very
strikingly how poor a manurial substance sewage is. Various methods have
been devised and experimented with for extracting these manurial
ingredients, and many methods are in operation in different parts of the
world. The methods of utilising sewage for agricultural purposes may be
broadly divided into two classes.

_Irrigation._

One of these, which may be classed under the heading of irrigation,
consists in pouring the sewage on to certain kinds of coarse green
crops. Sometimes the land is made to filter large quantities of sewage
by special arrangements of drains and ditches. The land is first
carefully and evenly graded down a gentle incline. At the top of the
field the sewage is conducted along an open ditch from which it is
permitted to escape, by the force of gravity, by several smaller
ditches running at right angles from the main ditch. By means of stops
which may be shifted at will, the sewage can be directed to flow over
different parts of the field. Modifications in this plan may be made so
as to suit the nature of the ground. In the case, for example, of a
steep incline, the field may be sewaged by means of what are known as
"catch-work" trenches running horizontally along the hill. In this way
the sewage is allowed to pass over the whole of the field, and is caught
at the bottom in a deep ditch, whence it is allowed to flow into the
nearest river or stream. This is the system which has been employed at
the famous Beddington Meadows, near Croydon.

Another method of distributing the sewage is by means of underground
pipes, which are laid in a sort of network over the ground to be
manured. At certain intervals pipes with couplings for hose are fitted
on, and by keeping a certain amount of pressure on the main pipes the
sewage may be distributed over the different parts of the field as it is
required.

A third modification is subsoil irrigation. This resembles the
last-named system, with this difference, that the pipes used are either
porous or perforated with small holes.

Total submersion can only be applied in the case of absolutely level
lands, and is practised to an enormous extent in Piedmont and Lombardy.

There has been little dispute as to the thorough efficiency of
irrigation--when conducted under favourable conditions--as a method of
purifying sewage and utilising to the full its constituents of manurial
value. It is the only method which has been conclusively shown to
extract from