But does the whole of the rain flow off at once into the sea in this way? A good deal of the rain which falls upon the land must sink underground and gather there. You may think that surely the water which disappears in that way must be finally withdrawn from the general circulation which we have been tracing. When it sinks below the surface, how can it ever get up to the surface again?
Yet, if you consider for a little, you will be convinced that whatever becomes of it underneath, it can not be lost. If all the rain which sinks into the ground be forever removed from the surface circulation, you will at once see that the quantity of water upon the earth’s surface must be constantly and visibly diminishing. But no such changes, so far as can be seen, are really taking place. In spite of the rain which disappears into the ground, the circulation of water between the air, the land, and the sea continues without perceptible diminution.
You are driven to conclude, therefore, that there must be some means whereby the water underground is brought back to the surface. This is done by springs, which gush out of the earth, and bring up water to feed the brooks and rivers, whereby it is borne into the sea. Here, then, are two distinct courses which the rainfall takes—one below ground, and one above. It will be most convenient to follow the underground portion first.
A little attention to the soils and rocks which form the surface of a country is enough to show that they differ greatly from each other in hardness, and in texture or grain. Some are quite loose and porous, others are tough and close-grained. They consequently differ much in the quantity of water they allow to pass through them. A bed of sand, for example, is pervious; that is, will let water sink through it freely, because the little grains of sand lie loosely together, touching each other only at some points, so as to leave empty spaces between. The water readily finds its way among these empty spaces. In fact, the sand-bed may become a kind of sponge, quite saturated with the water which has filtered down from the surface. A bed of clay, on the other hand, is impervious; it is made up of very small particles fitting closely to each other, and therefore offering resistance to the passage of water. Wherever such a bed occurs, it hinders the free passage of the water, which, unable to sink through it from above on the way down, or from below on the way up to the surface again, is kept in by the clay, and forced to find another line of escape.
Sandy soils are dry because the rain at once sinks through them; clay soils are wet because they retain the water, and prevent it from freely descending into the earth.
Now the rocks beneath us, besides being in many cases porous in their texture, such as sandstone, are all more or less traversed with cracks—sometimes mere lines, like those of a cracked window-pane, but sometimes wide and open clefts and tunnels. These numerous channels serve as passages for the underground water. Hence, although a rock may be so hard and close-grained that water does not soak through it at all, yet if that rock is plentifully supplied with these cracks, it may allow a large quantity of water to pass through. Limestone, for example, is a very hard rock, through the grains of which water can make but little way; yet it is so full of cracks or “joints,” as they are called, and these joints are often so wide, that they give passage to a great deal of water.
In hilly districts, where the surface of the ground has not been brought under the plow, you will notice that many places are marshy and wet, even when the weather has long been dry. The soil everywhere around has perhaps been baked quite hard by the sun; but these places remain still wet, in spite of the heat. Whence do they get their water? Plainly not directly from the air, for in that case the rest of the ground would also be damp. They get it not from above, but from below. It is oozing out of the ground; and it is this constant outcome of water from below which keeps the ground wet and marshy. In other places you will observe that the water does not merely soak through the ground, but gives rise to a little run of clear water. If you follow such a run up to its source, you will see that it comes gushing out of the ground as a spring.
Springs are the natural outlets for the underground water. But, you ask, why should this water have any outlets, and what makes it rise to the surface?
Let us suppose that a flat layer of some impervious rock, like clay, underlies another layer of a porous material, like sand. The rain which falls on the surface of the ground, and sinks through the upper bed, will be arrested by the lower one, and made either to gather there, or find its escape along the surface of that lower bed. If a hollow or valley should have its bottom below the level of the line along which the water flows, springs will gush out along the sides of the valley. The line of escape may be either the junction between two different kinds of rock, or some of the numerous joints already referred to. Whatever it be, the water can not help flowing onward and downward, as long as there is any passage along which it can find its way; and the rocks underneath are so full of cracks, that it has no difficulty in doing so.
But it must happen that a great deal of the underground water descends far below the level of the valleys, and even below the level of the sea. And yet, though it should descend for several miles, it comes at last to the surface again. To realize clearly how this takes place, let us follow a particular drop of water from the time when it sinks into the earth as rain, to the time when, after a long journey up and down in the bowels of the earth, it once more reaches the surface. It soaks through the soil together with other drops, and joins some feeble trickle, or some more ample flow of water, which works its way through crevices and tunnels of the rocks. It sinks in this way to perhaps a depth of several thousand feet, until it reaches some rock through which it can not readily make further way. Unable to work its way downward, the pent-up water must try to find escape in some other direction. By the pressure from above it is driven through other cracks and passages, winding up and down until at last it comes to the surface again. It breaks out there as a gushing spring.