From the sanitary standpoint, wells are of very great significance, especially in view of the fact that such a large proportion of people depend upon well water. It is generally understood that typhoid fever is more common in the country than in cities, in spite of what might reasonably be expected. What are some of the causes leading up to such a situation? The idea that water purifies itself after flowing a relatively short distance is, in many cases, far from being true, especially when we are dealing with underground water. Actual observations prove that germ-laden water may travel surprisingly far underground. Germ-laden water from barns, cesspools, or outhouses spreads notably on sinking to the water table and it is easy to see how so many wells become contaminated. On general principles, a geologist is especially wary of water from a well in a barn yard. The well for human use at least should be located out of reasonable range of such contamination. Under the condition of the diagram a well or spring some distance down the side of the hill may actually be unfit for use, though a serious situation is much less likely to develop there. Nor should one assume that by locating the well on the uphill side of the house, and the outhouses or cesspool on the downhill side, safety is assured. From what we have learned in regard to earth movements, and the tilting of strata from their original positions, we know how the movement of water in the saturated zone near the surface may be downhill roughly following the hill slope, while in a tilted porous layer of rock farther below the surface the movement of water may be in just the opposite direction. A well drilled into the solid rock for safety on the uphill side of a house might derive its water from this very same porous layer, whose water has been contaminated from a cesspool or other source down the side of the hill. Such a case is by no means a theory or a rarity. There is also real danger of contamination in cases where the water flows more like streams underground through cracks or fissures in hard or dense rocks, or through channels developed by solution in limestone. It may happen that water becoming contaminated from barn sites, cesspools, or outhouses finds its way along such a channel to the side or bottom of a well. The author well remembers the case of a farmer whose house, barn, and well were close together on a little limestone terrace and who continued to use the well water although he complained of its disagreeable taste, especially after a rain when he could "taste the barn in it." [Illustration: Fig. 22.--Diagram illustrating a danger of contamination of wells by impure underground water. S, soil; B, bedrock; P, porous stratum. Impure water from cesspool moves through porous layer to bottom of well. (By the author.)]