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clay drain as freely as sand. The fact is that the adjust ment of the distances between drains is very far from partaking of the nature of an exact science, and there is really very little known, by any one, of the principles on which it should be based, or of the manner in which the bearing of those principles, in any particular case, is af. fected by several circumstances which vary with each change of soil, inclination and exposure.
In the essays on drainage which have been thus far published, there is a vagueness in the arguments on this branch of the subject, which betrays a want of definite conviction in the minds of the writers; and which tenda quite as much to muddle as to enlighten the ideas of the reader. In so far as the directions are given, whether fortified by argument or not, they are clearly empirical, and are usually very much qualified by considerations which weigh with unequal force in different cases.
In laying out work, any skillful drainer will be guided, in deciding the distance between the lines, by a judgment which has grown out of his former experience; and which will enable him to adapt the work, measurably, to the requirements of the particular soil under consideration; but he would probably find it impossible to so state the reasons for his decision, that they would be of any genera! value to others.
Probably it will be a long time before rules on this subject, based on well sustained theory, can be laid down with distinctness, and, in the mean time, we must be guided by the results of practice, and must confine ourselves to a distance which repeated trial, in various soils, has proven to be safe for all agricultural land. In the drainage of the Central Park, after a mature consideration of all that had been published on the subject, and of a considerable previous observation and experience, it was decided to adopt a general depth of four feet, and to adhere as closely as possible to a uniform distance of forty feet. No instance was known of a failure to produce good results by drain. ing at that distance, and several cases were recalled where drains at fifty and sixty feet had proved so inefficient that intermediate lines became necessary. After from seven to ten years' trial, the Central Park drainage, by its results, has shown that, -although some of the land is of a very retentive character,—this distance is not too great; and it is adopted here for recommendation to all who have no especial reason for supposing that greater distances will be fully effective in their more porous soils.
As has been before stated, drains at that distance, (or at any distance,) will not remove all of the water of sat.uration from heavy clays so rapidly as from more porous soil; but, although, in some cases, the drainage may be insufficient during the first year, and not absolutely per. fect during the second and third years, the increased por. osity which drainage causes, (as the summer droughts make fissures in the earth, as decayed roots and other organic deposits make these fissures permanent, and as chemical action in the aërated soil changes its character,) will finally bring clay soils to as perfect a condition as they are capable of attaining, and will invariably render them excellent for cultivation.
The Direction of the Laterals should be right up and doron the slope of the land, in the line of steepest descent. For a long time after the general adoption of thoroughdraining, there was much discussion of this subject, and much variation in practice. The influence of the old rules for making surface or “catch-water” drains lasted for a long time, and there was a general tendency to make tile drains follow the same directions. An important require ment of these was that they should not take so steep an inclination as to have their bottoms cut out and their banks undermined by the rapid flow of water, and that they should arrest and carry away the water flowing down over the surface of hill sides. The arguments for the line of steepest descent were, however, so clear, ang drains laid on that line were so universally successful in practice, that it was long ago adopted by all, - save those novices who preferred to gain their education in draining in the expensive school of their own experience.
The more important reasons why this direction is thu best are the following: First, it is the quickest way to get the water off. Its natural tendency is to run straight down the hill, and nothing is gained by diverting it from this course. Second, if the drain runs obliquely down the hill, the water will be likely to run out at the joints of the tile and wet the ground below it; even if it do not, mainly, run past the drain from above into the land be low, instead of being forced into the tile. Third, a drain lying obliquely across a hillside will not be able to draw the water from below up the hill toward it, and the water of nearly the whole interval will have to seek its outlet through the drain below it. Fourth, drains run. ning directly down the hill will tap any porous water bearing strata, which may crop out, at regular intervals, and will thus prevent the spewing out of the water at the surface, as it might do if only oblique drains ran for a long distance just above or just below them. Very steep, and very springy hill sides, sometimes require very frequent drains to catch the water which has a tendency to flow to the surface; this, however, rarely occurs.
In laying out a plan for draining land of a broken sur face, which inclines in different directions, it is impossible to make the drains follow the line of steepest descent, and at the same time to have them all parallel, and at uniform distances. In all such cases a compromise must be made between the two requirements. The more nearly the parallel arrangement can be preserved, the less costly will the work be, while the more nearly we follow the steepest slope of the ground, the more efficient will each drain be. No rule for this adjustment can be given, but a careful study of the plan of the ground, and of its contour lines, will aid in its determination. On all irregular ground it requires great skill to secure the greatest efficiency consis. tent with economy.
The fall required in well made tile drains is very much ess than would be supposed, by an inexperienced person, to be necessary. Wherever practicable, without too great cost, it is desirable to have a fall of one foot in one hundred feet, but more than this in ordinary work is not especially to be sought, although there is, of course, no objection to very much greater inclination.
One half of that amount of fall, or six inches in one hundred feet, is quite sufficient, if the execution of the work is carefully attended to.
The least rate of fall which it is prudent to give to a drain, in using ordinary tiles, is 2.5 in 1,000, or three inches in one hundred feet, and even this requires very careful work.* A fall of six inches in one hundred feet is recommended whenever it can be easily obtained—not as being more effective, but as requiring less precision, and consequently less expense.
Kinds and Sizes of Tiles.-Agricultural drain-tiles are made of clay similar to that which is used for brick. When burned, they are from twelve inches to fourteen inches long, with an interior diameter of from one to eight inches, and with a thickness of wall, (depending on the strength of the clay, and the size of the bore,) of from one-quarter of an inch to more than an inch. They are porous, to the extent of absorbing a certain amount of water, but their porosity has nothing to do with their use for drainage,—for this purpose they might as well be of glass. The water enters them, not through their walls,
* Some of the drains in the Central Park have a fall of only 1 in 1,000, and they work perfectly; but they are large mains, laid with an amount of care, and with certain costly precautions, (including precisely graded wooden floors,) which could bardly be expected in privato work
but at their joints, which cannot be made so tight that they will not admit the very small amount of water that will need to enter at each space. Gisborne says:
“If an acre of land be intersected with parallel drains “ twelve yards apart, and if on that acre should fall the “very unusual quantity of one inch of rain in twelve “hours, in order that every drop of this rain may be dis“ charged by the drains in forty-eight hours from the com“mencement of the rain—(and in a less period that quan“tity neither will, not is it desirable that it should, filter “through an agricultural soil)—the interval between two “pipes will be called upon to pass two-thirds of a table“spoonful of water per minute, and no more. Inch pipes, “ lying at a small inclination, and running only half-full, “ will discharge more than double this quantity of water “ in forty-eight hours."
Tiles may be made of any desired form of section,—the usual forms are the “horse-shoe,” the “sole,” the “double-sole," and the “round.” The latter may be used with collars, and they constitute the “pipes and collars,” frequently referred to in English books on drainage.
Horse-shoe tiles, Fig. 13, are condemned by all modern engineers. Mr. Gisborne disposes of them by an argument
of some length, the quotation of which in these pages is
probably advisable, because Fig. 13.-HORSE-SHOE TILE.
. they form so much better conduits than stones, and to that extent have been so successfully employed, that they are still largely used in this country by “amateurs.”
“We shall shock some and surprise many of our readers, when we state confidently that, in average soils, and, still more, in those which are inclined to be tender, horse shoe tiles form the weakest and most failing conduit which has ever been used for a deep drain. It is so, howeyer; and a little thought, even if we had no experience, will tell us that it must be so. A doggrel sony, quite destitute of humor, informs Lis that tiles of this sort were used in 1760 at Grandesburg Hall, in Suf: