Material for the Drains.--Doubtless there are places and times when stone, or boards, or brush should be used in construction of underdrains, but they are relatively few in number. Such underdrains lack permanency, as a rule, though some stone drains are effective for a long time. If drain tile can be obtained at a reasonable price, it should be used even in fields that have an abundance of stone. Its use requires less labor than that of stone, and when properly laid on a good bottom, it continues effective. There is no known limit to the durability of a drain made of good tile. The Outlet.--The value of any drainage system is dependent upon the outlet. Its location is the first thing to be determined. If the land is nearly flat, a telescope level should be used to determine elevations of all low points in the land to be drained. The outlet should permit a proper fall throughout the length of the system, and it should not require attention after the work is completed. If it is in the bank of a stream or ditch, it should be above the normal level of the water in the stream. In times of heavy rainfall water may back up into the main with no injury other than temporary failure to perform its work, but continuous submersion will lead to deposits of silt that may close the tile. Locating Main and Branches.--There are various systems of drainage. Wherever a branch or lateral joins the main, the means of drainage is duplicated within the area that the main can drain, and the system should call for the least possible waste of this sort. It usually is best that the main take the center line of the low land, laterals being used to bring the water to the main from both sides, but there is less duplication of work when the main can be at one side of the wet land. Branches of the main may be needed to reach remote parts of the area. The Laterals.--Small lines of tile are used to bring the water to the main when the wet land extends beyond the influence of the main. The distance between these laterals depends upon their depth and the nature of the land. A tight clay soil will not let water pass laterally more than a rod or 20 feet, compelling the placing of the drains not over 40 feet apart, while an open soil may permit a distance of 60 or more feet between laterals.