DE RANCE: UNDEKGRoUND water-supply and river FL(X)DS. 211 
exhaustion" has to be increased, its vertical height increases by 
lowering the pumps to a lower level, and a larger concentric circle is 
added to the central one at the surface. At each successive lowering 
of the pumps, a larger concentric circle of contribution is added to 
the original area of abstraction. Wells of this class are not artesian, 
and the water in them does not rise at pressure. In porous rocks of 
great thickness, the plane of saturation is often at a considerable 
depth from tlie surface, the ainiual rainfall absorbed being balanced 
by the springs running off at the lowest level, but little being collected 
from the floods, the water passing over the outcrop of the porous 
strata too (quickly to sink into the strata. In these cases it would be 
possible to raise the height of the saturation level, increase the storage 
powers of the rock, and the yield of the springs by sinking " dumb- 
wells" into the porous strata, and draining the storm-water channels 
into them, in this manner the floods of wet periods may be collected, 
and distributed as springs during periods of drought, and the " dry- 
weather flow " of the streams increased. 
When Sandstone or (jrit is examined under the microscope, spaces 
are seen to exist between the grains of sand forming the rock. The 
size and extent of these spaces limit the capacity of the rock for ivater 
starage, water being stored in the interspaces just as water is stored 
in a tank full of shingle, into which water has been poured. The 
amount of water that can be held by a rock varies with the proportion 
of its cubic contents, that is made up of interspaces ; a cubic foot of 
sand or chalk will hold 2 gallons of water, a cubic foot of Bath Oolite 
a gallon-and-a-half, while a cubic foot of granite 0"18o, or a pint- 
and-a-half 
One inch of rainfall equals 1 gallon, tilling over 2 square feet, or 
22,427 gallons per acre, or 14,355,280 gallons per square mile. Such 
a rainfall spread over a year of 365 days, gives a daily average of 62 
gallons per day per acre, or 40,000 gallons per day per square mile. 
The amount that can be absorbed by the porous rocks varies from 
5 to 15 inches, and may be said to average 10 inches in sandstone, 
giving 400,000 gallons per day per square mile. This figure is a 
useful one in estimating the probable yield of a well. 
In limestone the quantity of water held by a given mass of rock 
also varies with the size of the infinitely fine gTains making it up, the 
