ikely to stop, until
a barrier is formed, and the drain stopped.

In speaking of the forms of tiles, the superiority of rounded openings
over those with flat bottom has been shown. The greater head of water in
a round pipe, gives it force to drive before it all obstructions, and so
tends to keep the drain clear.

_Obstructions at the Outlet._ The water from deep drains is usually very
clear, and cattle find the outlet a convenient place to drink at, and
constantly tread up the soft ground there, and obstruct the flow of
water. All earthy matter, and chemical solutions of iron, and the like,
tend to accumulate by deposit at the outlet. Frogs and mice, and insects
of many kinds, collect about such places, and creep into the drains. The
action of frost in cold regions displaces the earth, and even masonry,
if not well laid; and back-water, by flowing into the drains, hinders
the free passage of water.

All these causes tend to obstruct drains at the outlet. If once stopped
there, the whole pipe becomes filled with stagnant water, which deposits
all its earthy matter, and soon becomes obstructed at other points, and
so becomes useless. The outlet must be rendered secure from all these
dangers, at all seasons, by some such means as are suggested in the
chapter on the Arrangement of Drains.

_Obstruction by roots._ On the author's farm in Exeter, a wooden drain,
to carry off waste water from a watering place, was laid, with a
triangular opening of about four inches. This was found to be obstructed
the second year after it was laid; and upon taking it up, it proved to
be entirely filled for several feet, with willow roots, which grew like
long, fine grass, thickly matted together, so as entirely to close the
drain. There was a row of large willows about thirty feet distant, and
as the drain was but about two feet deep, they found their way easily
to it, and entering between the rough joints of the boards, not very
carefully fitted, fattened on the spring water till they outgrew their
new house.

A neighbor says, he never wants a tree within ten rods of any land he
desires to plow; and it would be unsafe to undertake to set limits to
the extent of the roots of trees. "No crevice, however small," says a
writer, "is proof against the entrance of the roots of water-loving
trees."

The behavior of roots is, however, very capricious in this matter; for,
while occasional instances occur of drains being obstructed by them, it