278 Ground Water, Wells and Farm Drainage. 



The capacity of a 6-inch well sunk 100 feet into sand- 

 stone having different sizes of sand grains but with uni- 

 form pore space of 32 per cent, and a temperature of 50 

 F. give computed flows under a pressure of four feet as 

 follows : 



345. Influence of Depth on the Capacity of a Well. When 

 other conditions are the same the greater the depth of a 

 well in the water-bearing beds the greater will be its capac- 

 ity because this increases the area of the section of the 

 sand or sandstone through which the water may enter the 

 well. 



If a 6-inch well is sunk just to the surface of a water- 

 bearing bed the area through which the water can enter it is 

 only 28.27 square inches. So, too, if a 6-inch well casing 

 ends in a water-bearing sand only so much water can enter 

 this well as can flow through a circle of sand 6 inches in 

 diameter. 



If the well penetrates the water-bearing bed one foot so 

 that water can enter the sides as freely as it enters the bot- 

 tom then the percolation surface will be increased to 



28.27 + 226.2 = 254.47 sq. in. 



making the section of flow nine times as great. Leaving 

 the bottom of the well out of consideration it is clear that 

 doubling the depth of the well in the water-bearing beds 

 doubles the area for water to enter and hence it is a matter 

 of the greatest importance to secure a sufficiently large per- 

 colating surface in the water-bearing beds. This capacity 

 increases in a somewhat slower ratio than the depth, as in- 

 dicated in the table below. 



