In bored wells, polluting water may enter through the uncemented joints of the tiling or through the joints in the staves of wooden tubing; in drilled or driven wells, through leaky joints or holes eaten in the iron casing by corrosive waters. By cementing the interior surface of stone-or brick-curbed wells, by replacing wood with cement or other impervious curbs and by substituting new pipes for leaky iron casings, the entrance of polluting water may be prevented. In the average home the water supply is most commonly taken from a well, the water from which comes through the earth from unknown sources, and the character of chemical salts or organic matter the water contains will depend on the kind of soil through which it passes before reaching the well. The water from wells, whether deep or shallow, is generally of relatively local origin, it being absorbed by the soil and carried to the water stratum by percolation. If the soil contains soluble mineral salts the water will contain these materials in quantities depending on the amount of the salts present in the earth. If the earth contains organic matter as pathogenic bacteria the water is likely to contain these bacteria in like numbers as they are present in the soil through which the water filters. [Illustration: FIG. 124.--Undesirable form of well curbing.] As usually encountered, the water-bearing earth occurs in sheets rather than in veins or streams. The movement of the water in such areas follows the contour of the earth and is influenced by the varying amount of rain or snowfall and the atmospheric pressure. The lateral movement is often only a few inches a day and in some places no lateral movement occurs at all. Underground streams of any kind are not usually found except in limestone regions. As a rule, a well is formed by digging or boring into the earth until a stratum of water-bearing soil is encountered, the type of the well being determined by the character of the earth and the location of the water-bearing soil. The water from the surrounding area fills the opening to the height of the saturated soil. As the water is pumped from the well it is replenished by the flow from the surrounding earth. If the soil is porous, as in the case of gravel, the water will refill the well almost as fast as it is taken away by the pump. If the soil is dense and the inward flow is slow, the well when once exhausted may be a long time in refilling.