hich may be best illustrated in the case
of cows which feed on the herbage of a meadow, the manure from the cows
furnishing food for the grass which otherwise would soon exhaust the
nutriment of the soil.

_Stream pollution._

The first fundamental principle of sewage disposal, therefore, is to
distribute the organic matter in the sewage so that these beneficent
bacteria may most rapidly and thoroughly accomplish their purpose.
During the last fifty years, a great deal of study has been expended on
this problem, and while it has not as yet been entirely solved, certain
essential features have been well established.

The most important factor promoting the activity of these agents of
decay is the presence of air, since in many ways it has been proved that
without air their action is impossible. Thus it has been shown that
discharging sewage into a stream, whether the stream be a slow and
sluggish one or whether it be a mountain stream churned into foam by
repeated waterfalls, has little other power to act on organic matter
than to hold it for transportation down stream, or to allow it to settle
in slower reaches until mud banks have been accumulated which will be
washed out again at the first freshet. Experiments have shown that the
agencies to which certain diseases are attributed, commonly known as
pathogenic bacteria, are frequently, if not always, found in sewage, and
that when these bacteria are discharged into streams they may be carried
with the stream hundreds of miles and retain all their power for evil,
in case the water is used for drinking purposes. No right-minded person
to-day will so abuse the rights of his fellow-citizens as deliberately
to pour into a stream such unmistakable poison as sewage has proved
itself to be. The fact is so well known that it is not worth while
pointing out examples. It is enough to say that some of the worst
epidemics of typhoid fever which this country has known have been traced
to the agency of drinking water, polluted miles away by a relatively
small amount of sewage.

In a number of states, laws have been passed which expressly prohibit
the discharge of sewage, even from a single house, into a stream of any
sort, even though the stream is on the land of the man thus discharging
sewage and where it would appear as if he alone might control the uses
of that stream. Unfortunately, the machinery of the law does not always
operate to detect and punish the breakers of the law,