on of
drainage, that they crack, and so open passages for the water to the
drains.

All gravels, black mud of swamps, and loamy soils of any kind, are
readily drained.

Occasionally, however--even in tracts of easy drainage, as a
whole--deposits are found of some combinations with iron, so firmly
cemented together, as to be almost impenetrable with the pick-axe, and
apparently impervious to water. Exceptional cases of this nature must
be carefully sought for by the drainer.

Whenever a wet spot is observed, seek for the cause, and be satisfied
whether it is wet because a spring bursts up from the bottom; or because
the subsoil is impervious, and will not allow the surface-water to pass
downward. Ascertain carefully the cause of the evil, and then skillfully
doctor the disease, and not the symptoms merely. A careful attention to
the theory of moisture, will go far to enable us properly to determine
the requisite frequency of drains.

DISTANCES DEPEND UPON THE DEPTH OF THE DRAINS.

The relations of the depth and distance of drains will be more fully
considered, in treating of the depth of drains. The idea that depth will
compensate for frequency, in all cases, seems now to be abandoned. It is
conceded that clay-soils, which readily absorb moisture, and yet are
strongly retentive, cannot be drained with sufficient rapidity, or even
thoroughness, by drains at any depth, unless they are also within
certain distances.

In a porous soil, as a general rule, the deeper the drain, the further
it will draw. The tendency of water is to lie level in the soil; but
capillary attraction and mechanical obstructions offer constant
resistance to this tendency. The farther water has to pass in the soil,
the longer time, other things being equal, will be required for the
passage. Therefore, although a single deep drain might, in ten days
lower the water-line as much as two drains of the same depth, or, in
other words, might draw the water all down to its own level, yet, it is
quite evident that the two drains might do the work in less
time--possibly, in five days. We have seen already the necessity of
laying drains deep enough to be below the reach of the subsoil plow and
below frost, so that, in the Northern States, the question of shallow
drainage seems hardly debatable. Yet, if we adopt the conclusion that
four feet is the least allowable depth, where an outfall can be found,
there may be the question still, whether, in very op