One part of the rain sinks under the ground, and you have traced its progress there until it comes to the surface again. You have now to trace, in a similar way, the other portion of the rainfall which flows along the surface in brooks and rivers. You can not readily meet with a better illustration of this subject than that which is furnished by a gently sloping road during a heavy shower of rain. Let us suppose that you know such a road, and that just as the rain is beginning you take up your station at some part where the road has a well-marked descent. At first you notice that each of the large heavy drops of rain makes in the dust, or sand, one of the little dints or rain-prints already described. As the shower gets heavier these rain-prints are effaced, and the road soon streams with water. Now mark in what manner the water moves. 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.