WATER SUPPLY 231 
and non-porous, or the increase in temperature, or some 
obstacle such as a fault or a dyke. If a well be sunk through 
the overlying impermeable layer, the water will rise in it to 
a height determined by the pressure-head. If the mouth of 
the well is lower than the water-charged part of the permeable 
rock, the water will discharge as a flowing well. If the head 
of the water does not force it to the top of the well it can be 
obtained by pumping or baling, or, as in the air-lift pump, 
by the injection of compressed air, the expansion of which 
lifts the water to the surface. 
One well-known variety of flowing well occurs. where a 
permeable bed is bent into a trough-like fold or syncline 
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F16. 59.—CIRCULATION OF WATER 
Circulation of water in a porous limestone, L, between two impermeable 
beds. U.S., Upper shale, and L.S., Lower shale; D, Dyke; W.T,, 
Water-table if D were absent and the limestone had a discharge at 
lower end. Hs.S., Hydrostatic Surface, owing to blockage of flow of 
water by the dyke, D; Hl, Level of Hydraulic Surface if dyke were 
absent, In the 5 wells the solid part indicates the water-level as it 
would stand in wells 1-4, if the dyke were absent. No. 4 would be a 
Jowing well; with the dyke present water in that well would rise in 
a pipe to Hs.S. No. 5 would be dry owing to the dyke. 
between two impermeable beds; when the deeper part of 
the water-bearing layer is reached by a bore the water 
overflows at the surface owing to the pressure of the water 
in the upper part of the * U-shaped sheet of permeable 
rock. Such wells were called artesian from those at Artois 
in Flanders. The term has been used so widely that it has 
lost its meaning. It is used in America for all deep wells. 
In the British well sinking industry it is used for bored 
wells in distinction to dug wells. Flowing wells due to the 
pressure of water at a higher level in a water-bearing bed in a 
synclinal are widespread, as in the London and Paris basins. 
Flowing wells due to simple water pressure also occur where 
the beds are inclined in one direction, as along the coast of