[Figs. 719.] [720.] [721.] represent the principal kinds of mason-work employed in the galleries and shafts of mines. [Fig. 722.] exhibits the walling in of the cage of an overshot water-wheel, as mounted within a mine. Before beginning to build, an excavation large enough must be made in the gallery to leave a space three feet and a half high for the workmen to stand in, after the brick-work is completed. Between the two opposite sides, cross beams of wood must be fixed at certain distances, as chords of the vault, over which the rock must be hollowed out to receive the arch-stones, and the centring must then be placed, covered with deals to receive the voussoirs, beginning at the flanks and ending with the key-stone. When the vault is finished through a certain extent, the interval between the arch and the rock must be rammed full of rubbish, leaving passages if necessary through it and the arch, for currents of water.

In walling galleries, attention must be paid to the direction of the pressure, and to build vertically or with a slope accordingly. Should the pressure be equal in all directions, a closed vault, like [fig. 719.], should be formed. For walls not far from the vertical, salient or buttressed arches are employed, as shown in [fig. 720.], called in German überspringende bogen; for other cases, twin-arches are preferred, with an upright wall between.

[Fig. 721.] is a transverse section of a walled drain-gallery, from the grand gallery of the Hartz; see also [fig. 722.] a is the rock which needs to be supported only at the sides and top; b, the masonwork, a curve formed of the three circular arcs upon one level; c, the floor for the watercourse. [Fig. 719.] is a cross section of a walled gallery, as at Schneeberg, Rothenburg, Idria, &c.; d, is the rock, which is not solid either at the flanks, roof, or floor; e, the elliptic masonwork; f, the wooden floor for the waggons, which is sometimes, however, arched in brick to allow of a watercourse beneath it.

[Fig. 720.] shows two vertical projections of a portion of a walled shaft with buttresses, as built at the mine Vater Abraham, near Marienberg. J is a section in the direction of the vein g h, to show the roof of the shaft. I, a section exhibiting the slope of the vein g h, into which the shaft is sunk; m is the wall of the vein; k is the roof of the same vein; n, buttresses resting upon the flanks of the shaft; g, great arcs on which the buttresses bear; y, vertical masonwork; z, a wall which divides the shaft into two compartments, of which the larger p is that for extracting the ore, and the smaller for the draining and descent of the miners.

[Fig. 722.] C D is the shaft in which the vertical crank-rods c g, e d, move up and down. F, is a double hydraulic wheel, which can be stopped at pleasure by a brake mounted upon the machine of extraction. G, is the drum of the gig or whim for raising the corves or tubs (tonnes); H, is the level of the ground, with the carpentry which supports the whim and its roof. k, is the key-stone of the ogee arch which covers the water-wheel; a, is the opening or window, traversed by the extremity of the driving shaft, upon each side of the water-wheel, through which a workman may enter to adjust or repair it; c b, line of conduits for the streams of water which fall upon the hydraulic wheel; c, g, double crank with rods, whose motion is taken off the left side of the wheel; e, d, the same upon the right side. The distance from H to F is about 22 yards.

[Figs. 723.] [724.] present two vertical sections of the shaft of a mine walled, like the roof of a cavern, communicating with the galleries of the roof and the wall of the vein, and well arranged for both the extraction of the ore, and the descent of the miners. The vertical partition of the shaft for separating the passage for the corves or tubs from the ladders is omitted in the figure, for the sake of clearness.

[Fig. 723 and 724 enlarged] (264 kB)