WATER SUPPLY 227 
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conditions similar to that in the pipes. The block of sand- 
stone would behave as a continuous chain of piezometers. 
If there were no outlet the water would saturate the block 
to the level of the water in the tank; the water-table would 
coincide with the hydrostatic surface. If there be a free 
outlet the water-table would be the hydraulic gradient from 
the water-level in the tank to the outlet, and would vary with 
the porosity of the sandstone. If the sandstone were re- 
placed by a block composed of layers of sand and clay the 
water-table would be an irregular surface; the water would 
rise to different heights according to the permeability of 
the material; the greatest possible height would be that of 
the water-level in the tank. If part of the block were heated 
by a lamp, then the water near it might be raised above the 
hydrostatic surface by gas-pressure. 
The resistance to the underground flow of water by friction 
limits the yield of wells. If a well be over-pumped a conical 
area around it is drained of water and the well fails until 
the * cone of exhaustion ” is again filled from the surrounding 
bed. Wells on the seashore often yield fresh water, as they 
drain land water flowing to the sea; but if such a well be 
over-pumped sea-water enters the cone of exhaustion and 
the well becomes brackish. 
The cone of exhaustion was used by George Stephenson in 
the construction of the Kilsby Tunnel near Northampton. 
It was feared that the tunnel could not be made until the 
whole of a water-logged bed had been pumped dry. Stephen- 
son realized that by pumping a continuous series of cones 
of exhaustion the tunnel could be constructed although a 
sloping bank of water rose above it on both sides. 
Tre Yierp oF WELLs—WATER - StoraAGE—The water- 
level in wells depends on the local hydraulic gradient. The 
yield of a well depends on the capacity of the adjacent beds 
or rocks as regards storage, permeability, and imbibition. 
The water-storage depends on the total amount of pores in 
a material, and varies with the uniformity of the grains. 
If a box be filled with shot of uniform size the total inter- 
space between them is the same whether the shot be large 
or small. With a mixture of large and small shot, as the 
small shot occupy the spaces between the larger, the total 
interspace is reduced. Similarly in rocks: the amount of