MINING. 117 
of the roof. After the uprights are placed, covering-boards are fastened 
upon the caps and upon the sides behind the uprights, in such a manner 
that they may lap over towards the rock. Where the pressure of the rock 
is not very great, the second upright is placed about three yards from the 
first. When the pressure is more considerable, an auxiliary upright is 
placed half way between the two. 
When the level is very wide and the cross-beam does not appear suffi- 
ciently strong to resist the pressure, it is strengthened by means of braces 
of joists, which meet under the centre of the cross-beam and rest on the sides 
of the uprights. Where a level is employed for ventilation, draining, and 
often even for mining itself, the timber-work has an appearance like that 
represented in pl. 25, jig. 18, in which both the uprights lean against 
each other at the top, and stand below on a horizontal beam or sill. This 
timber-work is simple and cheap, and requires but little room. This method 
of timbering is much used in the copper mines of Cornwall. Another kind 
of timber-work used in rubble-stone which has but little pressure, is repre- 
sented in jig.11b. It consists of four posts or planks, from 2 to 23 inches 
thick and 12 or 15 inches broad. These planks cover four sides of the level, 
and are so placed that the ends of the upright planks are behind the ends 
of the other two. The planks are held together at the corners by square 
blocks or pieces of joist, against which the upright planks are nailed. 
There should be no empty space between the planks and the ground in 
which the excavation is made. 
If the ground is so soft that it will not sustain itself at the least distance 
from the upright posts, the timber-work must, to a certain extent, precede 
the mining operations. The process adopted when certain strata of sand or 
clay are entirely pervaded by water, forming marshy or what is called com- 
pressible soil, is as follows. Two upright posts with a cap are placed in the 
level to be driven. If the bottom is not solid they must be placed upon a 
sill. When a square frame has thus been set up, a covering of plank piles, 
or sheeting piles, is driven in around the frame. The sheeting piles must 
always be introduced at a slight divergency, so that the whole piling may 
have the form of a truncated pyramid, the smaller end of which embraces 
the first frame set up. If the ground is not very soft, as soon as the piling 
is inserted the level may be driven onwards 20 or 24 inches, after which a 
second piling is placed exactly like the first. In driving the level care 
must be taken to keep the course perfectly true. The divergency of the 
piling must be preserved. The piling is kept at some distance from 
the second frame by wooden wedges, which are driven in between the piles 
and the frame. The piles are afterwards driven further into the ground by 
beetles, and then the mining of the level is further carried on until a third 
frame is set up. The piles should not be longer than 6 or 8 ft., and, there- 
fore, after the fourth frame is set up, new piles must be placed. The second 
set of piles lie on the frame and below the ends of the first, so that wedges 
may be driven in between them. As an example of this mode of working 
in marshy ground, we will describe the operations in the mine of argenti- 
ferous lead ore, called the Frederick mine, at Tarnowitz, in Upper Silesia 
ny 
