Trial-holes.
_b_ Clay-banks of lias or of boulder-clay.
_c_ A more porous warp-drift filling furrows between the
clay-banks.]

"The next object is to connect these furrows by drains laid across
them. The result is, that as the furrows and ridges here run along
the fall of the ground, which I have observed to be the case
generally elsewhere, the sub-mains follow the fall, and the
parallel drains cross it obliquely.

"The intervals between the parallel drains are irregular, varying,
in the same field, from 14 to 21, 31, and 59 feet. The distances
are determined by opening the diagonal drains at the greatest
distance from the trial-holes at which experience has taught the
practicability of its draining the hole. If it does not succeed in
accomplishing the object, another drain is opened in the interval.
It has been found, in many cases, that a drain crossing the
clay-banks and furrows takes the water from holes lying lower down
the hill; that is to say, it intercepts the water flowing to them
through these subterranean channels. The parallel drains, however,
are not invariably laid across the fall. The exceptions are on
ground where the fall is very slight, in which case they are laid
along the line of greatest descent. On such grounds there are few
or no clay-banks and furrows."

It would seem highly probable that the mode of drainage adopted at
Keythorpe, is indebted for its success at that place, to a geological
formation not often met with. At a public discussion in England, Mr. T.
Scott, a gentleman of large experience in draining, stated that "he
never, in his practice, had met with such a geological formation as was
said to exist at Keythorpe, except in such large areas as to admit of
their being drained in the usual _gridiron_ or parallel fashion."

It is claimed for this system by its advocates, that it is far cheaper
than any other, because drains are only laid in the places where, by
careful examination beforehand, by opening pits, they are found to be
necessary; and that is a great saving of expense, when compared with the
system of laying the drains at equal distances and depths over the
field.

Against what is urged as the Keythorpe system, several allegations are
brought.

In the first place, that it is in fact _no system_. Mr. Denton, having
carefully examined the Keythorpe e