(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:

[AK] It is probable that a composition of hydraulic cement and some
soluble material will be invented, by which a continuous pipe may be
laid in the bottoms of trenches, becoming porous as the soluble material
is removed by water.

CHAPTER III.

ADVANTAGES OF UNDER-DRAINING.

The advantages of under-draining are many and important.

1. It entirely prevents drought.

2. It furnishes an increased supply of atmospheric fertilizers.

3. It warms the lower portions of the soil.

4. It hastens the decomposition of roots and other organic matter.

5. It accelerates the disintegration of the mineral matters in the soil.

6. It causes a more even distribution of nutritious matters among those
parts of soil traversed by roots.

7. It improves the mechanical texture of the soil.

8. It causes the poisonous excrementitious matter of plants to be
carried out of the reach of their roots.

9. It prevents grasses from running out.

10. It enables us to deepen the surface soil.

By removing excess of water--

11. It rende