Agricola, Georgius
,
De re metallica
,
1912/1950
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tunnel vertically or inclined, in an uninterrupted c
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se. </
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<
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>The same is true
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when a tunnel runs straight on to a shaft. </
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<
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>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. </
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<
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>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. </
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<
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>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. </
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<
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>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. </
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<
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>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. </
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<
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>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. </
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<
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>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. </
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<
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>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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<
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>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. </
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<
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>This instrument, although it also has waxed circles, differs from the
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other, which I have described in the third book. </
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<
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>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. </
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<
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>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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<
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>Both have waxed circles, but the compass has seven circles, and the orbis
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only five circles. </
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<
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>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. </
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<
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>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. </
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<
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>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. </
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<
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>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. </
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<
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>That the
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instrument does not incline forward or backward, and in that way the </
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