harged. At the outlet, must be deposited whatever earth is brought
down by the drains; and, in many cases, the outlet must be at a swamp or
pond. If no decided fall can be obtained at the outlet, there must be
care to provide and keep an open ditch or passage, so that the
drainage-water may not be dammed back in the drains. It is advised,
even, to follow down the bank of a stream or river, so as to obtain
sufficient fall, rather than to have the outlet flooded, or _back-water_
in the drains. Still, there may be cases where it will be impossible to
have an outlet that shall be always above the level of the river or pond
which may receive the drainage water. If the outlet must be so situated
as to be at times overflowed, great care should be taken to excavate a
place at the outlet, into which any deposits brought down by the drain,
may fall. If the outlet be level with the ground beyond it, the smallest
quantity of earth will operate as a dam to keep back the water.
Therefore, at the outlet, in such cases, a small well of brick or
stonework should be constructed, into which the water should pour.
There, even if the water stand above the outlet, will be deposited the
earth brought along in the drain. This well must at times, when the
water is low, be cleared of its contents, and kept ready for its work.

The effect of back-water in drains cannot ordinarily be injurious,
except as it raises the water higher in the land, and occasions deposits
of earthy matter, and so obstructs the drains. We have in mind now, the
common case of water temporarily raised, by Winter flowage or by Summer
freshets.

It should be remembered that even when the outlet is under water, if
there is any current in the stream into which the drain empties, there
must be some current in the drain also; and even if the drain discharge
into a still pond, there must be a current greater or less, as water
from a level higher than the surface of the pond, presses into the
drains. Generally, then, under the most unfavorable circumstances, we
may expect to have some flow of water through the pipes, and rarely an
utter stagnation. If, then, the tiles be carefully laid, so as to admit
only well-filtered water, there can be but little deposit in the drain;
and a temporary stagnation, even, will not injure them, and a trifling
flow will keep them clean. Much will depend, as to the obstruction of
drains, in this, and indeed in all cases, upon the internal smoothness,