CHAPTER II 

 THE WORK OF THE ATMOSPHERE 



The atmosphere is nearly everywhere in contact with the land, 

 and it penetrates the soil and the rock beneath to great depths. 

 It affects the land in many ways, but only a few of the changes which 

 it brings about will be noticed here. 



1. MECHANICAL WORK. THE WORK OF THE WIND 



Dust 



All small particles of solid matter in the air are called dust. 

 The dust in the air may be seen on windy days in dry regions. It 

 is present, even when the air seems perfectly still. By letting light 

 into a dark room through a narrow crack, or a small hole, countless 

 dust motes may be seen in the air. Dust is found in the air even 

 over high mountains, and it is carried far from its sources. 



The presence of dust in the atmosphere may be shown in another 

 way. If a quantity of newly-fallen rain-water is evaporated, a little 

 sediment remains. This sediment is the dust brought down by the 

 falling drops. If fresh snow is melted and evaporated, there is a 

 slight residue of dust. Since all rains and snows bring down dust, 

 it must be present everywhere in the atmosphere. 



Sources. Much of the dust in the air is made of fine particles 

 of earthy matter blown up from streets, plowed fields, and surfaces 

 not well covered with vegetation. Where the surface is dry and 

 the wind strong, "clouds" or " whirls" of dust are sometimes swept 

 up by the rising currents of air, and may be seen for miles. The 

 solid particles of smoke (soot), the pollen of flowers, the spores of 

 plants which, like the puff-ball, do not blossom, are also abundant 

 in the atmosphere at times. 



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