118 TECHNOLOGY. 
(pl. 25, jigs. 15-17). When marshy ground is met with in driving the 
levels it is shut in to prevent the caving in of the sand and clay. A square 
frame, like that described above, is placed against the marshy soil which is 
to be driven through, and behind it cross-boards, forming a bulkhead, are 
placed, which are kept up by the frame (jigs. 15, 16). If the bottom is 
bad, the uprights are set on a wooden sill, formed of a half-round board 
split from a log 16 or 20 inches in diameter. The flat side is placed on the 
ground. Pieces of board are sometimes placed below the sill to give it 
more support. ‘The sill is made as long as possible, in order to rest in the 
earth on each side. After the sill has received its position, the two uprights 
are set up accurately perpendicular, and are bound together in the usual 
manner by a cap. The ends of the cap do not project beyond the uprights, 
as is seen in jig. 15. After the frame has been put in place, sheeting piles 
of plank are placed around the uprights and the cap, and are retained at 
the requisite distance from the frame by wedges. The wedges lie on the 
piles already fastened in, and by driving the wedges the position of the 
frame can be accurately adjusted. The piles are then driven in, com- 
mencing with the two which rest on the upper corners of the frame, being 
made broadest at the end which is driven in. The cap is first covered, and 
afterwards the uprights. In this mine, where the pressure was very strong, 
piles formed of plate-iron were used. 
To carry on the mining, the topmost of the planks which shut up the 
end of the level, as is shown in jig. 15, is lifted up or moved from side to 
side, and as much of the earth taken away as can be done without reaching 
the end of the piling. This plank is afterwards shoved further forward and — 
fastened by two short braces or ties to the last frame which has been set up. 
The plank when pushed forward is somewhat raised, that it may touch the 
piling with its upper edge. The section of the planks and side elevation 
of the braces or ties are shown in pl. 25, fig. 16. If the ground is very 
soft or marshy, the water is drained off, and this drainage so regulated that 
the water may be withheld at pleasure; otherwise it might fill the level and 
displace the frame. As soon as the workman sees that a sufficient quantity 
of water has flowed out, he presses back the plank, or stuffs in a bundle of 
straw. Short braces are driven in by hammers between the uprights and 
the plank, in order to push forward the latter. When the highest plank has 
in this way been shoved forward, there remains between it and the one 
which is under it, and which has not been pushed forward so far, an empty 
space, through which the mud sometimes flows out. This must be kept 
under command. In this way, by a successive pushing on of the planks, 
the end of the piling is almost reached. Then another main frame must 
be forced in and new piling fastened by wedges. If it is apprehended that 
the framework is not strong enough to resist the pressure, strong pieces or 
longitudinal beams are placed under the caps, and on the sills and between 
them perpendicular posts are placed, as is shown in jigs. 15 and 16. 
In ground where the pressure is very great much is accomplished if even 
a narrow passage can be effected, because by opening and draining the 
ground it is prepared for subsequent working. For this purpose, at Tarno- 
698 
