The boys knew little or nothing of the river south of the bridge, and frequented mainly that mile-long stretch of it between the bridge and the dam, beyond which there was practically nothing for many years; afterwards they came to know that this strange region was inhabited. Just above the bridge the Hydraulic emptied into the river with a heart-shaking plunge over an immense mill-wheel; and there was a cluster of mills at this point, which were useful in accumulating the waters into fishing-holes before they rushed through the gates upon the wheel. The boys used to play inside the big mill-wheel before the water was let into the Hydraulic, and my boy caught his first fish in the pool below the wheel. The mills had some secondary use in making flour and the like, but this could not concern a small boy. They were as simply a part of his natural circumstance as the large cottonwood-tree which hung over the river from a point near by, and which seemed to have always an oriole singing in it. All along there the banks were rather steep, and to him they looked very high. The blue clay that formed them was full of springs, which the boys dammed up in little ponds and let loose in glassy falls upon their flutter-mills. As with everything that boys do, these mills were mostly failures; the pins which supported the wheels were always giving way; and though there were instances of boys who started their wheels at recess and found them still fluttering away at noon when they came out of school, none ever carried his enterprise so far as to spin the cotton blowing from the balls of the cottonwood-tree by the shore, as they all meant to do. They met such disappointments with dauntless cheerfulness, and lightly turned from some bursting bubble to some other where the glory of the universe was still mirrored. The river shore was strewn not only with waste cotton, but with drift which the water had made porous, and which they called smoke-wood. They made cigars for their own use out of it, and it seemed to them that it might be generally introduced as a cheap and simple substitute for tobacco; but they never got any of it into the market, not even the market of that world where the currency was pins.