This water, on account of the many, many layers of soil through which it has filtered, and the long distance it has come, is usually fairly pure, so far as animal or vegetable impurities are concerned, though it is apt to have become too strong in certain salty and mineral substances, which give it a taste of salt, or iron, or sulphur.

If, however, it is free from these salty substances, it makes a very pure and wholesome drinking water; and if the upper part of the well shaft be lined with bricks and cement, so that the surface water cannot leak into it, it may be used with safety for drinking purposes even in the heart of a city.
[Illustration: THE DANGER SPOT ON THE FARM The milk inspector on visiting this dairy farm found that the well was receiving the drainage of both house and privy.

The well water was used for drinking and for washing the milk pails (seen behind the fence).] The Greatest Single Danger to Well Water.

The greatest single danger to the purity of well water is the privy vault.

This is doubly dangerous, first, because it is dug below the level at which the bacteria in the soil are most abundant and active, so that they cannot attack and break up its contents; and the impurities, therefore, are gradually washed down by the rain water into the soil, unchanged, and seep directly into the well.