PLUMBING AND WATER SUPPLY 461 



chalk, clay, etc. Water will soak through sand and chalk 

 thoroughly, but clay, on the contrary, will not allow water 

 to pass through it. As the result of various upheavals of 

 the earth, the layers are not horizontal but are tipped at 

 angles and are often exposed to the surface of the earth. 

 When a layer (stratum) of sand is supported by a layer of 

 clay, the water is held up as in a basin. Therefore, when a 

 hole is dug into the ground through the sand and not below 

 the clay, water is usually found to have percolated through 

 the ground and settled in the sand. Figure 206 shows a 



FIG. 206. Section of Earth showing Conditions which 

 Produce Artesian Wells. 



section of the earth and illustrates how flowing wells are 

 formed. The upper and third layers of earth are porous, the 

 second and fourth non-porous. The force of gravity causes 

 the water to sink into the lower porous layer and remain 

 at this level. 



This water comes from the rain, ice, or snow and takes 

 up considerable impurities as it passes along the ground. 

 The sand usually filters or removes these impurities, although 

 sometimes it fails to do so completely. This is most fre- 

 quently the case in a shallow well. A welt should be at least 

 150 ft. from the nearest known source of pollution. 



The water is raised from wells or from the source of supply 

 to reservoirs by means of pumps. 



