58a 



NATURE 



[April 22, 1880 



projecting rocks with two small bores 115 and 93 metres 

 long, and only then could observations satisfactorily be 

 made either as to direction or level. 



In each observatory was placed a large theodolite, and 

 after a very large number of angular measurements and 

 astronomical verifications, arrows and levelling-marks 

 were placed in the exact direction of the tunnel, so that 

 at every moment during the first stage of the tunnelling 



works the direction in which the miners were advancing 

 could be verified. 



In spite of the success of the verifications of M. Gelpke's 

 labours, the managers of the St. Gothard Company 

 thought it right to recommence the triangulation in 1874, 

 employing another engineer, M. Koppe, and a different 

 system. Instead of limiting himself to summits in the 

 neighbourhood, and having moderately-sized triangles 



-Plan of Triangulalion for the St. Gothard Tunnel, i 5s 's: 



(the longest side being under 7,000m.). M. Koppe preferred 

 to use as large triangles as possible, in order to connect 

 together by a minimum number of intermediary stations 

 the two tunnel openings. No doubt, since these openings 

 are at the bottom of valleys, a certain number of triangles 

 was necessary to descend from the heights ; but the sides of 

 triangles of from 10 to 15 kilometres connect directly the 



signals on the heights above Goeschenen with those of 

 the heights near Airolo, and several triangles for the 

 purpose of verification were laid down. The two opera- 

 tions proving the accuracy of the works were (1) the 

 correspondence within two seconds of M. Koppe's triangu- 

 lation with that of M. Gelpke ; (2) the following almost 

 direct and very interesting verification. M. Koppe started 



—Profile along the length of the Tunnel. 



a system of arrows from Airolo northwards along the axis the tunnel-axis. He thus ascended the flanks of a moun- 

 of the tunnel over the mountain, following the direction tain till he could observe in the distance the mast he 



given by the observatory to the highest attainable point 

 near Kastelhorn, where he erected a mast. Being unable 

 to reach this point from Goeschenen by going south, on 

 account of the local difficulties, he started northwards also 

 from that opening, in the direction of the continuation of 



had erected near Kastelhorn. Then observing the level 

 of Goeschenen with a theodolite, he moved his field-glass 

 in a vertical plane, and directing it upon Goeschenen, he 

 was delighted to see his mast almost in the centre of his 

 field of vision, at a distance from his estimated axis of 



