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the ordinary mounting a masonry pedestal with a perfectly 

 level top is employed to support the instrument during the 

 observations. The location is made by means of triangulation. 

 The various operations must be performed with the greatest 

 accuracy, and repeated several times in such a way as to reduce 

 the errors to a minimum, since the final meeting of the head- 

 ings depends upon their elimination. 



The St. (Jothard tunnel furnishes perhaps the best illus- 

 tration of careful work in locating the center line of long recti- 

 linear tunnels of any tunnel ever built. The length of this 

 tunnel is 9.25 miles, and the height of the mountain above it 

 is very great. The center line was located by triangulation by 



FIG. 2. Triangulation System for KtUabllnhlng the Cnter Line of the St. Gothurd Tunnel. 



two different astronomers using different sets of triangles, and 

 working at ditTeivni times. The set or system of triangles used 

 by Dr. Knppe, one of the observers, is sho\\n by Fig. -: it ei in- 

 sists of very large and quite small triangles combined, the 

 latter heing required beCBUee the entrances both at Airolo and 

 Goeschenen were so low as to permit only of a short sight 

 being taken. The apices of the triangles were located by means 

 of the contour maps of the Swiss Alpine flub. Kach angle 

 was read ten times, the instrument was collimated four times 

 for each reading, and was afterwards turned off "> or in t,, 

 avoid errors of graduation. The average of the errors in read- 

 ing was about oiie second of arc. The triangulation was 



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