ht to the upper edge
of a small piece of ground, usually sowed to grass, and allowed merely
to run out over the surface of the ground. There should be, however,
some method of alternating plots of ground, one with another, so that
the sewage is turned from one to the other every day. Each plot will
then have one day's application of sewage and one day's rest, and this
would complete the disposal, were it not for the interference of rain
and cold. The winter season practically puts a stop to this method of
treatment, and rainy weather reduces the power of the soil to absorb
sewage. For these two reasons, it is desirable to have one plot in
reserve, or three in all, and the area of each plot should be based on
the amount of sewage contributed. For a family of ten persons using
twenty-five gallons of water per day the total area provided should be
one tenth of one acre, or an area seventy feet square divided into three
plots. Figure 67 shows six beds arranged to care for the sewage of a
public institution in Massachusetts. As a guide to the amount of land
needed, it will be safe to provide at the rate of one acre for each
forty persons where the soil is a well-worked loam but underlaid with
clay. The effect of this irrigation on the grass will be to induce a
heavy, rank growth which must be kept down by repeated cutting or by
constant grazing. Both methods are practiced in England, and it may be
said in passing that no injury to stock from the feeding of such
sewage-grown grass has been recorded. The grass cut from such areas (and
the cutting is done every two weeks through the whole summer) is packed
into silos and fed to cattle through the winter with advantage. Or, if
grazing is resorted to to keep the grass down, the herd is alternated
with the sewage from one field to the other, so that the bed which has
received sewage one week is used for pasture the next week, and the
number of head which can thus be fed is astonishing. In order to secure
an even flow of sewage over such grass land as is here contemplated,
there must be a gentle slope to the field, and the ditch or drain
bringing the sewage to the field should run along its upper side.
Openings from the drain, controlled by simple stop planks, are provided
at intervals of about ten feet, and no attention is needed further than
the opening and closing of these admission gates.

[Illustration: FIG. 67.--Sewage beds.]

Another method of applying sewage to the surface