ll not reach these partings at all, as at A. In this
case, the water will continue to flow on in its accustomed channel,
and discharge its waters at B.

[Illustration: Fig. 34--DRAINS ACROSS THE SLOPE.]

"But again, even though it does reach these partings, as at C, a
considerable portion of water will escape from the drain itself,
and flow to the _lower level_ of its old point of discharge at D.
Whereas, a drain cut in the line of the slope, as from D to E,
intersects all these partings, and furnishes an outlet to them at a
lower level than their old ones."

These reasons are, it is true, applicable only to land of peculiar
structure; but there are reasons for selecting the line of greatest fall
for the direction of drains which are applicable to all lands alike.

"The line of the greatest fall is the only line in which a drain is
relatively lower than the land on either side of it." Whether we regard
the surplus water as having recently fallen upon the field, and as being
stopped near the surface by an impervious stratum, or as brought down on
these strata from above, we have it to be disposed of as it rests upon
this stratum, and is borne out by it to the surface.

If there is a decided dip, or inclination, of this stratum outward down
the slope, it is manifest that the water cannot pass backward to a cross
drain higher up the slope. The course of the water must be downward upon
the stratum on which it lies, and so all between two cross drains must
pass to the lower one. The upper drain could take very little, if any,
and the greater the inclination of this stratum, the less could flow
backward.

But in such case a drain down the slope gives to the water borne up by
these strata, an outlet of the depth of the drain. If the drain be four
feet deep, it cuts the water-bearing strata each at that depth, and
takes off the water.

In these cases, the different layers of clay or other impervious
"partings," are like the steps of a huge stairway, with the soil filling
them up to a regular grade. The ditch cuts through these steps, letting
the water that rests on them fall off at the ends, instead of running
over the edges. Drains across the slope have been significantly termed
"mere catch-waters."

If we wish to use water to irrigate lands, we carefully conduct it along
the surface across the slope, allowing it to flow over and to soak
through the soil. If we desire to ca