Springs. Springs are formed on the same plan as the deep well, but with the difference that the waterproof layer on top of which the water is running either crops out on the surface again, lower down the mountain, or folds upon itself and comes up again to the surface some distance away from the mountain chain, out on the level. This is why springs are usually found in or near mountainous or hilly regions. If the water of a spring has gone deep enough into, or far enough through, the layers of the earth, it may, like water of some of the artesian wells, contain certain salts and minerals, particularly soda, sulphur, and iron. Such springs are often highly valued as mineral water, healing springs, or baths, partly because of these salts, partly on account of their peculiar taste. Most of the virtues ascribed to mineral waters or springs are due, however, to their _pure water_, and its cleansing effects internally and externally when freely used. Springs are among the most highly prized sources of water supply, because they have gone underground sufficiently deep to become well filtered and cooled to a low temperature, and usually not far enough to become too heavily loaded with salts or minerals like the waters of the deep wells. It must, however, be remembered that they also come from rain-water, and that in hilly or broken regions the source of that rain water may be the surface of the ground only a few hundred yards up the hill or mountain, and impurities there may affect it. Much of the delightful sparkle of spring water is due, as in the case of the popular soda water, to the presence of carbon dioxid, only in spring water it is produced by the decomposition of vegetable matter in it. As springs usually break out in a hollow or at the foot of a hill, unless carefully closed in they are quite liable to contamination from rain water from the surrounding surface of the ground. Where springs of a sufficient size can be reached, or a sufficiently "live" series of deep wells can be bored, these furnish a safe source of water supply for cities. But of course not more than one city in five or ten is so favored. Mountain Reservoirs. Two other methods of securing a water supply are now generally adopted. One is to pick out some stream up in the hills or mountains, within fifteen miles or so of the city, and put in a dam, thus making a reservoir, or to enlarge some lake which already exists there. At the same time, the entire valley, or slope of the mountain, which this stream or lake drains of its surface water, is bought up by the Government, or turned into a forest reserve, so that no houses can be built or settlement of any kind permitted upon it. It can still be used for lumber supply, for pastures, and, within reasonable limits, for a great public hunting and fishing reserve and camping resort. [Illustration: A CITY WATER SUPPLY BROUGHT FROM THE FAR HILLS]