irect outward flow. The heavy
solids sink to the bottom and anaerobic bacteria, which develop only
where there is no oxygen, breed rapidly and break these up so that
they rise to the top and provide the ever present scum which excludes
all air and stimulates fermentation of the entire content. Meanwhile,
liquid from the tank is flowing into the disposal fields, which are
porous land tile laid in shallow trenches and covered with earth and
sod. Here some air is present and aerobic bacteria (those which thrive
where there is oxygen) develop and complete the process of
transforming the wastes into clear water.

Installing such a system is neither expensive nor complicated. The
tank itself should be large enough to hold the sewage of a household
for twenty-four hours. It can be bought ready to install, or built of
brick or concrete. Ready-made tanks are to be had of steel, concrete,
or vitrified tile. We installed one of steel (which is the cheapest)
some ten years ago and have found it most satisfactory. When it was
delivered, two husky truck-men placed it at the edge of the pit
prepared for it by the waiting plumber. They exhibited some curiosity
and the plumber explained briefly about the bacteria and its action.

"You mean one of these here bugs is into it already?" asked one of
them as he applied an awe-struck eye to the aperture in the top. He
apparently expected to find an insect akin to a full-size cockroach
running around inside, and either decided the light was poor or that
the plumber was a first-class liar, for he went off shaking his head
doubtfully.

The size of tank and length of disposal field is entirely a matter of
size of household. On an average, the daily volume can be reckoned on
the basis of fifty gallons per person and, for every fifty gallons of
tank capacity, there should be thirty feet of disposal field. Thus,
for a family of eight, a tank of five hundred gallons' capacity
connected with a disposal field of three hundred feet will be ample,
allowing for guests as well.

In installing this system, the tank itself can be as near the house as
ten or fifteen feet but the piping connecting it with the soil line of
the plumbing should be water tight. The best way is to use four-inch
cast iron pipe, calking all joints with oakum and lead. At a
convenient point between house and tank, this line of pipe should have
a "clean-out" fitting so that rags, solidified grease, or other
substances that might b