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ESSENTIAL FEATURES OF ARTESIAN WELLS.

The artesian stream has its source, its underground water way, its ascent through the well, and its final descent in the rill that runs away. It is peculiar mainly in its underground conditions. Upon these chiefly the ascending flow depends.

Typical Examples.—To fashion a simple idea of the common class of flowing wells, picture to the mind a pervious stratum through which water can readily pass. Below this let there be a water-tight bed, and let a similar one lie upon it so that it is securely embraced between impervious layers Suppose the edges of these layers to come to the surface in some elevated region (save that they may be covered with soil or loose surface material), while in the opposite direction they pitch down to considerable depths, and either come up again to the surface at some distance, thus forming a basin (Fig. 1) or else terminate in such a way (Fig.2) or take on such a nature (Fig. 3) that water can not escape in that direction. Now let surface waters penetrate the elevated edge of the porous bed, and fill it to the brim. That such beds are so filled is shown by ordinary wells, which commonly find a constant supply in them at no great depth. Now it is manifest that if such water-fat bed be tapped by boring at some point lower than its outcrop, the water will rise and flow at the surface, because of the higher head in the upper edge of the bed. If the surface water continually supplies the upper edge as fast as the water is drawn off below the flow will be constant.

Prerequisites.—The leading conditions upon which artesian flows depend are involved in this simple conception. Drawn out, they are as follows:

• I. A pervious stratum to permit the entrance and the passage of the water.
• II. A water-tight bed below to prevent the escape of the water downward.
• III. A like impervious bed above, to prevent escape upward, for the water, being under pressure from the fountain head, would otherwise find relief in that direction.
• IV. An inclination of these beds so that the edge at which the waters enter will be higher than the surface at the well.