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.