52 SCIENCE OF COMMON THINGS. 



Artesian wells. 

 SO3 What is an Artesian weU? 



Water is sometimes obtained by Coring into the 

 earth with a species of auger, until a vein or sheet of 

 water is found, which rises to the surface through the 

 cylindrical excavation. Such excavations are called 

 Artesian wells, because the method was first invented 

 and employed at Artois, in France. 



/(; 3O4 How do you account for the water rising to the surface in Arte- 

 sian, and sometimes in ordinary wells ? 



Strata which are pervious frequently alternate with 

 others which are not so ; or may form a basin, the 

 area of which is partially filled with clay, through 

 which water cannot pass ; in such a case it is obvious 

 that the bed of sand beneath the clay, fed by the rain 

 which descends on the uncovered margin of the basin, 

 must form a reservoir where the water will gradually 

 accumulate beneath the central layer of clay, through 

 which it cannot escape. If the bed of clay be pene- 

 trated by natural or artificial means, the water must 

 necessarily rise to the surface, and may even be thrown 

 up in a jet to an altitude which will depend on the 

 level of the fluid in the subterranean reservoir. 



Fig. 17. 



Thus, if a sandy stratum, a a (Fig. 17), acting as a filter, occupies an 

 inclined position between two other strata impervious to water, such 

 as clay, the water being absorbed by the superficial parts of the strata, 

 a at a a (which may be of very great extent), will penetrate through its 

 whole depth, and, finding no egress below on account of the basin-like 

 form of the stratum, or from its resting at the lower termination upon a 

 compact rock, will accumulate. The porous strata, therefore, becomes a 

 reservoir tp a greater or less extent, and if, by boring through the super- 

 incumbent mass, we form an opening into the stratum, as at 6, the water 



