tile. This defect of grade is shown in Fig. 17. If the second hundred feet
has an inclination of _more_ than three inches, (Fig. 18,) the removal of
silt will be even better secured than if the fall continued at the
original rate. Some silt will enter newly made drains, in spite of our
utmost care, but the amount should be very slight, and if it is evenly
deposited throughout the whole length of the drain, (as it sometimes is
when the rate of fall is very low,) it will do no especial harm; but it
becomes dangerous when it is accumulated within a short distance, by a
decreasing fall, or by a single badly laid tile, or imperfect joint,
which, by arresting the flow, may cause as much mischief as a defective
grade.

Owing to the general conformation of the ground, it is sometimes
absolutely necessary to adopt such a grade as is shown in Fig. 19,--even to
the extent of bringing the drain down a rapid slope, and continuing it
with the least possible fall through level ground. When such changes must
be made, they should be effected by angles, and not by curves. In
_increasing_ the fall, curves in the grade are always advisable, in
_decreasing_ it they are always objectionable, except when the decreased
fall is still considerable,--say, at least 2 feet in 100 feet. The reason
for making an absolute angle at the point of depression is, that it
enables us to catch the silt at that point in a silt basin, from which it
may be removed as occasion requires.

[Illustration: Fig. 19 - THREE PROFILES OF DRAINS, WITH DIFFERENT
INCLINATIONS.]

Fig. 19 - THREE PROFILES OF DRAINS, WITH DIFFERENT INCLINATIONS.

_A Silt Basin_ is a chamber, below the grade of the drain, into which the
water flows, becomes comparatively quiet, and deposits its silt, instead
of carrying it into the tile beyond. It may be large or small, in
proportion to the amount of drain above, which it has to accommodate. For
a few hundred feet of the smallest tile, it may be only a 6-inch tile
placed on end and sunk so as to receive and discharge the water at its
top. For a large main, it may be a brick reservoir with a capacity of 2 or
3 cubic feet. The position of a silt basin is shown in Fig. 19.

The quantity of silt which enters the drain depends very much on the soil.
Compact clays yield very little, and wet, running sands, (quicksands,) a
great deal. In a soil of the latter sort, or one having a layer of running
sand at th