posed in
any direction that is found convenient; but in undulating ground a
single field may require several distinct sets of drains lying at
different angles, so as to suit its several slopes. When a field is
ridged in the line of the greatest ascent of the ground, there is an
obvious convenience in adopting the furrows as the site of the drains;
but wherever this is not the case the drains must be laid off to suit
the contour of the ground, irrespective of the furrows altogether. When
parts of a field are flat, and other parts have a considerable
acclivity, it is expedient to cut a receiving drain near to the bottom
of the slopes, and to give the flat ground an independent set of drains.
In laying off receiving drains it is essential to give hedgerows and
trees a good offing, lest the conduit be obstructed by the roots.

When a main drain is so placed that parallel ones empty into it from
both sides, care should be taken that the inlets of the latter are not
made exactly opposite to each other. Much of the success of draining
depends on the skilful planning of these main drains, and in making them
large enough to discharge the greatest flow of water to which they may
be exposed. Very long main drains are to be avoided. Numerous outlets
are also objectionable, from their liability to obstruction. An outlet
to an area of from 10 to 15 acres is a good arrangement. These outlets
should be faced with mason work, and guarded with iron gratings.

The distance and depth apart of the parallel drains is determined
chiefly by reference to the texture of the soil. In an impervious clay
the flow of the water is much impeded and the water-table can be
controlled only by frequent lines of pipes. On such land it is customary
to lay them about 3 ft. from the surface and from 15 to 21 ft. apart. In
lighter soils the depth, and proportionately the distance apart, is
increased, but the drains are rarely more than 4 ft. 6 in. below the
surface, though they may be 75 or 100 apart. A fall of at least 1 in 200
is desirable.

There are various forms of under-drainage, some of them alluded to in
the historical section below, but by far the commonest is by means of
cylindrical or oval pipes of burnt clay about 1 ft. in length, sometimes
supplemented by collars, though nowadays the use of these is being
abandoned. Pipes vary in bore from 2 in. for the parallel to 6 in. for
the main drains.

In constructing a drain, it is of importance that t