Agricola, Georgius
,
De re metallica
,
1912/1950
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always drives forward another, they penetrate into the tunnel and change
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the air, whereby the miners are enabled to continue their work.</
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<
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>If heavy vapours need to be drawn off from the tunnels, generally three
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double or triple bellows, without nozzles and closed in the forepart, are placed
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upon benches. </
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<
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>A workman compresses them by treading with his feet, just
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as persons compress those bellows of the organs which give out varied and
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sweet sounds in churches. </
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<
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>These heavy vapours are thus drawn along the
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air-pipes and through the blow-hole of the lower bellows board, and are
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expelled through the blow-hole of the upper bellows board into the open
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air, or into some shaft or drift. </
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<
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>This blow-hole has a flap-valve, which the
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noxious blast opens, as often as it passes out. </
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<
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>Since one volume of air conĀ
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stantly rushes in to take the place of another which has been drawn out by
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the bellows, not only is the heavy air drawn out of a tunnel as great as 1,200
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feet long, or even longer, but also the wholesome air is naturally drawn in
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through that part of the tunnel which is open outside the conduits. </
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<
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>In this way
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the air is changed, and the miners are enabled to carry on the work they have
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begun. </
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<
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>If machines of this kind had not been invented, it would be necessary
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for miners to drive two tunnels into a mountain, and continually, at every
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two hundred feet at most, to sink a shaft from the upper tunnel to the
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lower one, that the air passing into the one, and descending by the shafts
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into the other, would be kept fresh for the miners; this could not be done
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without great expense.</
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<
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>There are two different machines for operating, by means of horses, the
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above described bellows. </
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<
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>The first of these machines has on its axle a
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wooden wheel, the rim of which is covered all the way round by steps; a
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horse is kept continually within bars, like those within which horses are held
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to be shod with iron, and by treading these steps with its feet it turns the wheel,
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together with the axle; the cams on the axle press down the sweeps which
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compress the bellows. </
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<
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>The way the instrument is made which raises the
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bellows again, and also the benches on which the bellows rest, I will explain
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more clearly in Book IX. </
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<
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>Each bellows, if it draws heavy vapours
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out of a tunnel, blows them out of the hole in the upper board; if they are
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drawn out of a shaft, it blows them out through its nozzle. </
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<
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>The wheel has
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a round hole, which is transfixed with a pole when the machine needs to be
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stopped.</
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<
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>The second machine has two axles; the upright one is turned by a horse,
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and its toothed drum turns a drum made of rundles on a horizontal axle;
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in other respects this machine is like the last. </
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<
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>Here, also, the nozzles of
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the bellows placed in the conduits blow a blast into the shaft or tunnel.</
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<
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>In the same way that this last machine can refresh the heavy air of a
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shaft or tunnel, so also could the old system of ventilating by the constant
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shaking of linen cloths, which Pliny
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20
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has explained; the air not only grows </
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