Page:Popular Science Monthly Volume 61.djvu/242

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from two to five cubic yards of gravel. As contrasted with fire-setting, the steam-thawing obviates the suffocating fumes of burning wood, or the danger of thawing the frozen roof in underground workings.

A recent innovation which I saw coming into operation was thaw- ing with water by means of the pulsometer pump, which seems to be economical since the water can be used over and over again; this process seems likely to come into more general use.

The gravel is raised from the shaft in buckets by windlass or steam- hoists, and in winter is dumped on to heaps for summer work, or in summer may be emptied straight into the sluice-boxes.

Much ingenuity has been exercised in the construction of self- dumping hoists, in which, by a single rope, the bucket is raised from the bottom of the shaft, caught by a traveling clutch, carried along a horizontal rope to the dump-heap or the sluice-boxes, where it is automatically tilted. All this is done by an engine in charge of one man, and saves much labor.

As regards the washing of the gravel, the old-fashioned hand- rockers are still to be seen in operation upon many of the gulches, but have been for the most part replaced by sluice-boxes. These are long wooden troughs made in 12-foot lengths and about ten inches broad; the bottom is lined with wooden riffles, consisting generally of longitudinal bars (Pole riffles), but sometimes of transverse bars (Hungarian riffles), by which the gold and heavy minerals are caught. Sometimes Auger riffles — planks with circular holes — are employed; mercury is seldom used. A sluice-head of 75 miners' inches, i. e.. 112 cubic feet of water per minute, is usual, and a fall of about eight inches in the 12-foot box length.

Water, which is very scarce in the district, and must be used economically, is conducted to the sluice-boxes by long wooden flumes, which are themselves a serious expense on account of the cost of wood (about $110 a thousand feet). Some of these flumes are half a mile in length ; in a wide valley, where the pay-streak is on the opposite side from the stream, it is necessary to raise it by centrifugal pumps to a height of 30 or 40 feet, and to convey it across the valley by a long flume.

In the final 'wash-up,' by which the gold-dust is extracted from the sluice-boxes, the riffles are taken out, and a copious stream of water is sent down, which carries away the fine gravel and leaves the gold, and a heavy black sand which accompanies it. This black sand consists mainly of magnetic ore, and it is removed partly by a magnet and partly by shaking with the hand and blowing with the mouth in a small metal tray.

The mining operations on the creeks and upon the hillsides are somewhat different. On the creek a shaft is sunk down to bed-rock,

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