great depth to which they are dug and because no overflow into the
surface layers of the soil is provided, the pores of the ground around
the cesspool become clogged and choked, and the cesspool becomes filled
with a thick, viscous, dark-colored, objectionable-looking, and
evil-smelling liquid.

The three principles which will avoid these conditions are, as already
stated, plenty of air, presence of bacteria normally found in the
surface layers of the soil, and intermittency of application.

In order to secure the operation of these three principles in the
application of sewage onto land, the sewage must be made to pass either
over the surface of the land in its natural condition in such a way that
the sewage may sink into the soil and be absorbed and at the same time
give up its manurial elements to whatever vegetation the soil produces;
or, as a modification of this principle, the sewage may be required to
pass through an artificial bed of coarse material by which the rate of
treatment may be considerably increased. In the latter case, although
probably the greater part of the action of the bacteria takes place in
the top twelve inches, it is customary to make the beds about three feet
thick, chiefly in order to prevent uneven discharge of the sewage
through the bed. Finally, wherever, for aesthetic reasons, it is
desirable that the sewage should not be in evidence, either before
passing through the natural soil or exposed in an artificial bed, the
practice may be resorted to of distributing the sewage through
agricultural tile drain laid about twelve inches below the surface. In
this way, the sewage is scattered through the top soil, where bacteria
are most active, without being apparent, and a front lawn thus treated
would not give any indication of its use.

Taking up now in order these three methods of treatment, we may consider
some of the details of construction. In spreading the sewage over the
lawn or in distributing it on the surface, due regard must be paid to
the kind of soil. Clay soils and peaty soils are useless for the
purpose of sewage disposal unless as the result of continuous
cultivation a few inches of top soil may have accumulated on the clay.
This top soil is adapted to sewage purification, provided the quantity
applied is not excessive.

_Surface application on land._

Two methods of operation may be pointed out. The sewage (and this is the
simplest method of disposal possible) may be broug