of the ground is to
lead it in channels between narrow beds on which vegetables have grown.
These beds are made about eight feet wide with two rows of root crops,
such as turnips or beets, set back about two feet from the edge. The
beds are made by properly plowing, the channels between the beds being
back-furrowed. Here, again, the principle of intermittent application is
essential, and the area to be provided is the same as already given for
the surface irrigation. Three beds should be provided, as before; but,
in general, no provision need be made for carrying off the sewage at the
lower end of the beds, since it may be safely assumed that all of the
sewage will be absorbed by the soil. Of course, a sandy soil will
absorb more water than a clay soil, and if the soil is entirely clay, it
is not suitable for such treatment. Sewage passed over the surface of
clay soil, however, will, in the course of a few months, so modify the
clay as to convert it into a loam, and in this way increase its
absorptive power.

When possible, it is desirable to have a plot of plowed ground over
which the sewage may pass before reaching the beds, so that the grosser
impurities may be left behind and harrowed in or plowed under. If proper
regard is paid to intermittent application, no danger from odors need be
feared, and the repeated plowing in will increase immensely the
fertility of the soil. Nor need one be afraid that all of the manurial
elements will be left behind on this plowed ground. About two thirds of
the organic matter in sewage is in solution, and this will be carried
onto the beds just as if passage over the plowed ground had not
occurred.

_Artificial sewage beds._

In order to secure a higher rate of discharge of sewage through the soil
it is best to arrange an artificial bed which shall be made of coarse,
sandy material which will allow a rate of at least 10 times that already
given. The best material out of which to make such an artificial bed is
a coarse sand; that is, a sand whose particles will not pass through a
sieve which has 60 meshes to the inch and which would pass through a
sieve of 10 meshes to the inch. Such an ideal sand will purify sewage at
the rate of 50,000 gallons per acre per day, or an acre will take care
of the sewage of at least 1000 persons. This means that it is necessary
to provide about 50 square feet for each person in the family, or a
family of 10 persons could have all the sewage taken car