Agricola, Georgius
,
De re metallica
,
1912/1950
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avoid wearing away their clothes and injuring their left shoulders they
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usually bind on themselves small wooden cradles. </
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<
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>For this reason, this
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particular class of miners, in order to use their iron tools, are obliged to bend
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their necks to the left, not infrequently having them twisted. </
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<
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>Now these
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veins also sometimes divide, and where these parts re-unite, ore of a richer and
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a better quality is generally found; the same thing occurs where the stringers,
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of which they are not altogether devoid, join with them, or cut them crosswise,
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or divide them obliquely. </
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<
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>To prevent a mountain or hill, which has in
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this way been undermined, from subsiding by its weight, either some natural
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pillars and arches are left, on which the pressure rests as on a foundation, or
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timbering is done for support. </
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<
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>Moreover, the materials which are dug out
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and which are devoid of metal are removed in bowls, and are thrown back,
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thus once more filling the caverns.</
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<
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>Next, as to
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venæ cumulatæ.
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<
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> These are dug by a somewhat different
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method, for when one of these shows some metal at the top of the ground,
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first of all one shaft is sunk; then, if it is worth while, around this one many
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shafts are sunk and tunnels are driven into the mountain. </
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<
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>If a torrent or
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spring has torn fragments of metal from such a vein, a tunnel is first driven
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into the mountain or hill for the purpose of searching for the ore; then
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when it is found, a vertical shaft is sunk in it. </
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<
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>Since the whole mountain, or
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more especially the whole hill, is undermined, seeing that the whole of it is
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composed of ore, it is necessary to leave the natural pillars and arches, or the
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place is timbered. </
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<
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>But sometimes when a vein is very hard it is broken by
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fire, whereby it happens that the soft pillars break up, or the timbers are
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burnt away, and the mountain by its great weight sinks into itself, and then
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the shaft buildings are swallowed up in the great subsidence. </
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<
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>Therefore,
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about a
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vena cumulata
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it is advisable to sink some shafts which are not sub
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ject to this kind of ruin, through which the materials that are excavated may
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be carried out, not only while the pillars and underpinnings still remain whole
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and solid, but also after the supports have been destroyed by fire and have
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fallen. </
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<
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>Since ore which has thus fallen must necessarily be broken by fire,
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new shafts through which the smoke can escape must be sunk in the abyss.
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</
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<
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>At those places where stringers intersect, richer ore is generally obtained
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from the mine; these stringers, in the case of tin mines, sometimes have in
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them black stones the size of a walnut. </
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<
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>If such a vein is found in a plain,
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as not infrequently happens in the case of iron, many shafts are sunk, because
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they cannot be sunk very deep. </
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<
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>The work is carried on by this method
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because the miners cannot drive a tunnel into a level plain of this kind.</
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<
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>There remain the stringers in which gold alone is sometimes found,
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in the vicinity of rivers and streams, or in swamps. </
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<
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>If upon the soil being
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removed, many of these are found, composed of earth somewhat baked and
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burnt, as may sometimes be seen in clay pits, there is some hope that gold
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may be obtained from them, especially if several join together. </
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>
<
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>But the
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very point of junction must be pierced, and the length and width searched
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for ore, and in these places very deep shafts cannot be sunk.</
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<
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>I have completed one part of this book, and now come to the other, in
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which I will deal with the art of surveying. </
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<
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>Miners measure the solid </
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</
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