MINING 



307 



they are meant to serve. The shafts intended to be stayed with timber are usually 

 square or rectangular, because this form, in itself more convenient for the miner, ren- 

 ders the execution of the timbering more easy. The woodwork consists generally of 

 rectangular frames, the spars of which are about eight inches in diameter, and placed 

 at a distance asunder of from a yard to a yard and a half. The spars are never placed 

 in contact, except when the pressure of the earth and the water is very great. The 

 pieces composing the frames are commonly united by a half-check, and the longer of 

 the two pieces extends often beyond the angles, to be rested in the rock. Whether 

 the shaft is vertical or inclined, the framework is always placed so that its plane may 

 be perpendicular to the axis of the pit. It happens sometimes in inclined shafts that 

 there are only two sides, or even a single one, which need to be propped. These are 

 stayed by means of cross beams, which rest at their two ends in the rock. When the 

 frames do not touch one another strong planks or stakes are fastened behind them to 

 sustain the ground. To these planks the frames are firmly connected, so that they 

 cannot slide. In this case the whole timbering will be supported, when the lower 

 frame is solidly .fixed, or when the pieces from above pass by its angles to be abutted 

 upon the ground. 



In the large rectangular shafts, which serve at once for extracting the ores, for the 

 discharge of the waters, and the descent of the workmen, the spaces destined for these 

 several purposes are in general separated by partitions, which also serve to increase the 

 strength of the timberings, by acting as buttresses to the planks in the long sides of 

 the framework. Occasionally a partition separates the ascending from the descend- 

 ing basket, to prevent their jostling. Lastly, particular passages are left for ven- 

 tilation. 



As it is desirable that the wood shall retain its whole force, only those pieces are 

 squared which absolutely require it. The spars of the frames in shafts and galleries 

 are deprived merely of their bark, which, by holding moisture, would accelerate the 

 decomposition of the wood. The alburnum of oak is also removed. 



Eesinous woods, like the pine, last much shorter than the oak, the beech, and the 

 cherry-tree ; though the larch is used with advantage. The oak has been known to 

 last upwards of 40 years ; while the resinous woods decay frequently in 10. The 

 fresher the air in mines, the more durable is the timbering. 



The fys. 1456, 1457 represent two vertical sections of a shaft, the one at right 



1456 



angles to the other, with the view of showing the mode of 

 sustaining the walls of the excavation by timbering. It is 

 copied from an actual mine in the Hartz. There we may 

 observe the spaces allotted to the descent of the miners by 

 ladders, to the drainage of the waters by pumps P, and rods 

 t, and to the extraction of the mineral substances by 

 baskets, a, b, c,f, h, k, are various cross timbers; A, c, E, 

 upright do. ; E, pump cistern ; v, w, corve-ways. The 

 shafts here shown, are excavated in the line of the vein 

 itself, the rock enclosing it being seen in the second figure. 



In a great many mines it is found advantageous to 

 support the excavations by brick or stone buildings, con- 

 structed either with or without mortar. These construc- 

 tions are often more costly than wooden ones, but they 

 last much longer, and need fewer repairs. They are em- 

 ployed instead of timberings, to support the walls and roof 

 of galleries, to line the sides of shafts, and to bear up the 

 roofs of excavations. 



Sometimes the two sides of a gallery are lined with vertical walls, 

 supported by an ogee vault, or an arch. If the sides of the mine are 



x 2 



1457 



and its 

 solid, a 



roof is 

 simple 



