THE USE OF INLAND WATERS most important inland waters comprise lakes, both salt and fresh, rivers, and canals. Like the oceans, these serve as (1) regu lators of temperature, (2) sources of moisture, (3) as an aid to health, (4) as a source of minerals, (5) as a source of food, (6) as barriers, and (7) as carriers of commerce. They also serve as (8) sources of water supply, (9) as a source of power, and (10) as a means of irrigation, fertilization, and drainage. This last pertains so largely to agriculture that it is deferred to Part IV.
Inland Waters as Regulators of Temperature.—As regulators of temperature even the largest lakes are of little importance com pared with oceans. Yet the southeastern shores of Lakes Michigan and Erie are great regions for grapes and other fruit because the water, which retains the heat of summer in the fall, warms the north west winds and prevents early frosts. Also in the spring the lakes retain the low temperature of winter and thus prevent the fruit trees from flowering too early and being nipped by the frost. In the same way Chicago is a much more healthful and vigorous city because in summer the hottest days are often relieved by lake breezes which blow like sea breezes in the afternoon. Even a small lake or a broad river has a slight cooling effect on the wind in summer and a warming effect in the autumn, when the water does not grow cold so fast as the land.
Inland Waters as Sources of Moisture.—In this respect lakes and rivers are no more important than as regulators of temperature. Nevertheless at the southern end of the Caspian Sea the northern slopes of the Elburz Mountains are very well watered by rain derived from this great salt lake, and form a striking contrast to the barren deserts on either side. The Caspian Sea, however, is so large as to be almost like a part of the ocean, and the high mountains at its southern end would cause rainfall even if it were dry. An inland body of water as large as Lake Michigan receives only a little more rain on its eastern or leeward side than on the windward side.
Smaller lakes have practically no effect on rainfall.
Inland Waters as Aids to Health.—When it comes to health and recreation inland waters take high rank, although not so impor tant as the ocean. How high they stand is evident from the way in which little summer houses skirt the shores of lakes, ponds, and rivers all over the United States. The boy who goes to the swimming hole on a hot summer day is illustrating the importance of inland waters in this respect. So, too, is his sister who takes her sewing down by the river to enjoy the cool breeze, and his college cousin who goes to Canada on a canoe trip. Few summer resorts are more famous than those around the Rangeley Lakes, at Lakes Champlain, George and Placid, and along the shores of the upper peninsula of Michigan. The Thousand Isles in the picturesque St. Lawrence River are equally noteworthy, as are Lakes Louise and Tahoe in the western mountains.
Inland Waters as a Source of Minerals.—Fresh-water lakes and rivers do not furnish minerals, but other inland bodies of water are a source of medicinal salts, iron ore, peat, salt, and potash. Many springs like those of Saratoga are full of dissolved minerals which have a most valuable healing quality. Swamps are the source of bog iron ore. To-day -this is not important, but the first iron foundry in America was established at Lynn in 1643 to smelt the ore from neighboring bogs. Swamps also furnish peat, which may be called a half mineralized vegetable product. Most of the world's coal appears to have been formed in ancient swamps which were part of the earth's inland waters. Salt lakes also furnish not only rock salt, such as is obtained by evaporating the water in little ponds on the shores of the Dead Sea, but also rarer minerals, such as potash, which is found abundantly in many little lakes in western Nebraska. Many important salt deposits such as those deep down in the earth near Syracuse, N. Y., were laid down millions of years ago in salt lakes that were gradually drying up.