caught by these upward currents, and carried to 

 great heights, and transported great distances. 



Dust 1 



Transportation of dust by the wind is nearly universal. No 

 house, no room, and scarcely a drawer is so tightly closed but that 

 dust enters, and the movements of dust in the open must be much 

 more considerable. The visible dustiness of the atmosphere in 

 dry regions during wind-storms is familiar proof of the efficiency 

 of the wind as a carrier of dust. 



Under special circumstances, opportunity is afforded for rough 

 determinations of the distance and height to which dust is carried 

 by the wind. Snow taken from snow-fields in high mountain 

 regions is found to contain a small amount of earthy matter. Some 

 of its particles are often volcanic dust, even when the place whence 

 the snow is taken is scores or even hundreds of miles from the 

 nearest volcano. In the great eruption of Krakatoa 2 in 1883, 

 large quantities of volcanic dust (pulverized lava) were projected 

 to great heights into the atmosphere. The coarser particles soon 

 settled; but many of the finer ones, caught by the currents of 

 the upper atmosphere, were carried around the earth in 15 days. 

 Through all its long journey, the dust was gradually settling from 

 the atmosphere, but not until it had traveled repeatedly round 

 the earth did its amount become so small as to cease, to make its 



1 For an excellent study of the erosion, transportation, and sedimenta- 

 tion performed by the atmosphere, see Udden, Jour, of Geol., Vol. II, pp. 

 318-331; also Pop. Sci. Mo., September, 1896. 



2 The Eruption of Krakatoa. Committee of the Royal Society, 1888. 



