BOOK V. 141 



tunnel vertically or inclined, in an uninterrupted course. The same is true 

 when a tunnel runs straight on to a shaft. But when each of them bends 

 now in this, now in that direction, if they have not been completely driven 

 and sunk, no living man is clever enough to judge how far they are deflected 

 from a straight course. But if the whole of either one of the two has been ex- 

 cavated its full distance, then we can estimate more easily the length of one, 

 or the depth of the other ; and so the location of the tunnel, which is below 

 a newly-started shaft, is determined by a method of surveying which I will 

 describe. First of all a tripod is fixed at the mouth of the tunnel, and likewise at 

 the mouth of the shaft which has been started, or at the place where the shaft will 

 be started. The tripod is made of three stakes fixed to the ground, a small 

 rectangular board being placed upon the stakes and fixed to them, and on 

 this is set a compass. Then from the lower tripod a weighted cord is let 

 down perpendicularly to the earth, close to which cord a stake is fixed in the 

 ground. To this stake another cord is tied and drawn straight into the tunnel 

 to a point as far as it can go without being bent by the hangingwaU or the 

 footwall of the vein. Next, from the cord which hangs from the lower tripod, 

 a third cord likewise fixed is brought straight up the sloping side of the 

 mountain to the stake of the upper tripod, and fastened to it. In order that 

 the measuring of the depth of the shaft may be more certain, the third cord 

 should touch one and the same side of the cord hanging from the lower tripod 

 which is touched by the second cord — the one which is drawn into the tunnel. 

 All this having been correctly carried out, the surveyor, when at length 

 the cord which has been drawn straight into the tunnel is about to be bent 

 by the hangingwaU or footwall, places a plank in the bottom of the tunnel 

 and on it sets the orbis, an instrument which has an indicator pecuUar 

 to itself. This instrument, although it also has waxed circles, differs from the 

 other, which I have described in the third book. But by both these 

 instruments, as weU as by a rule and a square, he determines whether the 

 stretched cords reach straight to the extreme end of the tunnel, or whether 

 they sometimes reach straight, and are sometimes bent by the footwall or 

 hangingwaU. Each instrument is divided into parts, but the compass into 

 twenty-four parts, the orbis into sixteen parts ; for first of aU it is divided 

 into four principal parts, and then each of these is again divided into four. 

 Both have waxed circles, but the compass has seven circles, and the orbis 

 only five circles. These waxed circles the surveyor marks, whichever instru- 

 ment he uses, and by the succession of these same marks he notes any 

 change in the direction in which the cord extends. The orbis has an open- 

 ing running from its outer edge as far as the centre, into which opening he 

 puts an iron screw, to which he binds the second cord, and by screwing it into 

 the plank, fixes it so that the orbis may be immovable. He takes care 

 to prevent the second cord, and afterward the others which are put up, 

 from being pulled off the screw, by employing a heavy iron, into an opening 

 of which he fixes the head of the screw. In the case of the compass, since 

 it has no opening, he merely places it by the side of the screw. That the 

 instrument does not incline forward or backward, and in that way the 

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