e of on an area
25 feet square. The same principle of intermittency of application,
however, must be observed by dividing the bed into three parts, so that
the sewage may be alternated from one bed to another. Practice has
indicated that it is better to shift from bed to bed about once a week
and to deliver the sewage onto each bed intermittently; that is, to
discharge a bucketful at a time with short intervals between, rather
than to allow a small stream to flow continuously onto a bed. Such a bed
should be about 3 feet deep, as already stated, and preferably should
have light concrete side walls and bottom, as shown in the sketch (Fig.
68). Ordinarily, the surface of the sand will be level, and the dose of
sewage applied to the bed will cover it a fraction of an inch deep, and
in the course of an hour or so will disappear into the sand and reappear
in the underdrains as clear water.

[Illustration: FIG. 68.--Sewage beds.]

In cold weather a thin sheet of sewage spread out over the surface of
the sand would freeze before penetrating the bed; therefore, in the
winter time, it is usual to furrow the beds; that is, dig furrows across
the beds 2 or 3 inches wide at the bottom and about 10 inches deep, so
that in the bottom of these furrows the sewage may be, partly at least,
protected against frost. It has been found that, if sewage is discharged
intermittently,--that is, in bucketfuls into such furrows,--the beds
open and allow the filtration of the sewage. To be sure, the
purification effected in cold weather is not quite that accomplished in
warm weather, but the results are sufficiently satisfactory, and no
nuisance ensues.

_Subsurface tile disposal._

The other method of distributing sewage over land is by means of
draintile placed in shallow trenches, so that the sewage may leach out
into the soil through the open joints of the pipe. These draintiles
receive the sewage intermittently, and by the constant rush of water are
presumably filled throughout their length. The sewage then gradually
works out of the joints into the surrounding soil, and the pipes are
empty and ready to receive another dose when next delivered.

Two essential points must be considered in the successful operation of
such a plant: the grade of the tile and the length of the tile.

The grade of the tile must be properly adjusted to the porosity of the
soil; that is, in open, porous, and gravelly soils a grade must be
steeper than in loamy