le by this operation.

These cracks are found, by observation, to commence at the drains, and
extend further and further, in almost straight lines, into the subsoil,
forming so many minor drains, or feeders, all leading to the tiles.
These main fissures have numerous smaller ones diverging from them, so
that the whole mass is divided and subdivided into the most minute
portions. The main fissures gradually enlarge, as the dryness increases,
and, at the same time, lengthen out; so that, in a very dry season, they
may be traced the whole way between the drains. The following cut will
give some idea of these cracks, or fissures, as they exist in a dry
time:

[Illustration: Fig. 98.--Cracking of Clays by Drainage.]

Mr. Gisborne says: "Clay lands always shrink and crack with
drought; and the stiffer the clay, the greater the shrinking, as
brick-makers well know. In the great drought thirty-six years ago,
we saw, in a very retentive soil in the Vale of Belvoir, cracks
which it was not very pleasant to ride among. This very Summer, on
land, which, with reference to this very subject, the owner stated
to be impervious, we put a walking-stick three feet into a
sun-crack without finding a bottom, and the whole surface was a
network of cracks. In the drained soil, the roots follow the
threads of vegetable mould which have been washed into the cracks,
and get an abiding tenure. Earth-worms follow either the roots or
the mould. Permanent schisms are established in the clay, and its
whole character is changed."

In the United States, the supply of rain is far less uniform than in
England, and much severer droughts are experienced. Thus the
contraction, and consequent cracking of the soil, must be greater here
than in that country.

In laying drains more than four feet deep, in the stiffest clay which
the author has seen, in a neighborhood furnishing abundance of brick and
potter's clay, these cracks were seen to extend to the very bottoms of
the drains, not in single fissures from top to bottom, but in
innumerable seams running in all directions, so that the earth, moved
with the pick-axe, came up in little cubes and flakes, and could be
separated into pieces of an inch or less diameter. This was on a ridge
which received no water except from the clouds, having no springs in or
upon it, yet so nearly impervious to water, that it remained soft and
muddy till late