With these tools, and a long light crowbar, for hard soils, trenches may be dug much more cheaply than with the common spade and pickaxe. Where there are large boulders in the soil, these draining tools may dig under them so that they will not have to be removed. When the trenches are dug to a sufficient depth, the bottoms must be made perfectly smooth, with the required descent (from six inches to a few feet in one hundred feet). Then the tiles may be laid in, so that their ends will correspond, be packed down, and the trenches filled up. Such a drain, if properly constructed, may last for ages. Unlike the stone drain, it is not liable to be frequented by rats, nor choked up by the soil working into it. The position of the tile may be best represented by a figure, also the mode of constructing stone drains. [Why are small stones better than large stones in the construction of drains? On what must the depth of under-drains depend?] It will be seen that the tile drain is made with much less labor than the stone drain, as it requires less digging, while the breaking up of the stone for the stone drain will be nearly, or quite as expensive as the tiles. Drains made with large stones are not nearly so good as with small ones, because they are more liable to be choked up by animals working in them.[AK] [Illustration: Fig. 6. _a_--Tile drain trench. _b_--Stone drain trench. _c_--Sod laid on the stone.] [Describe the principle which regulates these relative depths and distances. (Blackboard.) Which is usually the cheaper plan of constructing drains?] The _depth_ of the drains must depend on the distances at which they are placed. If but _twenty_ feet apart, they need be but _three_ feet deep; while, if they are _eighty_ feet apart, they must be _five_ feet deep, to produce the same effect. The reason for this is, that the water in the drained soil is not level, but is higher midway between the drains, than at any other point. It is necessary that this highest point should be sufficiently far from the surface not to interfere with the roots of plants, consequently, as the water line between two drains is _curved_, the most distant drains must be the deepest. This will be understood by referring to the following diagram. [Illustration: Fig. 7. _aa_--5 feet drains, 80 ft. apart. _bb_--3 feet drains, 20 ft. apart.] The curved line represents the position of the water. In most soils it will be easier to dig one trench five feet deep, than four trenches three feet deep, and the deep trenches will be equally beneficial; but where the soil is very hard below a depth of three feet, the shallow trenches will be the cheapest, and in such soils they will often be better, as the hard mass might not allow the water to pass down to enter the deeper drains. By following out these instructions, land may be cheaply, thoroughly, and permanently drained. FOOTNOTES: