The surface may have a considerable slope, while the lower strata lie nearly level, as if they had been cut through by artificial grading. The following figure from the Cyclopedia of Agriculture, with the explanation, fully illustrates this idea. "In many subsoils there are thin partings, or layers, of porous materials, interspersed between the strata, which, although not of sufficient capacity to give rise to actual springs, yet exude sufficient water to indicate their presence. These partings occasionally crop out, and give rise to those damp spots, which are to be seen diversifying the surface of fields, when the drying breezes of Spring have begun to act upon them. In the following cut, the light lines represent such partings. "Now, it will be evident, in draining such land, that if the drains be disposed in a direction transverse or oblique to the slope, it will often happen that the drains, no matter how skillfully planned, will not reach these partings at all, as at A. In this case, the water will continue to flow on in its accustomed channel, and discharge its waters at B. [Illustration: Fig. 34--DRAINS ACROSS THE SLOPE.] "But again, even though it does reach these partings, as at C, a considerable portion of water will escape from the drain itself, and flow to the _lower level_ of its old point of discharge at D. Whereas, a drain cut in the line of the slope, as from D to E, intersects all these partings, and furnishes an outlet to them at a lower level than their old ones." These reasons are, it is true, applicable only to land of peculiar structure; but there are reasons for selecting the line of greatest fall for the direction of drains which are applicable to all lands alike. "The line of the greatest fall is the only line in which a drain is relatively lower than the land on either side of it." Whether we regard the surplus water as having recently fallen upon the field, and as being stopped near the surface by an impervious stratum, or as brought down on these strata from above, we have it to be disposed of as it rests upon this stratum, and is borne out by it to the surface. If there is a decided dip, or inclination, of this stratum outward down the slope, it is manifest that the water cannot pass backward to a cross drain higher up the slope. The course of the water must be downward upon the stratum on which it lies, and so all between two cross drains must pass to the lower one. The upper drain could take very little, if any, and the greater the inclination of this stratum, the less could flow backward. But in such case a drain down the slope gives to the water borne up by these strata, an outlet of the depth of the drain. If the drain be four feet deep, it cuts the water-bearing strata each at that depth, and takes off the water.