Agricola, Georgius, De re metallica, 1912/1950

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    <archimedes>
      <text>
        <body>
          <chap>
            <p type="main">
              <s>
                <pb pagenum="141"/>
              tunnel vertically or inclined, in an uninterrupted c
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              se. </s>
              <s>The same is true
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              when a tunnel runs straight on to a shaft. </s>
              <s>But when each of them bends
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              now in this, now in that direction, if they have not been completely driven
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              and sunk, no living man is clever enough to judge how far they are deflected
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              from a straight course. </s>
              <s>But if the whole of either one of the two has been ex­
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              cavated its full distance, then we can estimate more easily the length of one,
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              or the depth of the other; and so the location of the tunnel, which is below
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              a newly-started shaft, is determined by a method of surveying which I will
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              describe. </s>
              <s>First of all a tripod is fixed at the mouth of the tunnel, and likewise at
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              the mouth of the shaft which has been started, or at the place where the shaft will
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              be started. </s>
              <s>The tripod is made of three stakes fixed to the ground, a small
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              rectangular board being placed upon the stakes and fixed to them, and on
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              this is set a compass. </s>
              <s>Then from the lower tripod a weighted cord is let
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              down perpendicularly to the earth, close to which cord a stake is fixed in the
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              ground. </s>
              <s>To this stake another cord is tied and drawn straight into the tunnel
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              to a point as far as it can go without being bent by the hangingwall or the
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              footwall of the vein. </s>
              <s>Next, from the cord which hangs from the lower tripod,
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              a third cord likewise fixed is brought straight up the sloping side of the
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              mountain to the stake of the upper tripod, and fastened to it. </s>
              <s>In order that
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              the measuring of the depth of the shaft may be more certain, the third cord
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              should touch one and the same side of the cord hanging from the lower tripod
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              which is touched by the second cord—the one which is drawn into the tunnel.
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              </s>
              <s>All this having been correctly carried out, the surveyor, when at length
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              the cord which has been drawn straight into the tunnel is about to be bent
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              by the hangingwall or footwall, places a plank in the bottom of the tunnel
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              and on it sets the orbis, an instrument which has an indicator peculiar
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              to itself. </s>
              <s>This instrument, although it also has waxed circles, differs from the
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              other, which I have described in the third book. </s>
              <s>But by both these
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              instruments, as well as by a rule and a square, he determines whether the
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              stretched cords reach straight to the extreme end of the tunnel, or whether
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              they sometimes reach straight, and are sometimes bent by the footwall or
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              hangingwall. </s>
              <s>Each instrument is divided into parts, but the compass into
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              twenty-four parts, the orbis into sixteen parts; for first of all it is divided
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              into four principal parts, and then each of these is again divided into four.
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              </s>
              <s>Both have waxed circles, but the compass has seven circles, and the orbis
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              only five circles. </s>
              <s>These waxed circles the surveyor marks, whichever instru­
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              ment he uses, and by the succession of these same marks he notes any
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              change in the direction in which the cord extends. </s>
              <s>The orbis has an open­
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              ing running from its outer edge as far as the centre, into which opening he
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              puts an iron screw, to which he binds the second cord, and by screwing it into
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              the plank, fixes it so that the orbis may be immovable. </s>
              <s>He takes care
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              to prevent the second cord, and afterward the others which are put up,
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              from being pulled off the screw, by employing a heavy iron, into an opening
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              of which he fixes the head of the screw. </s>
              <s>In the case of the compass, since
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              it has no opening, he merely places it by the side of the screw. </s>
              <s>That the
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              instrument does not incline forward or backward, and in that way the </s>
            </p>
          </chap>
        </body>
      </text>
    </archimedes>
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