The introduction of meters has been an important factor in reducing water waste. It is not the thought to limit the amount of water used or to deprive one of a sufficient amount, but there is nothing to be gained by an unnecessary waste. The introduction of meters in Milwaukee reduced the amount of water used per tap from 1,781 gallons per day to 644 gallons per day without putting any restrictions upon the consumers. It is an enormous task to provide a sufficient amount of pure clean water for a large city and certainly an effort should be made to avoid waste. SOURCES OF WATER SUPPLY Rain Water The sources of our water supply may be classified as: Rain and snow water, surface water and ground or subsoil water. The rain and snow water provide a supply for domestic purposes. This source cannot be relied upon in some sections of the country because of the variable amount of rain fall. Rain water is pure from the fact that it has been vaporized and then condensed the same as in distillation; but it does not remain long in this pure state since it is exposed to so many sources of pollution. This water is collected in receptacles called cisterns and tanks and used mostly for household purposes. It can readily be seen that water thus stored might become polluted from the surfaces with which it comes in contact. To prevent this special care would be necessary, and in the majority of cases the need for this precaution is not recognized. This source of water supply is not very desirable since its quantity is variable and also because of the difficulty in storing large quantities and in providing protection so that it will not become contaminated and thereby rendered unfit for household use. While rain water in its unpolluted state is a pure water, it is not suitable for drinking purposes until it is properly aerated. The air contains dust and other suspended impurities which will be precipitated with the rain and thus pollute the water. However, the amount of pollution is so small that it is of little sanitary importance. After the air has been cleared of these suspended impurities the rain water will be relatively pure. It will be noticed that after a rain the air seems clear and clean; this is due to the rain actually washing the air. It is necessary to filter rain water collected from the surface of roofs because roofs collect impurities from smoke and dust. The average filter used for this purpose is usually inadequate and receives such little care that it is of slight value.