58 Water and Land 



lated in the soil. This store of ground water amounts 

 according to recent estimates to some 25% of the bulk 

 of the first one hundred feet in soil depth. Thus it 

 equals a reservoir of water some 25 feet deep covering 

 the whole humid eastern United States. It is con- 

 tinuous over the entire country. Its fluctuations are 

 studied by means of measurements of wells, especially 

 by recording the depth of the so-called "water table." 

 On the maintenance of ground water stream-flow and 

 organic productiveness of the fields alike depend. 



