[Figs. 673], [674.] and [675.] exhibit a plan, a vertical section, and elevation, of one of these tables, taken in the direction of its length. The arbor or great shaft in prolongation from the stamps mill, is shown in section perpendicularly to its axis, at A. The cams or wipers are shown round its circumference, one of them having just acted on n.
These cams, by the revolution of the arbor, cause the alternating movements of a horizontal bar of wood o, u, which strikes at the point u against a table d, b, c, u. This table is suspended by two chains t, at its superior end, and by two rods at its lower end. After having been pushed by the piece o, u, it rebounds to strike against a block or bracket B. A lever p, q, serves to adjust the inclination of the movable table, the pivots q being points of suspension.
The ore-sand to be washed, is placed in the chest a, into which a current of water runs. The ore floated onwards by the water, is carried through a sieve on a sloping small table x, under which is concealed the higher end of the movable table d, b, c, u; and it thence falls on this table, diffusing itself uniformly over its surface. The particles deposited on this table form an oblong talus (slope) upon it; the successive percussions that it receives, determine the weightier matters, and consequently those richest in metal, to accumulate towards its upper end at u. Now the workman by means of the lever p, raises the lower end d a little in order to preserve the same degree of inclination to the surface on which the deposit is strewed. According as the substances are swept along by the water, he is careful to remove them from the middle of the table towards the top, by means of a wooden roller. With this intent, he walks on the table d b c u, where the sandy sediment has sufficient consistence to bear him. When the table is abundantly charged with the washed ore, the deposit is divided into three bands or segments d b, b c, c u. Each of these bands is removed separately and thrown into the particular heap assigned to it. Every one of the heaps thus formed becomes afterwards the object of a separate manipulation on a percussion table, but always according to the same procedure. It is sufficient in general to pass twice over this table the matters contained in the heap, proceeding from the superior band c u, in order to obtain a pure schlich; but the heap preceding from the intermediate belt b c, requires always a greater number of manipulations, and the lower band d b still more. These successive manipulations are so associated that eventually each heap furnishes pure schlich, which is obtained from the superior band c u. As to the lightest particles which the water sweeps away beyond the lower end of the percussion table, they fall into conduits; whence they are lifted to undergo a new manipulation.
[Fig. 676.] is a profile of a plan which has been advantageously substituted, in the Hartz, for that part of the preceding apparatus which causes the jolt of the piece o u against the table d b c u. By means of this plan, it is easy to vary, according to the circumstances of a manipulation always delicate, the force of percussion which a bar x y, ought to communicate by its extremity y. With this view, a slender piece of wood u is made to slide in an upright piece, v x, adjusted upon an axis at v. To the piece u a rod of iron is connected, by means of a hinge z; this rod is capable of entering more or less into a case or sheath in the middle of the piece v x, and of being stopped at the proper point, by a thumb-screw which presses against this piece. If it be wished to increase the force of percussion, we must lower the point z; if to diminish it, we must raise it. In the first case, the extremity of the piece u, advances so much further under the cam of the driving shaft t; in the second, it goes so much less forwards; whereby the adjustment is produced.
[Figs. 677.] and [678.] represent a complete system of sleeping tables, tables dormantes; such as are mounted in Idria. [Fig. 678.] is the plan, and [fig. 677.] a vertical section. The mercurial ores, reduced to a sand by stamps like those of [fig. 672.], pass into a series of conduits a a, b b, c c, which form three successive floors below the level of the floor of the works. The sand taken out of these conduits is thrown into the cells q; whence they are transferred into the trough e, and water is run upon them by turning two stopcocks for each trough. The sand thus diffused upon each table, runs off with the water by a groove f, comes upon a sieve h, spreads itself upon the board g, and thence falls into the slanting chest, or sleeping table i k. The under surface k of this chest is pierced with holes, which may be stopped at pleasure with wooden plugs. There is a conduit m, at the lower end of each table, to catch the light particles carried off by the water out of the chest i k, through the holes properly opened, while the denser parts are deposited upon the bottom of this chest. A general conduit n passes across at the foot of all the chests i k; it receives the refuse of the washing operations.