Agricola, Georgius, De re metallica, 1912/1950

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            <p type="caption">
              <s>
                <pb pagenum="210"/>
              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.</s>
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            <p type="main">
              <s>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. </s>
              <s>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. </s>
              <s>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. </s>
              <s>This blow-hole has a flap-valve, which the
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              noxious blast opens, as often as it passes out. </s>
              <s>Since one volume of air con­
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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. </s>
              <s>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. </s>
              <s>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.</s>
            </p>
            <p type="main">
              <s>There are two different machines for operating, by means of horses, the
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              above described bellows. </s>
              <s>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. </s>
              <s>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. </s>
              <s>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. </s>
              <s>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.</s>
            </p>
            <p type="main">
              <s>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. </s>
              <s>Here, also, the nozzles of
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              the bellows placed in the conduits blow a blast into the shaft or tunnel.</s>
            </p>
            <p type="main">
              <s>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
                <emph type="sup"/>
              20
                <emph.end type="sup"/>
              has explained; the air not only grows </s>
            </p>
          </chap>
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    </archimedes>
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