lines for our minor
drains, at right angles with the mains, because this is the simplest and
most systematic arrangement; but the natural ravines or water-courses in
fields, seldom run parallel with each other, or at right angles with the
slope of the hills, so that regular work like this, can rarely be
accomplished.

If the earth were constructed of regular slopes, or plains of uniform
character, we could easily apply to it all our rules; but, broken as it
is into hills and valleys, filled with stones here, with a bank of clay
there, and a sand-pit close by, we are obliged to sacrifice to general
convenience, often, some special abstract rule.

We prefer to run drains up and down the slope; but if the field be
filled with undulations, or hills with various slopes, we may often find
it expedient, for the sake of system, to vary this course.

If the question were only as to one single drain, we could adjust it so
as to conform to our perfect ideal; but as each drain is, as it were, an
artery in a complicated system, which must run through and affect every
part of it, all must be located with reference to every other, and to
the general effect.

Keeping in mind, then, the importance of some regular system that shall
include the whole field of operation, the work should be laid out, with
as near a conformity to established principles as circumstances will
permit.

ARRANGEMENT MUST HAVE REFERENCE TO THE FALL.

In considering what fall is necessary, and what is desirable, we have
seen, that although a very slight inclination may carry off water, yet
a proportionably larger drain is necessary as the fall decreases,
because the water runs slower.

"It is surprising," says Stephens, "what a small descent is
required for the flow of water in a well-constructed duct. People
frequently complain that they cannot find sufficient fall to carry
off the water from the drains. There are few situations where a
sufficient fall cannot be found if due pains are exercised. It has
been found in practice, that a water-course thirty feet wide and
six feet deep, giving a transverse sectional area of one hundred
and eighty square feet, will discharge three hundred cubic yards of
water per minute, and will flow at the rate of one mile per hour,
with a fall of no more than _six inches per mile_."

Messrs. Shedd and Edson, of Boston, have superintended some drainage
works in Milton, Mass