Water goes down, then, by its own weight, unless there is something through which it cannot readily pass, to bring it out at the surface. It will go into the drains, only because they are lower than the land drained. It will never go _upward_ to find a drain, and it will go toward a drain the more readily, in proportion as the descent is more steep toward it. To decide properly what direction a drain should have, it is necessary, then, to have a definite and a correct idea as to what office the drain is to perform, what water is to fall into it, what land it is to drain. Suppose the general plan to be, to lay drains forty feet apart, and four feet deep over the field, and the question now to be determined, as to the _direction_, whether across, or up and down the slope, there being fall enough to render either course practicable. The first point of inquiry is, what is expected of each drain? How much and what land should it drain? The general answer must be, forty feet breadth, either up and down the slope, or across it; according to the direction. But we must be more definite in our inquiry than even this. From _what_ forty feet of land will the water fall into the drain? Obviously, from some land in which the water is higher than the bottom of the drain. If, then, the drain run directly _across_ the slope, most of the water that can fall into it, must come from the forty feet breadth of land between the drain in question, and the drain next above it. If the water were falling on an impervious surface, it would all run according to the slope of the surface, in which case, by the way, no drains but those across, could catch any of it except what fell upon the drains. But the whole theory of drainage is otherwise, and is based on the idea that we change the course of the underground flow, by drawing out the water at given points by our drains; or, in other words, that "the water seeks the lowest level in all directions." Upon the best view the writer has been able to take of the two systems as to the direction of drains, there is but a very small advantage in theory in favor of either over the other, in soil which is homogeneous. But it must be borne in mind that homogeneous soil is rather the exception in nature than the rule. Without undertaking to advance or defend any peculiar geological views of the structure of the earth, or of the depositions or formations that compose its surface, it may be said, that very often the first four feet of subsoil is composed of strata, or layers of earth of varying porosity. Beneath sand will be found a stratum of clay, or of compact or cemented gravel, and frequently these strata are numerous and thin. Indeed, if there be not some stratum below the soil, which impedes the passage of water, it would pass downward, and the land would need no artificial drainage. Quite often it will be found that the dip or inclination of the various strata below the soil is different from that of the surface.