Looking at the road more narrowly, you remark that it is full of little roughnesses—at one place a long rut, at another a projecting stone, with many more inequalities which your eye could not easily detect when the road was dry, but which the water at once discloses. Every little dimple and projection affects the flow of the water. You see how the raindrops gather together into slender streamlets of running water which course along the hollows, and how the jutting stones and pieces of earth seem to turn these streamlets now to one side and now to another.
Toward the top of the slope only feeble runnels of water are to be seen. But further down they become fewer in number, and at the same time larger in size. They unite as they descend; and the larger and swifter streamlets at the foot of the descent are thus made up of a great many smaller ones from the higher parts of the slope.
Why does the water run down the sloping road? why do rivers flow? and why should they always move constantly in the same direction? They do so for the same reason that a stone falls to the ground when it drops out of your hand; because they are under the sway of that attraction toward the center of the earth, to which, as you know, the name of gravity is given. Every drop of rain falls to the earth because it is drawn downward by the force of this attraction. When it reaches the ground it is still, as much as ever, under the same influence; and it flows downward in the readiest channel it can find. Its fall from the clouds to the earth is direct and rapid; its descent from the mountains to the sea, as part of a stream, is often long and slow; but the cause of the movement is the same in either case. The winding to and fro of streams, the rush of rapids, the roar of cataracts, the noiseless flow of the deep sullen currents, are all proofs how paramount is the sway of the law of gravity over the waters of the globe.
Drawn down in this way by the action of gravity, all that portion of the rain which does not sink into the earth must at once begin to move downward along the nearest slopes, and continue flowing until it can get no further. On the surface of the land there are hollows called lakes, which arrest part of the flowing water, just as there are hollows on the road which serve to collect some of the rain. But in most cases they let the water run out at the lower end as fast as it runs in at the upper, and therefore do not serve as permanent resting places for the water. The streams which escape from lakes go on as before, working their way to the seashore. So that the course of all streams is a downward one; and the sea is the great reservoir into which the water of the land is continually pouring.
The brooks and rivers of a country are thus the natural drains, by which the surplus rainfall, not required by the soil or by springs, is led back again into the sea. When we consider the great amount of rain, and the enormous number of brooks in the higher parts of the country, it seems, at first, hardly possible for all these streams to reach the sea without overflowing the lower grounds. But this does not take place; for when two streams unite into one, they do not require a channel twice as broad as either of their single water-courses. On the contrary, such an union gives rise to a stream which is not so broad as either of the two from which it flows. But it becomes swifter and deeper.
Let us return to the illustration of the roadway in rain. Starting from the foot of the slope, you found the streamlets of rain getting smaller and smaller, and when you came to the top there were none at all. If, however, you were to descend the road on the other side of the ridge, you would probably meet with other streamlets coursing down-hill in the opposite direction. At the summit the rain seems to divide, part flowing off to one side, and part to the other.
In the same way, were you to ascend some river from the sea, you would watch it becoming narrower as you traced it inland, and branching more and more into tributary streams, and these again subdividing into almost endless little brooks. But take any of the branches which unite to form the main stream, and trace it upward. You come, in the end, to the first beginnings of a little brook, and going a little further you reach the summit, down the other side of which all the streams are flowing to the opposite quarter. The line which separates two sets of streams in this way is called the water-shed. In England, for example, one series of rivers flows into the Atlantic, another into the North Sea. If you trace upon a map a line separating all the upper streams of the one side from those of the other, that line will mark the water-shed of the country.
But there is one important point where the illustration of the road in rain quite fails. It is only when rain is falling, or immediately after a heavy shower, that the rills are seen upon the road. When the rain ceases the water begins to dry up, till in a short time the road becomes once more firm and dusty. But the brooks and rivers do not cease to flow when the rain ceases to fall. In the heat of summer, when perhaps there has been no rain for many days together, the rivers still roll on, smaller usually than they were in winter, but still with ample flow. What keeps them full? If you remember what you have already been told about underground water, you will answer that rivers are fed by springs as well as by rain.
Though the weather may be rainless, the springs continue to give out their supplies of water, and these keep the rivers going. But if great drought comes, many of the springs, particularly the shallow ones, cease to flow, and the rivers fed by them shrink up or get dry altogether. The great rivers of the globe, such as the Mississippi, drain such vast territories, that any mere local rain or drought makes no sensible difference in their mass of water.
In some parts of the world, however, the rivers are larger in summer and autumn than they are in winter and spring. The Rhine, for instance, begins to rise as the heat of summer increases, and to fall as the cold of winter comes on. This happens because the river has its source among snowy mountains. Snow melts rapidly in summer, and the water which streams from it finds its way into the brooks and rivers, which are thereby greatly swollen. In winter, on the other hand, the snow remains unmelted; the moisture which falls from the air upon the mountains is chiefly snow; and the cold is such as to freeze the brooks. Hence the supplies of water at the sources of these rivers are, in winter, greatly diminished, and the rivers themselves become proportionately smaller.