When water, not in very great body, runs in a rocky bed much interrupted by hollows, so that it can rest every now and then in a pool as it goes along, it does not acquire a continuous velocity of motion. It pauses after every leap, and curdles about, and rests a little, and then goes on again; and if in this comparatively tranquil and rational state of mind it meets with any obstacle, as a rock or stone, it parts on each side of it with a little bubbling foam, and goes round; if it comes to a step in its bed, it leaps it lightly, and then after a little splashing at the bottom, stops again to take breath. But if its bed be on a continuous slope, not much interrupted by hollows, so that it can not rest, or if its own mass be so increased by flood that its usual resting-places are not sufficient for it, but that it is perpetually pushed out of them by the following current, before it has had time to tranquilize itself, it of course gains velocity with every yard that it runs; the impetus got at one leap is carried to the credit of the next, until the whole stream becomes one mass of unchecked, accelerating motion. Now when water in this state comes to an obstacle, it does not part at it but clears it, like a race-horse; and when it comes to a hollow, it does not fill it up and run out leisurely at the other side, but it rushes down into it and comes up again on the other side, as a ship into the hollow of the sea. Hence, the whole appearance of the bed of the stream is changed, and all the lines of the water altered in their nature. The quiet stream is a succession of leaps and pools; the leaps are light and springy, and parabolic, and make a great deal of splashing when they tumble into the pool; then we have a space of quiet curdling water, and another similar leap below. But the stream when it has gained an impetus takes the shape of its bed, never stops, is equally deep and equally swift everywhere, goes down into every hollow, not with a leap, but with a swing, not foaming, nor splashing, but in the bending line of a strong sea-wave, and comes up again on the other side, over rock and ridge, with the ease of a bounding leopard; if it meet a rock three or four feet above the level of its bed, it will neither part nor foam, nor express any concern about the matter, but clear it in a smooth dome of water, without apparent exertion, coming down again as smoothly on the other side; the whole surface of the surge being drawn into parallel lines by its extreme velocity, but foamless, except in places where the form of the bed opposes itself at some direct angle to such a line of fall, and causes a breaker; so that the whole river has the appearance of a deep and raging sea, with this only difference that the torrent-waves always break backward, and sea-waves forward....