Along the roadside, the steady, down-pouring rain collects into pools and rills, or sinks out of sight in the ground. The tiny streams search out the easiest grade and run down the road, digging little gullies as they go. Soon these rills meet and, joining their muddy currents, flow on with greater speed down the hillside until they reach the bottom of the valley and go to swell the brook which flows on, through sunshine or rain. The water which sinks into the ground passes out of sight for a time, but its journey is also downward toward the brook, though the soil, acting as a great sponge, holds it back and makes it take a slower pace than the rushing surface water. This slower-moving underground water percolates through the soil until it comes to a layer of rock, clay, or other impervious substance, along the slope of which it flows until it is turned again to the surface in the form of a spring. Perhaps this spring is one of those clear, cold pools, with the water bubbling up through its sandy bottom, from which we love to drink on a hot summer's day; or, again, it is a swampy spot on the hillside where the cat-tails grow. In whatever form it issues from the ground, a tiny rill carries away its overflow, and this sooner or later joins the brook. The brook, we see, is simply the collected rainfall from the hillsides, flowing away to join the river. It grows larger as other brooks join it, and becomes a creek and finally a river. But where is the dividing line between brook, creek, and river? So gradually does the brook increase in volume that it would be difficult to draw any dividing line between it and the larger streams. And so with the rills that formed the brook: each is a part of the river, and the names rill, brook, creek, and river are merely relative terms. Brooks are but rivers on a small scale; and if we study the work that a brook is doing, we shall find it engaged in cutting down or building up, just as the river does, although, owing to the smaller size of the brook, we can see most of these operations in a short distance. Let us take our way through the wet grass and dripping trees to the brookside and see what work it is doing. The countless rain-born rills are pouring their muddy water into the brook and to-day its volume is much greater than when it is fed, as it is in fair weather, by the slower-moving underground water of the springs. It roars along with its waters no longer clear but full of clay and sand ("mud" as we call it).