Wells, which are the chief source of supply in the country, should be kept away from barnyards, stables, cesspools, and the waste waters from dairy and kitchen, to preserve them from pollution. Many cases of typhoid fever and other serious diseases have been directly traced to a violation of this rule. The ground surrounding the well should be raised so as to throw all surface water twelve or fifteen feet away from the well. See also what is said on this subject in the chapter on Dwellings. Springs usually furnish the purest and best water. Coming from a considerable depth, spring-water loses, in its passage through the earth, most if not all its organic matter, and rises to the surface clear, cool, pure, and sparkling. The spring should be walled and covered, and otherwise protected from surface drainage. =Cisterns.=--Rain-water collected in the country, and under favorable conditions, is comparatively pure and wholesome. In the cities, it contains such a large amount of organic matter and other impurities, washed out of the air and off the roofs by the rain and snow, that it is generally unfit for drinking without being filtered. On account of its softness, rain-water is very desirable for washing and other domestic purposes, but owing to the absence of mineral constituents it is flat and insipid to the taste. In New Orleans and other southern cities, where cisterns are largely used, the water is rendered cool and palatable by the use of large quantities of artificial ice. =Ice.=--It was formerly supposed that in the process of freezing all deleterious matter contained in the water was excluded. Several outbreaks of disease in New England led to an investigation, which showed that the ice used had been taken from ponds whose waters contained large quantities of sewage and other impurities. A change in the source of the ice supply resulted in an immediate check of the disease. Recent research has shown that typhoid bacilli, after being frozen in a block of ice for 103 days, may still be alive when released. =Diseases Caused by Drinking Polluted Water.=--A polluted water supply affects not one, but usually many persons, and notable epidemics have resulted. In consequence, more diligent inquiry has been instituted by Municipal, State, and National Boards of Health, and the evidence adduced is of the most positive character. Typhoid fever, cholera, dysentery, and diarrhœa have been clearly traced to the use of impure drinking water, and other related diseases are suspected of having a similar origin, although the evidence is not so conclusive.