The Supply and Uses of Water 197 



III. RURAL WATER SUPPLY 



77. Cisterns. A supply used in certain parts of the country 

 is the surface water which is collected in cisterns or tanks as 

 it flows from roofs of buildings. If this water is thoroughly 

 filtered and stored in non-porous cisterns, it is useful for nearly 

 all domestic purposes. It is, however, peculiarly subject to 

 contamination, and is too often imperfectly filtered. 



A cistern to be safe must be below the frost level in the ground 

 and made of material which is absolutely impervious to surface 

 water. Filters of charcoal and screens should be carefully 

 supervised. To render water safe, it should be kept in a still- 

 storage cistern for a sufficient period of time and then be filtered 

 into another tank for use. Why should rain water require such 

 careful treatment? Is it not pure as it falls? 



What rain water may contain. As it rises from land and 

 water surfaces, water vapor is entirely free from all impurities. 

 This is nature's way of purifying water, and man follows this 

 method in getting a really pure water for many uses. The 

 change from a liquid to a vapor condition, called vaporization, is 

 the first stage of distillation. In securing pure water, man con- 

 trols the conditions in which condensation takes place. Nature's 

 method of rain production is, however, different. When vapor 

 condenses in the atmosphere, the minute water particles accu- 

 mulate such impurities as dust and as they grow larger and fall 

 through the air, they gather up more and more dust, pollen, 

 spores, and bacteria, and absorb such gases as carbon dioxid and 

 ammonia. Thus when rain reaches the earth, it is laden with a 

 freight of undesirable character. Rain water with its absorbed 

 carbon dioxid has great solvent power upon mineral substances. 

 Besides the mass of vegetable and other organic matter collected, 

 it dissolves and carries away mineral substances from painted 

 roofs and similar places. Such water certainly needs to be 

 purified. Generally only the latter part of a rainfall is turned 

 into the cistern, the first part being allowed to waste, thus assur- 



