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 a less period that quantity neither will, not is it desirable
that it should, filter through an agricultural soil)--the interval between
two pipes will be called upon to pass two-thirds of a tablespoonful of
water per minute, and no more. Inch pipes, lying at a small inclination,
and running only half-full, will discharge more than double this quantity
of water in forty-eight hours."
Tiles may be made of any desired form of section,--the usual forms are the
"horse-shoe," the "sole," the "double-sole," and the "round.
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