240 



The Realm of Nature 



CHAP. 



water is termed a spring, and its origin is indicated at s 

 (Fig. 49). If a pit is dug through the upper rock, as at 

 W, deep enough to pass below the limit of saturation /, 



FIG. 49. The origin of springs. (After Prestwich.) The darker shading repre- 

 sents rocks impervious to water, the light shading shows permeable rocks. 

 W, a surface well ; the curves on the shaded part show different positions 

 of the limit of saturation ; s's, springs ;f, fault. 



water will ooze in from all sides, and a surface well will be 

 formed from which water may be lifted by a bucket or 

 pump. The limit of saturation rises in wet weather, but 

 sinks in a dry season. When it rises from / to /' the water 

 in the well deepens, when it sinks to the lowest curve 

 shown, the well becomes dry, and if the height is not 

 sufficient to overcome the resistance of capillarity the 

 springs also cease to flow. When layers of permeable 

 and impermeable rocks occur one above another, the 

 water which soaks into the permeable rocks at the surface 

 filters down along the junction with the impermeable 

 layer, and if a fissure or fault occurs (/ in the figure) 

 so that the permeable layer is brought against an im- 

 permeable wall, the water 

 will be forced up along the 

 crack and will reach the 

 surface as a fault-spring if 

 the ground -level is below 

 that of the limit of satura- 

 tion. Artificial bores driven 



FIG. 5 o.-Artesian wells, pp, permeable through an impermeable 

 rocks ; L, /, limits of saturation, show- stratum of rock to reach the 



ing level beyond which water from the , . , , 



bores aaa cannot rise. water-bearing strata below 



are termed Artesian wells, 



from the old name of part of the north of France where 

 they were largely used. By this means a copious water- 



