r alone excepted, than one free from such
swamps.

_Pollution of water by animals._

Animal pollution usually comes from the presence on the watershed of
domestic animals, that is, cows, sheep, and horses, or from manure
spread on fields draining into the brook, or from barns or barnyards
close by the water. It is the presence of this sort of pollution that
furnishes the other kind of organic matter not to be distinguished by
chemical analysis from the organic matter just referred to, but vastly
more objectionable.

Drainage from houses and barns is responsible for the same kind of
animal pollution, and while it is difficult to prove by statistics that
such pollution is always dangerous to health, it is sufficiently
repulsive from an aesthetic standpoint to be done away with whenever
possible. Such pollution applies only to surface water, such as brooks
or lakes, and the best method of detecting and evaluating this pollution
is to make a careful inspection of the watershed.

If it is proposed to use the water from a certain stream for drinking
purposes, the first step should be to examine carefully the area
draining into the stream, to detect, if possible, all opportunities for
animal wastes to find their way directly into the stream and to note
whether fields sloping rapidly to the streams are manured; to see
whether the stream flows through pasture land in which cows are kept,
and especially to note whether houses with their accompanying
outbuildings are near enough the brook so that water may at any time
wash impurities down into the stream. Whenever a brook flows through
woodland free from all animal pollution and not subject to pollution
before entering the wood, the water is probably as pure as that in any
spring or well.

On the contrary, when the water in a brook flows through a meadow used
for pasture or through gullies, the sides of which are manured, or in
the vicinity of houses and barns, the water is probably unfit for
drinking purposes. This can be realized by standing at the edge of a
barnyard and watching the rain falling first on the roof of the barn,
then in larger quantities from the eaves on to the manure pile into the
yard below, then accumulating in pools of reddish black concentrated
liquid, until the volume is sufficient to form small rills which
gradually assemble into a fair-sized stream. Similarly, the pig-pen
drainage is washed out from under or even through the building, and,
after co