ter-bearing strata which continue to exude
wetness after the higher portions are dry.

[Illustration: Fig. 35.--The vertical dotted lines show the
position of parallel drains.]

"A consideration, too, of the varying inclinations of surface, of
which instances will frequently occur in the same field,
necessitates a departure from uniformity, not in direction only,
but in intervals between drains. Take, for instance, the ordinary
case of a field, in which a comparatively flat space will intervene
between quickly rising ground and the outfall ditch. It is clear
that the soak of the hill will pervade the soil of the lower
ground, let the system of drainage adopted be what it may; and,
therefore, supposing the soil of the hill and flat to be precisely
alike, the existence of bottom water in a greater quantity in the
lower lands than in the higher, will call for a greater number of
drains. It is found, too, that an independent discharge or relief
of the water coming from the hill, at B, should always be provided,
in order to avoid any impediment by the slower flow of the flatter
drains.

[Illustration: Fig. 36.]

"Experience shows that, with few exceptions, hollows, or 'slacks,'
observable on the surface, as at B B, have a corresponding
undulation of subsoil and that any system which does not provide a
direct release for water, which would otherwise collect in and
draw towards these spots, is imperfect and unsatisfactory. It is
found to be much more safe to depend on relief drains, than on the
cutting of drains sufficiently deep through the banks, at A A, to
gain a fall at a regular inclination.

"Still, in spite of experience, we often observe a disregard of
these facts, even in works which are otherwise well executed to a
depth of four feet, but fettered by methodical rules, and I feel
compelled to remark, that it has often occurred to me, when I have
observed with what diligent examination the rules of depth and
distance have been tested, that if more attention had been paid to
the _source_ of injury, and to the mode of securing an effective
and permanent _discharge_ of the injurious water, much greater
service would be done."

In conclusion, as to distances, we should advise great caution on the
part of beginners in laying out their drain