In various localities we hear of springs in seemingly paradoxical situations on tops of hills and even mountains. Such a mystery is not difficult to clear up. In the first place, such springs are rarely at the summit of the hill or mountain. A case well known to many persons is the small, but never-failing spring a little below the summit of Mount Whiteface, a peak in the Adirondacks, rising 3,000 feet above the general level of the immediately surrounding country. In this case, a mass of highly fractured rock, subjected to much rainfall and lying above the level of the spring, is sufficiently large easily to contain and give forth enough water to account for several such springs. In rare cases, however, springs or flowing wells are located on summits, and in such places it is only necessary to bear in mind some of the principles above set forth, but mainly the facts that water may travel under pressure long distances underground, and that the point of emergence may be on a hill which is actually lower than the source of the water far away. The economic significance of underground waters is forcibly brought to our attention when we realize that 75 per cent of the people of the United States depend upon wells for their water supply. Many city supplies, most farm supplies, and much irrigation water come from wells. The 3,000,000 people of Iowa, for example, are dependent upon underground waters from wells varying in depth from a few feet to several thousand feet. [Illustration: Fig. 20.--Ideal section illustrating the chief requisite conditions of artesian wells. A, a porous stratum; B and C, impervious beds below and above A, acting as confining strata; F, the height of the water level in the porous bed A, or, in other words, the height of the reservoir or fountainhead; D and E, flowing wells springing from the porous water-filled bed A. (After U. S. Geological Survey.)] Most wells are simply dug to depths a little below the water table. In humid climate regions the depths seldom exceed fifty feet. The water encountered in such wells is rarely under pressure. In some regions of deep soils or loose formations, wells are actually bored with an auger to depths of as great as 200 feet. Deep wells in relatively hard rocks are always drilled to depths of even thousands of feet. In such cases the purpose is to strike either a porous rock layer charged with water, or a crack or fissure filled with water, the water almost always being under pressure (sometimes very great), under such conditions. These are called artesian wells whether the water under pressure actually flows out at the surface or not.