THE ST. GOT HARD TUNNEL. 



53 1 



48,750 feet long. The engineers of the tunnel of Mont Cenis had a 

 point at the highest part of the ground from which they could see at 

 once objects indicating the positions of both openings. No such ad- 

 vantage existed at St. Gothard, and some of the summits are so steep 



Fig. 1. Profile along the Length op the Tunnel. 



and high that it is impracticable to attempt any direct tracing of the 

 line over the mountain. The relative position of the two openings and 

 the direction of the tunnel had then to be calculated indirectly, from 

 trianomlations. The directions and levels were thus ascertained. Ob- 

 servatories were then placed at the tunnel-mouths to serve as direc- 

 tion-points for the miners. At Goeschenen it was necessary, in order 

 to get a long enough line of sight, to make borings of considerable 

 length through two projecting rocks. The surveys, originally made 

 by M. Gelpke, were verified by a second series of triangulations made 

 in 187-i by another engineer, M. Koppe, on a different system. M. 

 Gelpke had based his surveys on summits in the neighborhood, and had 

 used triangles of only moderate size. M. Koppe made his triangles as 

 large as possible, so that he might connect the two openings of the 

 tunnel by a minimum number of intermediary stations. The two tri- 

 angulations gave results agreeing within two seconds of direction with 

 each other. M. Koppe also verified his survey practically by project- 

 ing a line from the opening at Airolo toward a mast which he set up 

 at the highest attainable point along the axis of the tunnel. He could 

 not go toward this point from Goeschenen, so he went backward in 

 the direction of the continuation of the tunnel-axis, ascending the 

 flanks of the mountain till he coitld observe his mast. Then, having 

 directed his glass toward Goeschenen, he raised it vertically to the 

 level of the mast, when he saw it almost in the center of his field of 

 vision. The direction within the tunnel was verified by means of 

 field-glasses fixed within the observatories, so far as they would an- 

 swer, then by means of lamps hung on the line of the axis. The direc- 

 tion was, moreover, carefully verified from the observatories two or 

 three times a year. 



The borings were made almost entirely by machines, and it was the 

 policy of M. Louis Favre, the contractor for the tunnel, to dispense 

 with hand-boring as far as possible. The machines were driven by 

 water-power transmitted into the tunnel by means of the compressed- 



