., where, after obtaining permission to drain
through the land of an adjacent owner, not interested in the operation,
they could obtain but three inches fall in one hundred feet, or a half
inch to the rod, for three quarters of a mile, and this only by blasting
the ledges at the outlet. This fall, however, proves sufficient for
perfect drainage, and by their skill, a very unhealthful swamp has been
rendered fit for gardens and building-lots. In another instance, in
Dorchester, Mass., Mr. Shedd informs us that in one thousand feet, they
could obtain only a fall of two inches for their main, and this, by nice
adjustment, he expects to make sufficient. In another instance, he has
found a fall of two and a half inches in one hundred feet, in an open
paved drain to be effectual.

It is certainly advisable always to divide the fall as even as possible
throughout the drains, yet this will be found a difficult rule to
follow. Very often we have a space of nearly level ground to pass
through to our outfall; and, usually, the mains, in order that the minor
drains may be carried into them from both sides, must follow up the
natural valleys in the field, thus controlling, in a great measure, our
choice as to the fall. We are, in fact, often compelled to use the
natural fall nearly as we find it.

It is thought advisable to have the mains from three to six inches lower
than the drains discharging into them, so that there may be no
obstruction in the minor drains by the backing up of water, and the
consequent deposition of sand or other obstructing substances. Wherever
one stream flows into another, there must be more or less interruption
of the course of each. If the water from the minors enters the main with
a quick fall, the danger of obstruction in the minor, at least, is much
lessened. A frequent cause of partial failure of drains, is their not
having been laid with a regular inclination. If, instead of a gradual
and uniform fall, there should be a slight rising in the bed of a drain,
the descending water will be interrupted there till it accumulate so
high as to be above the level of the rising. At this point, therefore,
the water must have a tendency to press out of the drains, and will
deposit whatever particles of sand or other earthy matter it may bring
down.

Drains must, therefore, be so arranged, that in cutting them, their beds
may be as nearly as possible, straight, or, at least, have a constant,
if not a regular an