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.
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.