o requirements. The more nearly the parallel arrangement can
be preserved, the less costly will the work be, while the more nearly we
follow the steepest slope of the ground, the more efficient will each
drain be. No rule for this adjustment can be given, but a careful study of
the plan of the ground, and of its contour lines, will aid in its
determination. On all irregular ground it requires great skill to secure
the greatest efficiency consistent with economy.

The _fall_ required in well made tile drains is very much less than would
be supposed, by an inexperienced person, to be necessary. Wherever
practicable, without too great cost, it is desirable to have a fall of one
foot in one hundred feet, but more than this in ordinary work is not
especially to be sought, although there is, of course, no objection to
very much greater inclination.

One half of that amount of fall, or six inches in one hundred feet, is
quite sufficient, if the execution of the work is carefully attended to.

The least rate of fall which it is prudent to give to a drain, in using
ordinary tiles, is 2.5 in 1,000, or three inches in one hundred feet, and
even this requires very careful work.(8) A fall of six inches in one
hundred feet is recommended whenever it can be easily obtained--not as
being more effective, but as requiring less precision, and consequently
less expense.

*Kinds and Sizes of Tiles.*--Agricultural drain-tiles are made of clay
similar to that which is used for brick. When burned, they are from twelve
inches to fourteen inches long, with an interior diameter of from one to
eight inches, and with a thickness of wall, (depending on the strength of
the clay, and the size of the bore,) of from one-quarter of an inch to
more than an inch. They are porous, to the extent of absorbing a certain
amount of water, but their porosity has nothing to do with their use for
drainage,--for this purpose they might as well be of glass. The water
enters them, not through their walls, but at their joints, which cannot be
made so tight that they will not admit the very small amount of water that
will need to enter at each space. Gisborne says:

"If an acre of land be intersected with parallel drains twelve yards
apart, and if on that acre should fall the very unusual quantity of one
inch of rain in twelve hours, in order that every drop of this rain may be
discharged by the drains in forty-eight hours from the commencement of the
rain--(and in