Page:Popular Mechanics 1928 01.pdf/79

 twenty feet farther—all in search of eight cents' worth of gold. Sand and gravel brought up by the scoops are dumped first into a huge steel hopper at the very top of the superstructure of the dredge. Thence they pass to a screen, which, revolving at a steady pace, separates the fine material from the coarse and discards the latter.



Water under heavy pressure is then automatically sprayed on the gravel and the finer, gold-bearing sand is thus washed through the screen into a distributor, from which it flows in a slow stream over rows of riffles, improvements over the sluice boxes of the hand miner who first worked these "diggings." In these riffles, quicksilver lies in wait to pounce on every grain of gold, so that, when the sand and gravel leave the device, they are free of all values. In the meantime, the coarser gravel, from which the nuggets and flakes of gold had fallen away in the moving screen, is passed onto belt conveyors, which carry it over long arms and pile it at either side or to the rear of the dredge. When the engineer of this mastodon wishes it to move, he shifts a switch, and one or the other of two forty-five-ton steel legs—called "spuds"—languidly extends itself, buries its point fifteen or twenty feet in the river bed or bank, and, like a dancer turning on her toe, the 2,500 tons of dredge swings about. The second "spud" finds a hold, and the great machine either settles down to resume hunting for gold or continues "walking" on its twin steel legs.

It is estimated by engineers and mining