Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/770

Rh 7-16 used in river diggings in the north-west of America. A rectangular trough of boards, whose dimensions depend chieﬂy on the size of the planks available, is set up on the higher part of the ground at one side of the claim to be worked, upon trestles or piers of rough stone-work, at such an inclination that the stream may carry olf all but the largest stones, which are kept back by a grating of boards about 2 inches apart at a. The gravel, which in this par- ticular instance is froin 12 to 16 feet thick, and with an average breadth to the river of ‘.25 to 30 feet, is dug by hand and thrown in at the upper end, the stones kept back being removed at intervals by two men with four-pronged steel forks. The floor of the sluice is laid with riflles made of strips of wood 2 inches square laid parallel to the direction of the current (as at b, and in cross section at c), and at other points (I with boards having transverse notches ﬁlled with mercury. These were known originally as Hungarian riflles. The bottom of the working. which is below the drainage level of the valley, is kept dry by a Chinese bucket pump e, attached to a rough undershot wheel driven by the current in the sluice. The sluice boxes are made in lengths, and united together spigot and faucet fashion, so that they may easily be removed and re-erected as the different parts of the claim are progressively exhausted. In the larger and more permanent erections used in hydraulic mining, the upper ends of the sluices are often cut in rock or lined with stone blocks, the grating stopping the larger stones being known as a “ grizzly.” In order to save very ﬁne and especially rusty particles of gold, so-called “ under-current sluices ” are used; these are shallow wooden tanks, 50 square yards and upwards in area, which are placed somewhat below the main sluice, and communicate with it above and below, the entry being protected by a grating so that only the ﬁner material is admitted. These are paved with stone blocks or lined with mercury rililes, so that from the greatly reduced velocit.y of flow, due to the sudden increase of surface, the ﬁner particles of gold may collect. In order to save ﬁnely-divided gold, amalga- mated copper plates are sometimes placed in a nearly level position, at a considerable distance from the head of the sluice, the gold which is retained in it being removed from time to time. Sluiees are often made double, and they are usually cleaned up,—that is, the deposit rich in gold is removed from tl1em,—once a week. The gold is then re- covered by “panning ” The application of a jet of water to the removal of auriferous gravels by the so-called hydraulic system of mining has already been noticed at vol. iv. p 701.1 This method has for the most part been conﬁned to the country of its invention, California, and the western territories of America, where the conditions favourable for its use are more fully developed than elsewhere,—notabl y the presence of thick banks of gravel that cannot be utilized by other methods, and abundance of water, even though considerable work may be required at times to make it available. The general conditions to be observed in such workings maybe brieﬂy stated as follows :——( 1) The whole of the auriferons gravel, down to the “bed rock,” must be removed,—that is, no selection of rich or poor parts is possible; (2) this must be accomplished by the aid of water alone, or at times by water supplemented by gunpowder; (3) the con- glomerate must be mechanically disintegrated without interrupting the whole system ; (4) the gold must be saved without interrupting the continuous flow of water; and (5) arrangements must be made for disposing of the vast masses of impoverished gravel. ‘ Much valuable information on this subject will also be found in the Fifth Annual Report of the United States Cnnwz1'ss.,"nlr’79 of JIim'n_r/ Slatist.-'c.e, V/ashington, 1873, p. 390. GOLD The general appearance of an hydraulic gold working is seen in fig. 3, the water being brought from a ditch on the high ground, and through a line of pipes to the dis- tributing box, whence the branch pipes supplying the ' FIG. 3.—Hydraulic Gold Working. three jets diverge. The stream issues through a nozzle resembling that of a ﬁre engine (ﬁg. 4), which is mo'al»l»_‘ in a horizontal plane around the vertical axis (1, and in .1 vertical plane on the spherical joint and centre 1-, so that the direction of the jet may be varied * through considerable angles by simply mov- ing a handle. The material of the bank, being loosened by the cutting action of the water, crumbles into holes, or “ caves in,” and the superin- cumbent mass, often with large trees and stones, falls into the lower ground. The stream, laden with stones and gravel, passes into the sluices, where the gold is recovered in the manner already described. Under the most advantageous conditions the loss of gold may be estimated at 15 or 20 per eent., the amount recovered representing a value of about two shillings per ton of gravel treated. The loss of mercury is about the same, from 5 to G cwt. being in constant use per mile of sluice. About 1 cwt. is added daily in at least two charges. The average half-yearly consumption is estimated at about one l11mdred flasks of 7-1 lb each, after allowing for the amount recovered in clearing up and dis- tillation of the amalgam. The latter operation is pe1‘fo1'mc(l at intervals of seven or fourteen days in the upper length; of the sluice, and half-yearly in the lower parts. The dressing or mechanical preparation of vein stall‘ con- taining gold is generally similar to that of other ores, except that the precious metal should be removed from the waste substances as quickly as possible, even although other minerals of value that are subsequently recovered may be Fig. -1. present. This is usually done by amalgamation with .'nercury. In all cases the quartz or other vein stuff must be reduced to a very fine powder as a prelinnnary to further operations. This may be done in several '{Ly.<, €.f7., either (1) by the Mexican crusher or (12-2-u.»-Ira, in which the grinding is effected upon a bed of stone, ov.r which heavy blocks of stone attached to cross arms are dragged by the rotation of the arms about a central spindle, motion being furnished by mulcs or other power, or (1?) by the Chilian mill or trapic/ze, also known as the edge- runner, where the grinding stones roll upon the ﬂoor, at the same time turning about a central upright,—cou- trivances which are mainly used for the preparation of silver ores; but by far the largest proportion of the gold quartz of California and Australia is reduced by (3) the stamp mill, which is similar in principle to that used in Europe for the preparation of tin and other ores, but has