Such matter always has a marked influence on a chemical analysis of the water, shows large amounts of nitrogenous matter, and apparently indicates a polluted supply; but, if the reason for this apparent pollution lies in the presence of a swamp, no danger to health therefrom is to be apprehended. Such water also is less subject to decay or putrefaction, and if a water-supply for a house is to be taken from a small pond, a gathering ground containing swamps is likely to furnish a more satisfactory water, color 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.