Encyclopaedia Britannica > Volume 8, DIA-England

136
DRAINING.
Draining. To drain land means simply to make land dry. This
definition is satisfactory in regard to the ancient art of
draining, the object of which was to render land which was
wetted by springs, or had become marshy by stagnant
water, or subjected to the periodical overflowings of rivers,
sufficiently dry to permit the exercise of arable culture.
It is not so applicable to the modern practice of drain-
ing. Draining is now practised on land that is appa¬
rently dry, as well as on really wet land. Land that con¬
tains a superabundance of water produces plants of a
peculiar nature, such as rushes, flags, sprots, and many
others; and obstructs the operations of arable culture.
The very existence of such plants is sufficiently indica¬
tive of the wet state of land. But land that contains
no such superabundance of water as to obstruct its arable
culture, but which, by its inherent wetness, may never¬
theless prevent the luxuriant growth of the cereal grains
and the cultivated grasses, requires draining as much as
wet land, though wetness is not so apparent in it as in
the other. The deficiency of the crop on apparently dry
lands is naturally attributed to unskilful husbandry, till
experience proves that no management, however skilful,
can counteract the baneful influence of concealed stag¬
nant water. Observation teaches us that stagnant water,
whether on the surface or immediately under the surface
of land, injures all the useful classes of plants. How the
injury arises is not very clear; perhaps by obstructing
perspiration and intro-susception, and thus diseasing their
roots; or, what is more probable, it prevents their food
from being presented in a nutritious state, by checking
the chemical decomposition of the substances which supply
the food of plants. Whether the true cause of the bad
effects of stagnant water on the vegetation of useful plants
will ever be discovered, the future progress of vegeto-phy-
siological science can alone determine. In the mean time,
experience assures us that draining will remove these
bad effects.
It thus appears there are two species of draining; the
one draws oft' the larger bodies of water which are collect¬
ed from the discharge of springs in isolated spots, and the
other absorbs the superabundance of water from and un¬
der the surface of the land. The first is called under-
draining, because it intercepts the passage of the water
from springs at some distance under ground. The second
makes channels for conveying away the water which falls
on the surface of the ground, and is therefore called sur¬
face-draining. Surface-draining is again divided into two
kinds; the one consists only in making small open channels
and furrows immediately on the surface of the ground;
the other is effected by means of small drains constructed
at a short distance under the surface of the ground, in
order to collect the water which would otherwise remain
on a retentive subsoil at the bottom of the plough furrow.
This latter kind derives its name from the particular con¬
struction of the drain.
The theory on which these different kinds of draining
are founded is sufficiently explicit. WRere the upper sur¬
face of land is at all permeable to water, and where it
rests on beds of matter of different depths, of various
lengths and breadths, each possessing a consistency of re¬
tentiveness or permeability, the water produced from rain,
snow, or dew, in its progress along the porous bed, will
be interrupted and retained by the retentive beds, and
will accumulate in them in larger or smaller quantities,
according as they present a basin shape, over the edge of
which it will burst through the upper surface in the form
of springs. Hence the necessity of under-draining to
draw off the wmter contained in the basin-shaped curves
of the retentive layers. Where the upper surface is per¬
meable, and the subsoil immediately under it retentive,
water will accumulate on the subsoil, to the injury of
plants growing on the surface soil. Hence the origin of
surface-draining, by laying land in ridges, and forming
small open channels, along which the water percolating
through the surface soil may have a free passage. Where
the subsoil is so porous as to permit the water to perco¬
late through it beyond the reach of the plough, these open
surface gutters will have no effect in removing the super¬
abundant water. Hence the second class of surface drains
must then be constructed to absorb it. Where the sur¬
face soil and the subsoil are both permeable, they will
hold water only by capillary attraction; and what is not
so retained will sink down into the inferior permeable
beds by its gravity. Hence in such land draining is un¬
necessary. Of these excessive supplies of water, that
from springs, is most injurious to the vegetation of useful
plants, as being colder, and generally more permanent in
their operation. Capillary attraction will retain as much
water in the surface soil as vegetation requires, excepting
under the extraordinary occurrence of excessive drought.
Hence the abstraction of water by draining is quite inde¬
pendent of its supply as a manure, as a meliorator of the
soil, as a menstruum for food, or as a regulator of tempera¬
ture to plants.
These statements prove the necessity of a knowledge of
geology for a perfect understanding of the principles of
draining; and fortunately practice in this department of
rural economy is not at variance with scientific principle.
On the contrary, he who is best acquainted with geology
and the principles of hydraulics, will constitute the most
scientific drainer. But such a character cannot yet be
easily acquired by any man; for that department of geo-
logy> the knowledge of which is most applicable to the art
of draining, is the most difficult to be understood, and has
hitherto been the least explored. Most of the arable soil
is contained within the tertiary and alluvial formations of
rocks. 1 he intricate relations of the newer beds, which
compose the alluvial class of rocks, and which are the most
intimately connected with arable culture and draining,
present almost insurmountable obstacles to the thorough
acquirement of the art of draining. They at least throw
an uncertainty over its operations; and this uncertainty
must continue till the relations of the alluvial rocks are
discovered to be as fixed as those of the other classes.
Perhaps this certainty is unattainable; because it maybe
that the newer members of the alluvia strata do not bear
a strictly relative position to one another. But till this
fact is ascertained one way or the other, draining must be
conducted in a great measure in these alluvial deposites by
trial. In undertakings on trial, error generally ensues, and
unnecessary expense is often incurred. One very unfor¬
tunate circumstance, arising from the uncertainty attend¬
ing draining, is the uselessness of the experience acquired
in one set ot operations, in guiding to a means of securing
a more certain result in another. As uncertainty attends
on the number and depth, and even direction, of the drains
which are required to dry one field sufficiently, so a similar
uncertainty prevails over the similar operations in the ad¬
joining field. Every drainer will concur in the justness
of these remarks. A deeper study by geologists in this