Instead of this, the soil to a great depth is stored with water which cannot get away, except by the slow process by which the springs draw it off. This explains the steady flow of rivers. What should we do for wells if it were not for the water basins that lie below the surface? A shallow well may go dry. Its owner digs deeper, and strikes a lower "vein" of water that gives a more generous supply. In the regions of the country where the drift soil, left by the great ice-sheet, lies deepest, the glacial boulder clay is very far down. The surface water, settling from one level to another, finally reaches the bottom of the drift. Wells have to be deep that reach this water bed. The water follows the slope of this bed and is drained into the ocean, sometimes by subterranean channels, because the bed of the nearest river is on a much higher level. So we must not think that the springs contain only the water that feeds the rivers. They contain more. The layers of clay at different levels, from the surface down to the bottom of the drift, form water basins and make it possible for people to obtain a water supply without the expense of digging deep wells. The clayey subsoil, only a few feet below the surface, checks the downward course of the water, so that the sun can gradually draw it back, and keep a supply where plant roots can get it. The vapour rising keeps the air humid, and furnishes the dew that keeps all plant life comfortable and happy even through the hot summer months. Under the drift lie layers of stratified rock, and under these are the granites and other fire-formed rocks, the beginning of those rock masses which form the solid bulk of the globe. We know little about the core of the earth, but the granites that are exposed in mountain ridges are found to have a great capacity for absorbing water, so it is not unlikely that much surface water soaks into the rock foundations and is never drained away into the sea. The water in our wells is often hard. It becomes so by passing through strata of soil and rock made, in part, at least, of limestone, which is readily dissolved by water which contains some acid. Soil water absorbs acids from the decaying vegetation,--the dead leaves and roots of plants. Rain water is soft, and so is the water in ponds that have muddy basins, destitute of lime. Water in the springs and wells of the Mid-Western States is "hard" because it percolates through limestone material. In many parts of this country the well water is "soft," because of the scarcity of limestone in the soil.