124 TECHNOLOGY. 
which span the gallery into which the shaft enters. When the shaft is 
very wide, as when the ascent-shaft, service-shaft, and water-shaft, are 
contained in one, a single arch would require a great excavation into the 
strata over-head, and great thickness; it is preferable, therefore, to build 
one or two partition-walls, and to arch each part of the shaft separately. 
When at any part of the mines a powerful spring is encountered which 
cannot be cut off at a higher point, it may sometimes be backed up by a 
dam or bulkhead until it breaks out at the surface. This method can only 
be resorted to when there are no clefts in the formation through which the 
water might issue at another point, or perhaps even at a greater depth. A 
bulkhead for this purpose consists generally of strong beams of oak timber, 
closely fitted and caulked with moss or oakum with the aid of caulking- 
irons (pl. 24, jig. 29), and then wedged with wooden wedges, which are 
inserted by the aid of a kind of chisel, shown in jig. 28. A bulkhead of 
this kind, built in the lead mine of Huelgoet, in France, is represented in 
Jigs. 26 and 27. For the purpose of caulking on the water-side, a hole was 
left in the centre, which was afterwards closed with a wedge-shaped block 
of beech wood. The space behind the bulkhead was filled with concrete 
made with hydraulic cement. The ends of the beams bear against a rectan- 
gular offset in the rock on both sides of the bulkhead, which is, moreover, 
stiffened by braces on the outside. 
When a shaft in a wet mine is abandoned, it is often necessary to shut 
off the water from the mine by a bulkhead, across the shaft below the wet 
strata, in the solid formation. The part of the shaft above the bulkhead is 
generally filled up with rock. A horizontal dam or bulkhead of this kind 
may be built of masonry or timber; in the former case the spherical form 
is to be preferred; a bulkhead of timber is represented in pl. 24, jigs. 22 
and 23. The ends of the beams and the side beams are bevelled, and 
rest on a shoulder in the rock, as seen in the plate, where they are wedged 
tight ; the centre or key-beam is held in its proper position by means of a 
strong iron eye-bolt, b, while the whole bulkhead is caulked and firmly 
wedged up. Any subsequent fleam of the timber is prevented by bracing 
on top of the bulkhead. 
6. WorkING THE MIEs. 
Mines may be divided into two classes according as they are worked. In the 
first the economical minerals are found in connexion with the gangue and 
sterile rocks, which are separated in the mines, and are suitable for walling 
and protecting the passages. The second class embraces those mines in 
which the deads are not serviceable for the above purpose. To the first 
class belong most metallic mines, and to the second, mines of coal, salt, &c. 
Mines may be worked either by open or subterranean excavation. The 
former are the least expensive, when not pushed to a great depth; and are 
preferred for mineral deposits lying near the surface, for building materials, 
lime, &e. | 
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