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. coll 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 
