Agricola, Georgius
,
De re metallica
,
1912/1950
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shaft will reach the bottom of the tunnel when it has been sunk a further
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eight fathoms.</
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<
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>A TRIANGLE HAVING ALL ITS ANGLES ACUTE AND ITS THREE SIDES UNEQUAL.</
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<
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>This is the method of the surveyor in measuring the mountain, if the
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principal vein descends inclined into the depths of the earth or the transverse
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vein is vertical. </
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>
<
s
>But if they are both inclined, the surveyor uses the same
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method, or he measures the slope of the mountain separately from the slope
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of the shaft. </
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>
<
s
>Next, if a transverse vein in which a tunnel is driven does not
<
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cut the principal vein in that spot where the shaft is sunk, then it is necessary
<
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for the starting point of the survey to be in the other shaft in which the
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transverse vein cuts the principal vein. </
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>
<
s
>But if there be no shaft on that spot
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where the outcrop of the transverse vein cuts the outcrop of the principal
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vein, then the surface of the ground which lies between the shafts must
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be measured, or that between the shaft and the place where the outcrop of
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the one vein intersects the outcrop of the other.</
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<
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>Some surveyors, although they use three cords, nevertheless ascertain
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only the length of a tunnel by that method of measuring, and determine
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the depth of a shaft by another method; that is, by the method by
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which cords are re-stretched on a level part of the mountain or in
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a valley, or in flat fields, and are measured again. </
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>
<
s
>Some, however, do
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not employ this method in surveying the depth of a shaft and the
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length of a tunnel, but use only two cords, a graduated hemicycle
<
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18
<
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and a
<
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rod half a fathom long. </
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>
<
s
>They suspend in the shaft one cord, fastened
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from the upper pole and weighted, just as the others do. </
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>
<
s
>Fastened to the
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upper end of this cord, they stretch another right down the slope of the mountain
<
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to the bottom of the mouth of the tunnel and fix it to the ground. </
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>
<
s
>Then to
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the upper part of this second cord they apply on its lower side the broad part
<
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/>
of a hemicycle. </
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>
<
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>This consists of half a circle, the outer margin of which is
<
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covered with wax, and within this are six semi-circular lines. </
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<
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>From the </
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