_Percolation in Natural Soils._—In artificial percolation experiments, as well as during a fall of rain, the gradual settling of the fully wetted soil-column produces a compacting of that portion of the mass, that increasingly impedes the downward penetration. The effect of this under natural conditions is readily seen in the fact that after the first, rapid absorption of falling rain by the soil when in good tilth, there is a gradual slackening of the process even when the rain is fine and slow, causing a perceptible increase of the runoff until, should the rain continue for some time, the absorption becomes so slow as to cause all, or nearly all the water to drain off the surface. The soil is then called “saturated,” having really arrived at that point right at the surface, and to a depth varying according to the duration and amount of rain, and the natural perviousness of the land. When the rain ceases, the visible saturation of the surface usually soon disappears in cultivated soils, and the zone of saturation begins to descend. The progress of this descent may be very strikingly observed in a series of holes (post-holes) dug or bored across a ridge; as indicated in the subjoined schematic diagram, in which the successive dotted lines represent the levels of the descending “bottom water” at successive intervals, as derived from the observation of the water levels in the several holes.[86] [86] The exact record of these observations was unfortunately destroyed by fire; the soil was a heavy clay, and it took ten days before the water disappeared from the lowest hole. [Illustration: FIG. 42.—Percolation in clay land after heavy rain.]