WATER SUPPLY 365 



cast rings and covers that are convenient for the manhole 

 in a well cover. Metal well covers have been made and 

 placed on the market, but are used only to a limited extent. 



If at all practicable, the well should go down through a 

 stratum of clay or rock which does not permit the passage 

 of water. Where the wall passes through such a stratum, 

 the 6-inch ring of concrete should extend from the wall 

 into the face of this stratum. This cuts off all downward 

 seepage of surface water in the vicinity of the well. Such a 

 well will be much less liable to contamination than one dug 

 through porous material only. Since the outbreak of con- 

 tagious diseases has in numerous instances been traced to 

 surface wells, every possible means should be taken to pre- 

 vent any contamination of the underground stratum which 

 furnishes the household with water. 



Deep wells. As the development of an agricultural district 

 improves, there is a tendency to replace shallow wells with 

 deep wells 2 to 6 inches in diameter and 75 to 200 feet below 

 the surface. Such, wells usually penetrate deep strata of 

 impervious material before reaching water, and there is 

 almost no danger of contamination from surface drainage. 

 If there is a large opening on the outside of the pipe or a very 

 porous soil, it might be feasible to fill this opening with thin 

 cement mortar at the time of the completion of the well or 

 to excavate down a few feet and place a ring of concrete 

 around the tube. Ordinarily, however, such precautions 

 are not necessary for the reason that water, attempting to 

 work down the pipe to such a great depth, will carry sand 

 and silt with it, which will permanently seal any opening 

 outside of the tube. The only means by which such a water 

 supply can become polluted is by turning drainage water 

 into one of these wells, or by having the water strata at 

 some distant point above the well come to the surface; 

 but such a condition seldom exists. These small, deep wells 

 are to be recommended for improving sanitary conditions 

 wherever they can be had. Since wells of this class contain 



