ain, and the
period always much limited in which they can be prosecuted at all; the
compactness and toughness of the soil renders each operation more
arduous, and its repetition more necessary than in the case of dry land.
The surface must necessarily be thrown into ridges, and the furrows and
cross-cuts cleared out after each process of tillage, and upon this
surface-drainage as much labour is expended in twenty years as would
suffice to make under-drains enough to lay it permanently dry. With all
these precautions the best seed time is often missed, and this usually
proves the prelude to a scanty crop, or to a late and disastrous
harvest. The cultivation of the turnip and other root crops, which
require the soil to be wrought to a deep and free tilth, either becomes
altogether impracticable and must be abandoned for the safe but costly
bare fallow, or is carried out with great labour and hazard; and the
crop, when grown, can neither be removed from the ground, nor consumed
upon it by sheep without damage by "poaching."

The roots of plants require both air and warmth. A deep stratum through
which water can percolate, but in which it can never stagnate, is
therefore necessary. A waterlogged soil is impenetrable by air, and
owing to the continuous process of evaporation and radiation, its
temperature is much below that of drained soil. The surface of the water
in the supersaturated soil is known as the "water-table" and is
exemplified in water standing in a well. Water will rise in clay by
capillarity to a height of 50 in., in sand to 22 in. Above the
"water-table" the water is held by capillarity, and the percentage of
water held decreases as we approach the surface where there may be
perfect dryness. Draining reduces the "surface tension" of the capillary
water by removal of the excess, but the "water-table" may be many feet
below. Drains ordinarily remove only excess of capillary water, an
excess of percolating water in wet weather.

In setting about the draining of a field, or farm, or estate, the first
point is to secure a proper outfall. The lines of the receiving drains
must next be determined, and then the direction of the parallel drains.
The former must occupy the lowest part of the natural hollows, and the
latter must run in the line of the greatest slope of the ground. In the
case of flat land, where a fall is obtained chiefly by increasing the
depth of the drains at their lower ends, these lines may be dis