water from the river percolating through the sand, or by ground
water on its way to the stream and possibly contaminated with the
sewage of the town?

At what height does underground water stand in the wells of your
region? Does it vary with the season? Have you ever known wells to
go dry? It may be possible to get data from different wells and to
draw a diagram showing the ground-water surface as compared with
the surface of the ground.

FISSURE SPRINGS AND ARTESIAN WELLS. The DEEPER ZONES OF FLOW lie in
pervious strata which are overlain by some impervious stratum.
Such layers are often carried by their dip to great depths, and
water may circulate in them to far below the level of the surface
streams and even of the sea. When a fissure crosses a water-
bearing stratum, or AQUIFIER, water is forced upward by the
pressure of the weight of the water contained in the higher parts
of the stratum, and may reach the surface as a fissure spring. A
boring which taps such an aquifer is known as an artesian well, a
name derived from a province in France where wells of this kind
have been long in use. The rise of the water in artesian wells,
and in fissure springs also, depends on the following conditions
illustrated in Figure 29. The aquifer dips toward the region of
the wells from higher ground, where it outcrops and receives its
water. It is inclosed between an impervious layer above and water-
tight or water-logged layers beneath. The weight of the column of
water thus inclosed in the aquifer causes water to rise in the
well, precisely as the weight of the water in a standpipe forces
it in connected pipes to the upper stories of buildings.