h hill, when they will receive the laterals at their greatest
fall; but, as these are too far apart to completely drain the valley
between them, and are located on land higher than the center of the
valley, a drain, (_B_,) should be run up, midway between them.

The collecting drain, _A_, will receive the laterals from the hill to the
west of it, as far up as the 10-foot contour line, and, above that
point,--running up a branch of the valley,--it will receive laterals from
both sides. The drain, _B_, may be continued above the dividing point of
the valley, and will act as one of the series of laterals. The drain, _C_,
will receive the laterals and sub-mains from the rising ground to the east
of it, and from both sides of the minor valley which extends in that
direction.

Most of the valley which runs up from the easterly side of the swamp must
be drained independently by the drain _E_, which might be carried to the
silt basin, did not its continuation directly to the outlet offer a
shorter course for the removal of its water. This drain will receive
laterals from the hill bordering the southeasterly side of the swamp, and,
higher up, from both sides of the valley in which it runs.

In laying out these main drains, more attention should be given to placing
them where they will best receive the water of the laterals, and on lines
which offer a good and tolerably uniform descent, than to their use for
the immediate drainage of the land through which they pass. Afterward, in
laying out the laterals, the use of these lines as local drains should, of
course, be duly considered.

*The Lateral Drains* should next receive attention, and in their location
and arrangement the following rules should be observed:

1st. They should run down the steepest descent of the land.

2d. They should be placed at intervals proportionate to their depth;--if 4
feet deep, at 40 feet intervals; if 3 feet deep, at 20 feet intervals.

[Illustration: Fig. 20 - MAP WITH DRAINS AND CONTOUR LINES.]

Fig. 20 - MAP WITH DRAINS AND CONTOUR LINES.

3d. They should, as nearly as possible, run parallel to each other.

On land of perfectly uniform character, (all sloping in the same
direction,) all of these requirements may be complied with, but on
irregular land it becomes constantly necessary to make a compromise
between them. Drains running down the line of steepest descent cannot be
parallel,--and, consequently, the interva