GROUND-WATER 33 



The rise of vapor from the ground may be proved in a very 

 simple way. If a rubber blanket be spread on the ground on a 

 summer night, or if a pan be inverted on the soil, the under side of 

 the blanket or pan will often be dripping wet in the morning, before 

 the heat of the sun affects it. Had the cool blanket or the cool 

 metal not been there to stop it, the moisture from below would have 

 escaped into the air above as water vapor. It is escaping unseen 

 all day and all night, and every day and every night, over all land 

 surfaces wherever the air in the soil and below it is more moist than 

 the air above. In this and other ways the ground-water is con- 

 stantly used up. Rainfall and snowfall, on the other hand, keep 

 renewing its supply. 



It is probable that nearly all of the water which sinks beneath 

 the surface, comes up again sooner or later, in some one of these 

 various ways; but a very small amount of it unites with the solid 

 mineral matter, as in iron rust (p. 25). 



The rate at which ground-water moves depends chiefly on (1) 

 the porosity of the rock or soil, and (2) the pressure of the water. 

 The rate at which water seeps through soils from irrigating ditches 

 in the arid lands of the West, is in most cases, from one to eight feet 

 per day; but in very porous soils, it is sometimes as much as 50 feet 

 per day. In a widespread formation of sandstone which underlies 

 southern Wisconsin and northern Illinois, the rate of movement of 

 ground-water has been estimated at half a mile a year. At this rate 

 rain-water which enters this formation 100 miles from Chicago 

 would reach that city in about 200 years. 



Springs 



All water issuing from beneath the surface is seepage. Water 

 issuing through a natural opening in such quantity as to make a 

 distinct current is a spring. Springs may occur wherever there are 

 natural passageways through which the ground-water may reach 

 the surface. Two cases are illustrated by Fig. 22. In one case the 

 water descends through a more or less porous bed of rock, c, to a 

 layer, a, which is compact. The water flows along this layer until 

 the layer comes to the surface (outcrops), and there the water flows 

 out as a spring, s. In the other case, the water moves underground 



