THE ECOXOmC ASPECTS OF SOIC 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 im])ortance because of its relation to plant life, has 
only indirect value in the problem of land erosion. It serves to 
diminish this wearing h\^ favoring in the diy seasons the de- 
velopment of a mat of vegetation which in the period of rains 
ju’otects 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- 
sideral)le height in the form of drops of varied bulk, averaging 
perha[)s rather more than one-twentieth of an inch in diameter. 
They are generall}' large enough to acquire a considerable velocit}’’ 
on their wa}^ to the earth, though their momentum is much 
diminished l)y the friction they encounter in passing through 
the air. Striking the earth, they appl}' 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 uj) 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 hare, plowed field, on a declivity of not 
more than five feet in the hundred, or less than the average slo])e 
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, he noted on a field which has 
been plowed and rolled that here and there a small fiat stone or 
a potsherd lies on top of a little earthen column. A\’e sec at 
once that the natural roof has protected the earth beneath and 
caused it to he left behind in the process of erosion which has 
overtaken the soil of the neighlxjring surface. 
