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214 BLASTING GRAVEL BANKS.
of
a blow-out. The tamping must be firmly rammed by wooden mauls, so that
it will not settle from the roof of the drift. In order to guard
against failure through defective fuse it is customary to use two or
three lines, which are simultaneously ignited.
Firing
by electricity has the advantage of requiring less tamping and of
permitting it to be placed in the cross drifts between the two chambers
of powder, which are simultaneously fired—a result that could not be
effected by fuse. The force from the explosion from the two chambers,
acting upon the tamping from opposite sides, prevents its being blown
out; and therefore when drifts are fired in this way it is necessary to
tamp but a short distance in the cross drifts and but a few feet in the
main drift.
Owing,
however, to the many failures arising from defective batteries and
connections, the miners generally have abandoned the use of the
electric battery.
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