Agricola, Georgius
,
De re metallica
,
1912/1950
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out because of the hardness or other difficulty, and the drift or tunnel is
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low, a heap of dried logs is placed against the rock and fired; if the drift or
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tunnel is high, two heaps are necessary, of which one is placed above the
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other, and both burn until the fire has consumed them. </
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<
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>This force does not
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generally soften a large portion of the vein, but only some of the surface.
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</
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<
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>When the rock in the hanging or footwall can be worked by the iron tools
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and the vein is so hard that it is not tractable to the same tools, then the
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walls are hollowed out; if this be in the end of the drift or tunnel or above
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or below, the vein is then broken by fire, but not by the same method; for
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if the hollow is wide, as many logs are piled into it as possible, but if narrow,
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only a few. </
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<
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>By the one method the greater fire separates the vein more
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completely from the footwall or sometimes from the hangingwall, and by the
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other, the smaller fire breaks away less of the vein from the rock, because in
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that case the fire is confined and kept in check by portions of the rock which
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surround the wood held in such a narrow excavation. </
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<
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>Further, if the
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excavation is low, only one pile of logs is placed in it, if high, there are
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two, one placed above the other, by which plan the lower bundle being
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kindled sets alight the upper one; and the fire being driven by the draught
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into the vein, separates it from the rock which, however hard it may be, often
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becomes so softened as to be the most easily breakable of all. </
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<
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>Applying this
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principle, Hannibal, the Carthaginian General, imitating the Spanish miners, </
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