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.
In these cases, the different layers of clay or other impervious "partings," are like the steps of a huge stairway, with the soil filling them up to a regular grade. The ditch cuts through these steps, letting the water that rests on them fall off at the ends, instead of running over the edges. Drains across the slope have been significantly termed "mere catch-waters."
If we wish to use water to irrigate lands, we carefully conduct it along the surface across the slope, allowing it to flow over and to soak through the soil. If we desire to carry the same water off the field as speedily as possible, we should carry our surface ditch directly down the slope.
Now, looking at the operation of drains across the slope, and supposing that each drain is draining the breadth next above it, we will suppose the drain to be running full of water. What is there to prevent the water from passing out of that drain in its progress, at every point of the tiles, and so saturating the breadth below it? Drainpipes afford the same facility for water to soak out at the lower side, as to enter on the upper, and there is the same law of gravitation to operate in each case. Mr. Denton gives instances in which he has observed, where drains were carried across the slope, in Warwickshire, lines of moisture at a regular distance below the drains. He could ascertain, he says, the depth of the drain itself, by taking the difference of height between the line of the drain at the surface, and that of the line of moisture beneath it. He says again:
"I recently had an opportunity, in Scotland, of gauging the quantity of water traveling along an important drain carried obliquely across the fall, when I ascertained with certainty, that, although the land through which it passed was comparatively full of water, the drain actually lost more than it gained in a passage of several chains through it."
So far as authority goes, there seems, with the exception of some advocates of the Keythorpe system, of which an account has been given, to be very little difference of opinion. Mr. Denton says:
"With respect to the direction of drains, I believe very little difference of opinion exists. All the most successful drainers concur in the line of the steepest descent, as essential to effective and economical drainage. Certain exceptions are recognized in the West of England, but I believe it will be found, as practice extends in that quarter, that the exceptions have been allowed in error."