Page:The Journal of geology (IA journalofgeology21894univers).pdf/335

The chief circumstance on which this inefficiency depends is the small weight of the air, which is only about ⁠1/813⁠ as heavy as water. Moving with the same velocity it will strike with a force only ⁠1/813⁠ 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 velocity 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 sedimentation, and at the land above water and the coast lines alone, as being areas of erosion. Whatever be the height of the atmosphere, 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 atmospheric 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.