THE PURITY OF DRINKING-WATERS. 133 



Cisterns. Cisterns, to hold rain-water caught from the roofs of 

 houses, have frequently been used as a source of water and are, to a 

 certain extent, so used to-day, particularly in localities where the 

 natural waters of the soil are very hard. These cisterns are just as 

 dangerous as wells; sometimes more so. They are generally 

 placed where it is almost sure that they will become contaminated in 

 some way, and actual examination of such cisterns usually shows 

 the B. coli present, indicating sewage contamination. Instances 

 are also known where they have been the means of distributing 

 typhoid fever. 



Stored Water in Reservoirs or Lakes. These constitute a 

 far better source for drinking-water, and under ordinary circum- 

 stances are perfectly safe. Even the water of a contaminated 

 stream will become free from dangerous disease germs when it has 

 been stored for a few weeks. This is partly because the bacteria 

 sink to the bottom, and are not likely to get into the water mains; 

 but it is chiefly because the disease germs cannot live very long in 

 water. Typhoid germs cannot live more than six weeks (usually 

 not so long) in ordinary water, and if it be stored so long before it is 

 used, it will be free from this danger, even though at first it was 

 sewage contaminated. The stored water of reservoirs thus con- 

 stitutes the best large supply of water. It may be something of a 

 surprise to be told that stored water is purer and safer than running 

 water, but study and experience have shown this to be positively the 

 case. 



Springs. These are thoroughly reliable sources of drinking- 

 water if they are properly guarded. The water comes from under- 

 ground and has filtered through the soil for unknown distances. 

 There may be cases, it is true, where the filtering is through only a 

 thin layer of soil, insufficient to purify. But if such cases exist, 

 they are very unusual, and examination shows spring-water to be 

 free from disease germs, unless carelessly contaminated after the 

 water leaves the soil. The spring should be classed with the artesian 

 well in this respect, and is the best source of water that can be 

 obtained. 



Filtered Water. The rapidly extending contamination of 



