30 PHYSIOGRAPHY 



sinks into the ground becomes ground-water, and that which flows 

 off over the surface, without sinking, is the immediate run-off. Much 

 of the ground-water comes to the surface again, as in springs, and 

 joins the immediate run-off in the streams. 



GROUND-WATER 



Its existence. The abundance of water in the ground is well 

 known in many ways. (1) In farming regions, there are wells on 

 almost every farm. Illinois has more than 250,000 farms, and the 

 number of wells in the state is probably double the number of farms. 

 The number of wells in the United States must be several millions, 

 and the amount of water drawn out through them each day is very 

 great; yet the wells rarely go dry. (2) In deep mines huge pumps 

 are often kept at work all the time, in order to keep the mines dry 

 enough for men to work in. (3) Springs are common in many 

 regions, and their water comes from beneath the surface. These 

 facts show that the amount of ground-water is large. 



Its source. Rain-water and snow-water are continually sinking 

 beneath the surface, and as we know of no other source whence it 

 might come, it is thought that the water in the ground is rain and 

 melted snow which has sunk beneath the surface. The connection 

 between rain and ground-water is shown by the fact that some wells 

 and springs go dry in times of drought, but have water again when 

 the drought is broken by renewed rainfall. 



Descent of ground-water. Rain-water sinks into the soil and 

 rock through pores and cracks. The rocks near the surface have 

 more and larger pores and cracks than those at greater depths. 

 Pores and cracks become very small at the depth of a few thousand 

 feet, and it is probable that none exist below a depth of five or six 

 miles. If this is true, water does not descend more than five or 

 six miles. 



The ground-water surface. If a series of wells were dug in a flat 

 region where the soil and the rocks below are everywhere the same, 

 the wells would have to be dug to about the same depth in order to 

 secure a constant supply of water. This is illustrated by Fig. 20. 

 If the well at a is dug to a given depth, a well at b will need to be 



