SCIENCE OF COMMON THINGS. 51 



Origin of springs. How water collects in wells. 



sides and bottom the velocity is diminished by the 

 friction of the water against the banks, bars, etc. 



S99 Wliat is the origin of springs 1 



The water falling upon the earth sinks downwards 

 through the sand and porous materials, until an imper- 

 vious bed of clay or rock is reached. Here the water 

 accumulates, and finally bursts out at some point where 

 the impervious bed or strata comes to the surface in 

 consequence of a valley or excavation. 



Suppose a (Fig. 16) to 

 be a gravel hill, and b 

 a strata of clay or rock, 

 impervious to water. 

 The fluid percolating 

 through the gravel 

 would reach the imper- 

 ~~ " vious strata, along 



which it would run un- 

 til it found an outlet at c at the foot of the hill, where a spring would be 

 formed. 



SOO Why does not the water ooze out everywhere along the line of June* 

 tion of the two formations the gravel and the rock or clay so as to form 

 one continuous land soak, instead of a few springs only, and these far distant 

 from one another ? 



For two reasons : first, on account of rents and fis- 

 sures in the layers of rock, which act as natural drains 

 secondly, the existence of inequalities in the surface 

 of the impermeable stratum, which lead the water, as 

 valleys do on the external surface of a country, into 

 certain low levels and channels. 



3O1 Why does the water coUect in an ordinary well f 



An ordinary well consists of an excavation continued 

 until a stratum or layer of clay or rock is reached that 

 is permanently saturated with water. They are not 

 commonly supplied by springs, but merely by the 

 draining of the , water which exists within the circuit 

 of a few yards into a cavity. 



SOS Why do wells and springs fail oftentimes in dry weather f 



Because they are supplied by the water fallina as 

 rain, which percolates from the surface of the earth. 



