EROSION PERFORMED BY THE ATMOSPHERE. 319 



The chief circumstance on which this inefficiency depends is 

 the small weight of the air, which is only about n^ as heavy as 

 water. Moving with the same velocity it will strike with a force 

 only -g^ as great as that with which water will strike. The 

 effectiveness of the impact, however, or the striking force, 

 increases as the square of the velocity and thus when the veloc- 

 ity of the wind is 28 ( — 813 = 28) times greater than that of a 

 current of water, the impinging force of the two currents is the 

 same. Velocities 28 times greater than those of many rivers are 

 not uncommon in the air a small distance above the ground. But 

 the lightness of the air enables even a scanty vegetation to 

 greatly slacken the speed in the currents immediately in contact 

 with the ground. This slackening of the impinging current 

 is apparently sufficient to effectually protect even loose soil 

 from wind erosion under ordinary circumstances. Such is. 

 at least the case where the soil is moist and where the land 

 is level. 



As an erosive agent, the atmosphere is at a disadvantage also 

 in another respect. Lakes never erode their bottoms below the 

 plane of wave action, and even in rivers erosion is greatest at the 

 shores where this plane meets the land surface. Were it not for the 

 wave action, the erosion by continental waters, as well as by the 

 waters of the oceans, would be greatly reduced in its efficacy. 

 In fact we generally look at that part of the surface of the earth 

 which is under water, as being an area of deposition and sedi- 

 mentation, and at the land above water and the coast lines alone, 

 as being areas of erosion. Whatever be the height of the atmos- 

 phere, it does not appear likely that its upper limit is a well 

 defined plane with waves as on the sea. But even if it be, this wave 

 plane would be high above the most elevated point on the earth's 

 surface. There is, therefore, no plane of wave-erosion in the atmos- 

 pheric sea. Such work of this kind as is performed by the air 

 can only be compared with that which takes place in the ocean 

 far below its plane of wave-action, and rather in its abysmal 

 region. Evidently this is not very great, if of any consequence 

 at all. 



