308 



MINING 



arch is sufficient to sustain the roof, and at other times the whole surface of a gallery 

 is formed of a single elliptic vault, the great axis of which is vertical ; and the bottom 

 is surmounted by a -wooden plank, under which the waters run off: see* fig. 1458. 



Walled shafts also are sometimes constructed in a circular or elliptic form, which is 

 better adapted to resist the pressure of the earth and waters. Rectangular shafts of 

 all dimensions, however, are frequently walled. 



The sides of an excavation may also be supported by filling it completely with 

 rubbish. Whenever the sides need to be supported for some time without the neces- 

 sity of passing along them, it is often more economical to stuff them up with rubbish, 

 than to keep up their supports. In the territory of Liege, for example, there have 

 been shafts thus filled up for several centuries ; and which are found to be quite entire 

 when they are emptied. The rubbish is also useful for forming roads among steep 

 strata, for closing air-holes, and forming canals of ventilation. 



Figs. 1458, 1459, 1460, represent the principal kinds of mason-work employed in 

 the galleries and shafts of mines. Fig. 1461 exhibits the walling in of the cage of an 



1459 



1458 



1461 



overshot water-wheel, as mounted within a mine. Before beginning to build, an ex- 

 cavation large enough must be made in the gallery to leave a space three feet and a 

 half high for the workman 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 keystone. 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. 1458, should be formed. For walls not far from 

 the vertical, salient or buttressed arches are employed, as shown in fig. 1460, called in 

 German uberspringende Bogen ; for other cases, twin-arches are preferred, with an 

 upright wall between. 



Fig. 1459 is a transverse section of a walled drain-gallery, from the grand gallery 

 of the Hartz ; see &\sofig. 1461. a is the rock which needs to bo 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 water-course. Fig. 1458 is a cross-section of a walled gal- 

 lory, 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 ; /, the wooden floor 



