Page:Popular Mechanics 1928 01.pdf/78

 mately 135,000,000 cubic yards more of material than were excavated in the cutting of the Panama canal.



Just as the placer miner of the days of gold in California shoveled the gravel into a crude rocker, poured water over it, and depended on gravity to drop the gold to the bottom, where mercury picked it up, so the dredge, taking a ton where the miner lifted a shovelful, goes through the same process mechanically. Five men, masters of the metal monster, handle more than 15,000 tons of gravel and sand in the twenty-four-hour day. There are, of course, three shifts of five men each in that length of time, but the crew of the dredge is never more than five men at once.

Two permanent channels, each 500 feet wide and nearly three miles long, have been dredged to a depth of more than sixty feet, and two others, which will be of approximately the same length and width, are now being made. The gravel, cleaned of all the gold, is piled in solidly packed walls along these channels, furnishing what the gold diggers' owners claim to be permanent holding walls, between which flood waters and freshets are carried away from the agricultural lands on either side.

All the work so far accomplished has been done on the barren waste of tailings, which accumulated in the days of unrestricted hydraulic mining, when millions of yards of this waste material was dumped annually along the upper reaches of the Yuba river, where the modern mechanical method of mining is being applied.

Each of the six dredges is larger than many a deep-sea steamer, being 165 feet long, 68 feet wide, and weighing approximately 2,500 tons. A set of huge steel scoops, hung on an endless chain running over the "trunk" of the metal mastodon, reaches far down into the pile of gravel or the buried river bed and lifts out the dripping gravel. There are ninety-six of these scoops on the chain of each dredge, and each scoop carries one ton of material, when filled.

As the loaded chain moves sluggishly upward, dumping its content into the ever-open mouth of the mastodon, there follows a tremendous air-shaking medley of sounds, which becomes continuous as the dredge gnaws into the gravel. It is the crunching, groaning, roaring, grinding, clattering of stones from the size of a man's head down to that of a buckshot, all falling on moving metal screens and thence off these onto the shaking tables, until all has been sifted, the stones carried away and the heavier gold dropped through to be left in the grip of a pool of quicksilver.

The average depth to which the scoops go is sixty-two feet, but when a deeper pocket of gravel is found, the mastodon sends its great trunk down as much as