National Geographic : 1896 Oct
THE ECONOMIC ASPECTS OF SOIL EROSION 337 the surface of the lands. There, owing to the fact that the air is more or less uplifted, the precipitation of the water vapor is more favored, and the proportion of rainfall is usually greater than it is upon the surface of the ocean. Falling upon the land, the condensed moisture comes down in one or another of three forms-as dew, as rain, or as snow. The dew, though it has much geological importance because of its relation to plant life, has only indirect value in the problem of land erosion. It serves to diminish this wearing by favoring in the dry seasons the de velopment of a mat of vegetation which in the period of rains protects the earth in a very effective way from the temporary streams which gather during heavy showers. The importance of this form of precipitation is great, but it is so limited that we may, with this brief statement, dismiss it. The normal form of falling water is rain. In this mode of precipitation we usually find the fluid descending from a con siderable height in the form of drops of varied bulk, averaging perhaps rather more than one-twentieth of an inch in diameter. They are generally large enough to acquire a considerable velocity on their way to the earth, though their momentum is much diminished by the friction they encounter in passing through the air. Striking the earth, they apply to it what energy they have by virtue of their velocity. If we observe what takes place on recently tilled earth, we readily note certain important conse quences arising from this immediate assault of the rain. As soon as the soil is moistened, each stroke acts to break up the clods, bringing the material into the condition of mud, in which it is readily borne away by the rills which, if the shower be heavy, quickly form in such numbers as to interlace the surface. In a few moments these little streams, at first obscure, gather into distinct rills, which, with quickly swinging curves, carve out a model of a new drainage system. In the course of an hour of very rapid downfall a bare, plowed field, on a declivity of not more than five feet in the hundred, or less than the average slope of land, may-have an average of one-third of an inch of its sur face soil removed to the channels of the streams which drain it. It may, after such a time of rain, be noted on a field which has been plowed and rolled that here and there a small flat stone or a potsherd lies on top of a little earthen column. We see at once that the natural roof has protected the earth beneath and caused it to be left behind in the process of erosion which has overtaken the soil of the neighboring surface.