Wells 95 



Moreover, they do not exclude infiltration of sewage 

 liquids below the water-table, and, when the distance 

 from cess-pools, stables, or privy-vaults is slight, pol- 

 lution may occur in a manner noted in connection with 

 open wells. When pollution is excluded, the water 

 from driven wells contains but few bacteria, and, from 

 deeper wells, it may be practically germ-free. 



Artesian wells. The water of artesian wells is de- 

 rived from sources more or less distant, and makes its 

 way between two impervious strata towards lower 

 levels. When the upper stratum is punctured, the water 

 is driven to the surface with a force proportional to the 

 elevation of the region whence it is derived. A porous 

 layer of sand or gravel, situated between two layers of 

 rock or clay, and placed in a vertical position, may thus 

 collect large quantities of rain- or snow-water and con- 

 vey it hundreds of miles to the plain below. Owing, 

 however, to the efficient filtering action of the water- 

 bearing stratum, the comparatively small number of 

 microorganisms derived from the rain or snow are not 

 carried far, and artesian waters are, therefore, generally 

 very poor in bacteria. 



BACTERIA IN THE WATER OF CISTERNS AND TANKS 



In regions where the ground-water is difficult to 

 reach, or where it contains a large proportion of unde- 

 sirable substances, like bicarbonate of lime, or other 

 salts, the collection and use of rain-water is generally 

 resorted to. Its freedom from lime makes it highly 

 desirable for laundry purposes. When carefully collected 



V OF THE 



UNIVERSITY 



