of gravity upon underground water may locally tend toward
a state of equilibrium in which there is little movement. In such a case
the water is substantially ponded, and moves only when tapped by
artificial openings.

WELLS AND SPRINGS

Underground water becomes available for use by means of springs and
through wells or bore holes. Water rises to the surface in natural
springs at points where the pressure or _head_, due to its entrance into
the ground at a higher level, is sufficient to force it to the surface
after a longer or shorter underground course. The movement may be all
downward and lateral to the point of escape, or it may be downward,
lateral, and upward. Ordinarily, the course of spring waters does not
carry them far below the surface. Heat and gases may be added beneath
the surface by contact with or contributions from cooling igneous rocks.
These may accelerate the upward movement of spring waters, and yield
thermal and gas-charged waters, as in the springs and geysers of
Yellowstone Park.

When a well is sunk to tap the underground water supply, the water may
not rise in the artificial opening but may have to be lifted to the
surface.

If, however, the water is confined beneath an impervious stratum and is
under pressure from the water of higher areas, a well opening may simply
allow it to move upward under its own pressure or head. This pressure
may carry it upward only a few feet or quite to the surface or beyond,
in which latter case the well is called an _artesian_ well. The
essential condition for an artesian circulation is a porous zone,
inclining downward from the surface beneath an impervious stratum which
tends to confine and pond the water. The water at any point in the
water-bearing rock is under pressure which is more or less equivalent to
the weight of the column of water determined by the difference in height
between this point and the point of entrance or feeding area of the
water. If the feeding area is higher than the collar of the well, the
water will rise quite to the surface; if not, it will rise only part
way. Capillary resistance, however, may and usually does lessen the
theoretical pressure so figured.

The flow in deep artesian circulations is ordinarily a slow one. For the
artesian wells of southern Wisconsin, it has been calculated that waters
entering the outcrop of the southward dipping sandstone and limestone
layers in the northern part of the state have required two o