THE ATMOSPHERE AS A MECHANICAL AGENT. 159 



The basement on which it rests — the earth's uneven surface — varies 

 widely in temperature, and this variation passes to extremes in the higher 

 mountains, whatever the zone. The atmosphere's own temperature, even 

 in the tropics, is at the freezing-point at a height of less than four miles. 

 Through these and other conditions the atmosphere has its varying belts 

 of greater and less depth, — that is, of higher and lower barometric pres- 

 sure, — its areas of high and low pressure moving in great circuits, and, 

 as a consequence, winds, storms, cyclones, tornadoes, in its fruitless effort 

 toward a state of equilibrium. These winds are its chief means of mechani- 

 cal work. 



The Mechanical Work of the Atmosphere. — The atmosphere works mechan- 

 ically (1) by denudation, or, as it has been termed, deflation, with or without 

 abrasion ; (2) by transportation ; (3) by deposition ; and (4) through its 

 pressure. The work and the results are called Eolian, from AtoAos, the god 

 of the ivinds. 



The force of the wind, measured by the pressure on a square foot, in- 

 creases with the square of the velocity. At 5 miles an hour, the pressure 

 is about 2 ounces to the square foot ; at 10 miles, which is that of a light 

 breeze, 8 ounces ; at 20 miles, a good steady breeze, 2 pounds ; at 40 miles, 

 a strong gale, 8 pounds ; at 60 miles, 18 pounds ; at 100 miles, 50 pounds. 

 The work done is dependent largely on the form of the surface struck. 

 This is well shown in the anemometer made of hemispherical cups : the 

 difference between the pressure on the concave and convex sides being such 

 that the cups move one third as fast as the wind, whereas with flat disks 

 there would be no motion. A velocity of 186 miles an hour (or 170 pounds 

 to the square foot) has been registered by the anemometer. 



While the lighter winds, and especially the great currents, like the trades, 

 have a degree of regularity in movement, the storm winds, on which geo- 

 logical work mainly depends, are hurrying bodies of air of inconstant force, 

 breadth, and direction. A single storm includes all the courses of the com- 

 pass, and all degrees of force, from lulls to extremest violence ; and when 

 most constant, these winds are still made up of fitful blasts. Under such con- 

 ditions, abrasion, transportation, and deposition should be greatly mixed; 

 and this is a striking feature of the results. 



Eolian Denudation or Deflation. 



Denudation, or wear by wind-force, is carried on (1) by simple wind-impact 

 and (2) by impact when the air is loaded with sand or other material. 



1. By simple impact. — The lighter work of the winds is the taking up 

 of dust from roads, sand-fields, sand-hills, and sea-beaches, to drift away to 

 some other place. The streets of most cities and the roads of the country 

 often afford examples of the work on dry^ windy days. It is to be noted, 

 however, that a rather strong wind is required for this light deflation unless 

 moving wheels first stir up the dust. The result is due to the direct impulse 



