Another contrivance which soon came into use was the "cradle." This was a long box, sometimes only a hollowed-out log. At the top was a sieve which sifted out the stones. Nailed to the bottom of the cradle were small cleats of wood, or "riffles," which kept the water from running so fast as to sweep the gold out of the cradle with it. The cradle was placed on rockers and was also tilted slightly. The miner shoveled the gravel into the top of the cradle and his partner rocked it. The sieve kept back the stones, the water broke up the lumps of earth and gravel and washed them down the cradle, and the grains of gold were stopped by the riffles, and sank to the bottom. Sometimes the "pay dirt" continued under a stream. To get at it, the miners often built a little canal and turned the water into a new channel; then they could work on the former bed of the river. Before many years had passed, the gold that was near the surface had been gathered. The miners then followed the streams up into the mountains, and found that much of the gold had come from beds where in ancient times rivers had flowed. There was gold still remaining in these beds, but it was poorly distributed, the miners thought. Sometimes there would be quite an amount in one place, and then the miner would dig for days without finding any more. Even worse than this was the fact that these gravel beds were not on the top of the ground, but were covered up with soil and trees. Evidently the slow work with pans and cradles would not pay here; but it occurred to some one that if a powerful stream of water could be directed against the great banks of earth, as water is directed against a burning building, they would crumble, the dirt could be washed down sluices, and the gold be saved. This was done. Great reservoirs were built high up in the mountains, and water was brought by means of ditches or pipes to a convenient place. Then it was allowed to rush furiously through a hose and nozzle, and the great stream coming with tremendous force was played upon the banks of gravel. The banks crumbled, the gravel was washed into a string of sluices, or long boxes with riffles to catch the gold. Soon the miners found that if quicksilver was put into these sluices, it would unite with the gold and make a sort of paste called "amalgam." Then if this amalgam was heated, the quicksilver would be driven off in the form of gas, and the gold would remain in a beautiful yellow mass. [Illustration: HYDRAULIC GOLD MINING A placer mine at Gold Point, California, where tremendous streams of water under high pressure are busy washing away the side of a gold-bearing hill.]