140 TRANSACTIONS iSgQ-'oo 



The accompanying diagram will serve to explain what is 

 meant. By this construction surface water is prevented, by the 

 impermeable clay backing, from getting entrance to the well 

 until it has percolated through the earth to the line of level of 

 the ground-water. 



It will be quite clear that no one of the wells in Fig. 3 can 

 be free from unfiltered, and consequently dangerous water, unless 

 this precaution is taken, since even those which reach the 

 ground-water may be polluted by the soakage of unfiltered sur- 

 face water. 



If now we study the gathering ground i to 2 in Fig. 3, we 

 have a wild, rocky, and probably unsettled tract of land, free 

 from animal impurity, and comparatively free from vegetable 

 decay. Moreover, its distance from the point, at which the water 

 collected on it is used (W^), ensures thorough filtration, and we 

 can see at once why the water of this deep stratum should be 

 eminently pure and wholesome. Such water is, for obvious rea- 

 sons, apt to contain more mineral matter in solution, and may 

 even conform to the type of a true mineral water. Unless this 

 be the case, it is evidently a very desirable domestic supply, and 

 wells like W^ are always to be preferred when attainable. Even 

 these, however, must be protected against soakage contamination, 

 to which they ar& just as liable as those of any other type. This 

 study has shown us that shallow wells, obtaining as they do, 

 their supply from unfiltered soakage can never be safe for domes- 

 tic use, although favorable circumstances may prevent them from 

 becoming actively disease-producing ; that ground- water wells, if 

 properly protected from local contamination by soakage, are gen- 

 erally safe ; while deep water wells, guarded from local soakage, 

 are safest of all. 



It has seemed to me desirable, and even necessary to say 

 what I have, by way of introduction to the special phase of this 

 subject, to which I now ask your attention. Large towns and 

 cities, as a rule, obtain their water supplies from some single 

 source, a river, lake or spring, so that each family in a city of, 

 say 5,000 families, is supplied with water of the same kind as the 

 rest. It consequently becomes a matter of small cost to each 

 family, to take care of this common source of supply, to have it 

 examined from time to time, chemically and otherwise. More- 

 over, their is generally a Board of Water Commissioners ap- 



