THE PURITY OF DRINKING-WATERS IT43 
all the while that his well has “the finest water in the country.” 
The only safeguard is either to abandon the well entirely, or to 
have such an absolute isolation between his vault and his well as 
to make communication between them by soil drainage an absolute 
impossibility. 
Since the water in the well is likely to become contaminated 
with typhoid bacteria, if excreta are thrown upon the ground or 
are placed in a vault in the vicinity of the well which is used for 
drinking or dairy purposes, especial care should be taken that 
no surface rivulets in time of rain should run toward the well. 
If they do, contamination of the water by surface drainage is 
almost certain. 
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 gener- 
ally placed where it is almost sure that they will become con- 
taminated in some way, and actual examination of such cisterns 
usually shows the B. colt present, indicating sewage contamina- 
tion. 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 
