in may be seen, and so that it may not be
liable to be clogged by the accumulation of silt, (or mud,) in the open
ditch into which it flows.

2. The main drain should, usually, be run as nearly in the lowest part of
the principal valley as is consistent with tolerable straightness. It is
better to cut across the point of a hill, to the extent of increasing the
depth for a few rods, than to go a long distance out of the direct course
to keep in the valley, both because of the cost of the large tile used in
the main, and of the loss of fall occasioned by the lengthening of the
line. The main should be continued from the outlet to the point at which
it is most convenient to collect the more remote sub-mains, which bring
together the water of several sets of laterals. As is the case in the
tract under consideration, the depth of the main is often restricted, in
nearly level land, toward the upper end of the flat which lies next to the
outlet, by the necessity for a fall and the difficulty which often exists
in securing a sufficiently low outlet. In such case, the only rule is to
make it as deep as possible. When the fall is sufficient, it should be
placed at such depth as will allow the laterals and sub-mains which
discharge into it to enter at its top, and discharge above the level of
the water which flows through it.

[Illustration: Fig. 10 - STONE PIT TO CONNECT SPRING WITH DRAIN.]

Fig. 10 - STONE PIT TO CONNECT SPRING WITH DRAIN.

3. Subsidiary mains, or _sub-mains_, connecting with the main drains,
should be run up the minor valleys of the land, skirting the bases of the
hills. Where the valley is a flat one, with rising ground at each side,
there should be a sub-main, to receive the laterals from _each_ hill side.
As a general rule, it may be stated, that the collecting drain at the foot
of a slope should be placed on the line which is first reached by the
water flowing directly down over its surface, before it commences its
lateral movement down the valley; and it should, if possible, be so
arranged that it shall have a uniform descent for its whole distance. The
proper arrangement of these collecting drains requires more skill and
experience than any other branch of the work, for on their disposition
depends, in a great measure, the economy and success of the undertaking.

4. Where springs exist, there should be some provision made for collecting
their water in pits filled with loose stone, gravel,