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Rh in France, which are sunk through an impermeable stratum down into a water-bearing stratum which overlies an impermeable stratum. Obviously ordinary wells can supply water very cheaply, but, since impurities readily reach them, there is great risk of contamination. The same does not apply to deep wells, such water being usually free from organic impurities. In ordinary wells, and in deep wells, the water requires pumping to the surface; in artesian wells, on the other hand, the water usually spouts up to a greater or less height above it.

The mechanical appliances employed in boring for water are practically the same as in boring for petroleum (q.v.). The upper part of a deep well may be of brick, the continuation being lined with steel pipes, or, better, it may be lined with metal for its entire length.