Illustration: DRAIN (‡ DIAGRAM).
Illustration: DRAIN-PIPES AND TILES.
Drains are constructed for the purpose of carrying off from the
surface of the ground all superfluous fluid, whether it may arise
from want of a proper exit for the rain, from springs rising in the
ground to be drained, or from water coming from some higher level.
Drains are made in towns to carry off the rain from the surface, and
all liquid refuse from the houses. When the surface of any portion of
land has to be drained of its accumulated water, where the soil is of
an impervious nature, like clay, it is usual to cut deep furrows from
the higher to the lower parts and fill these up with gravel stones
or brushwood; the moisture which collects in these is made to flow
into the lowest part, and there form a pond, from which it evaporates
as fast as the drains supply it; or if the quantity be too great, a
main drain or ditch carries it onwards to some stream or river. The
furrows before being filled up with any porous substance are often
laid down with “drain-tiles;” tiles bent into the form of half a
cylinder, and placed with two edges on a flat tile at the bottom of
the drain, or pieces of tubing made of tile. The water finds its way
readily into these through the crevices. In some cases, water can be
kept from flowing over lands and forming marshes, by embankments.
These are more especially useful in land lying below the level of
rivers and lakes; immense tracts are thus preserved in Holland. When
these embankments are very extensive, there is a constant filtration
of the water through them; drains are therefore constructed to
collect it into certain spots, and mills erected to pump it over
the banks. These are usually small windmills, working a large wooden
screw in a cylindrical barrel, causing the water to rise up the
cylinder into a trough, from which it is poured over the embankment;
these mills require little or no attention, and work at all times
when the wind blows. When water accumulates in valleys lying above
the sea, deep cuttings or even tunnels may be necessary to carry off
the water; at Neufchâtel in Switzerland, very extensive tracts of
land have been reclaimed, by boring a tunnel through the surrounding
hills, and thus letting off the water from the lakes formed by the
rain.
Sometimes land may be drained by boring through the clay or other
impervious soil, down to a porous stratum, such as sand or gravel.
A knowledge of the structure of the whole district is necessary to
determine whether this is likely to be of service or not. By boring
deep pits in the drains cut in peat mosses, the weight of the mosses
will cause the water to be squeezed out, which rising in these holes
flows off by the drain, producing the same effect as if it were cut
to the depth of the pits. Drains are frequently made by means of
narrow curved spades, but the draining plough effects the purpose
more rapidly, and is very generally used.
The drainage of London and other large towns consists of small drains
from each house, made of earthenware tubes leading to a sewer of
brickwork, running along each street and uniting with larger ones.
The question of effectually draining London is a very difficult one,
schemes of various kinds being at this time propounded, which would
cost several millions of pounds to put into practice; at present all
the sewage runs into the river Thames, and it is a matter of serious
importance to obviate this, by carrying it off to some other place.