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"The tract drained by this system, though very swampy "before being drained, is now dry enough to walk upon, 'almost immediately after a storm, except when underlaid "by a stratum of frozen ground."
The area drained by the main at which these gaugingt were made, is about ten acres, and, in deference to the prevailing mania for large conduits, it had been* laid with 6-inch sole-tile. The greatest recorded discharge in 24 hours was (August 25th,) less than 100,000 gallons from the ten acres,—an amount of water which did not half fill the tile, but which, according to the tables referred to, would have entirely filled it.
In view of all the information that can be gathered on the subject, the following directions are given as perfectly reliable for drains four feet or more in depth, laid on a well regulated fall of even three inches in a hundred feet:
For 2 acres 1J inch pipes (with collars.)
For 40 acres 2 3£" "or one 5-inch sole-tile.
For 50 acres 6" "sole-tile.
For 100 acres 8" "or two 6-inch sole-tiles.
It is not pretended that these drains will immediatelj remove all the water of the heaviest storms, but they will always remove it fast enough for all practical purposes, and, if the pipes are securely laid, the drains will only be benefited by the occasional cleansing they will receive when running "more than fulL" In illustration of this statement, the following is quoted from a paper communicated by Mr. Parkes to the Royal Agricultural Society ot England in 1843:
"Mr. Thomas Hammond, of Penshurst, (Kent,) now "uses no other size for the parallel drains than the inch "tile in the table (No. 5,) having commenced with No. "4,* and it may be here stated, that the opinion of all the M farmers who have used them in the Weald, is that a bore "of an inch area is abundantly large. A piece of 9 acres, "now sown with wheat, was observed by the writer, 36 "' hours after the termination of a rain which fell heavily "and incessantly during 12 hours on the 7th of Novem"ber. This field was drained in March, 1842, to the depth "of 30 to 36 inches, at a distance of 24 feet asunder, the "length of each drain being 235 yards.
"Each drain emptied itself through a fence bank into M a running stream in a road below it; the discharge "therefore was distinctly observable. Two or three of "the pipes had now ceased running; and, with the ex"ception of one which tapped a small spring and gave a "stream about the size of a tobacco pipe, the run from "the others did not exceed the size of a wheat straw "The greatest flow had been observed by Mr. Hammond "at no time to exceed half the bore of the pipes. The "fall in this field is very great, and the drains are laid is "the direction of the fall, which has always been the prao"tice in this district. The issuing water was transpa M rently clear; and Mr. Hammond states that he has "never observed cloudiness, except for a short time after '' very heavy flushes of rain, when the drains are quickly "cleared of all sediment, in consequence of the velocity "and force of the water passing through so small a channel. "Infiltration through the soil and into the pipes, must. "in this case, be considered to have been perfect; and "their observed action is the more determinate and valua'ble as regards time and effect, as the land was saturated .' with moisture previous to this particular fall of rain, 'and the pipes had ceased to run when it commenced 'This piece had, previous to its drainage, necessarilj "been cultivated in narrow stetches, with an open watei
• Mo 5 was one Inch Id diameter; No. 4, abont ljtf Inches.
"furrow between them; but it was now laid i^aite plain, "by which one-eighth of the continuation of acreage ha* "been saved. Not, however, being confident as to the "soil having already become so porous as to dispense en"tirely with surface drains, Mr. Hammond had drawn "two long water furrows diagonally across the field. On "examining these, it appeared that very little water had •' flowed along any part of them during these 12 hours of "rain,—no water had escaped at their outfall; the entire "body of rain had permeated the mass of the bed, and "passed off through the inch pipes; no water perceptible "on the surface, which used to carry it throughout. The "subsoil is a brick clay, but it appears to crack very "rapidly by shrinkage consequent to drainage."
Obstructions.—The danger that drains will become obstructed, if not properly laid out and properly made, is very great, and the cost of removing the obstructions, (often requiring whole lines to be taken up, washed, and relaid with the extra care that is required in working in old and soft lines,) is often greater than the original cost of the improvement. Consequently, the possibility of tile drains becoming stopped up should be fully considered at the outset, and every precaution should be taken to prevent so disastrous a result.
The principal causes of obstruction are silt, vermin, and roots.
Silt is earth which is washed into the tile with the water of the soil, and which, though it may be carried along in suspension in the water, when the fall is good, will be deposited in the eddies and slack-water, which occur whenever there is a break in the fall, or a defect in the laying of the tile.
When it is practicable to avoid it, no drain should have a decreasing rate of full as it approaches its outlet.
If the first hundred feet from the upper end of the
drain has a fall of three inches, the next hundred feet should not have less than three inches, lest the diminished velocity cause silt, which required the speed which that fall gives for its removal, to be deposited and to choke the tile. This defect of grade is shown in Fig. 17. If the second hundred feet has an inclination of more than three inches, (Fig. 18,) the removal of silt will be even better secured than if the fall continued at the original rate. Some silt will enter newly made drains, in spite of our utmost care, but the amount should be very slight, and if it is evenly deposited throughout the whole length of the drain, it will do no especial harm; but it becomes dangerous when it is accumulated within a short distance, by a decreasing fall, or by a single badly laid tile, or imperfect joint, which, by arresting the flow, may cause as much mischief as a defective grade. The use of muslin bands practically prevents the entrance of silt.
Owing to the general conformation of the ground, it is Bometimes absolutely necessary to adopt such a grade as is shown in Fig. 19,—even to the extent of bringing the drain down a rapid slope, and continuing it with the least possible fall through level ground. When such changes must be made, they should be effected by angles, and not by curves. In increasing the fall, curves in the grade are always advisable, in decreasing it they are always objectionable, except when the decreased fall is still considerable,—say, at least 2 feet in 100 feet. The reason for making an absolute angle at the point of depression is, that it enables us to catch the silt at that point in a silt basin, from which it may be removed as occasion requires.
A Silt Basin is a chamber, below the grade of the drain, into which the water flows, becomes comparatively quiet, and deposits its silt, instead of carrying it into the tile beyond. It may be large or small, in proportion to the amount of drain above, which it has to accommodate. For a few hundred feet of the smallest tile, it may be only a