NATURE 



581 



THURSDAY, APRIL 22, 1880 



THE ST. GOTHARD TUNNEL 



THE news that the two advance borings had met in 

 the middle of the mountain traversed by the St. 

 Gothard tunnel resounded like a joyful echo in every 

 civilised country. It announced the success of the 

 greatest work hitherto attempted by man ; and, on the 

 completion of so important an event, the scientific public 

 must with good reason insist on having some details of 

 the gigantic labours which have excited the attention of 

 all intelligent men, and which, though well nigh finished, 

 still demand much skill and energy. 



The St. Gothard Tunnel is intended to form part of the 

 railway connecting the North Sea with the Mediter- 

 ranean; the ports of Belgium, Holland, and Germany 

 with Genoa ; the Rhine basin with that of the Po, by 

 passing through the chain of the Alps at its most central 

 point. If this point has been chosen in preference to any 

 other, it is because the line through St. Gothard is the 

 most direct. A proof that the passage is well selected is 

 that, if one compares all the roads crossing the Swiss 

 Alps, he will find that the St. Gothard one is more 

 frequented by travellers either walking or riding than all 

 the others together. 



If, therefore, the Mont Cenis railway preceded that of 

 St. Gothard, it is because there were political reasons 

 for its accomplishment ; it is because science had not 

 yet discovered the means of boring, in a relatively short 

 time, long tunnels without shafts. But the experiments 

 made in Mont Cenis being conclusive, the governments, 

 cities, companies, and private persons who were interested 

 all set to work, and the great international line subsidised 

 by Italy, Germany, and most of the Swiss cantons was 

 commenced in real earnest. 



Before the Company was formed, a most thorough 

 inspection was made on the spot, and the engineers con- 

 cluded that the great inevitable tunnel could have its 

 openings only near the small village, Goeschenen (Canton 

 Uri), on the north, and near Airolo (Canton Tessin) on the 

 south. Goeschenen is 1,109 metres above sea-level, and 

 672 above the Lake of the Four Cantons. Airolo is 1,145 

 above the sea. Considerable works, therefore, are neces- 

 sary to reach so great elevations by means of a railway. 

 But to lower the level of the tunnel by only a few metres 

 while awkwardly increasing the length, and to ascend 

 higher to lessen that length, only alleviated the difficulty 

 to a trifling extent, especially on the south slope, where 

 Airolo is almost the only possible point of entrance. 



Both openings being thus decided upon, the first thing 

 to do was to lay down the plan of the tunnel and ascertain 

 its direction and level. Thanks to the excellent topogra- 

 phical map possessed by Switzerland, it was evident, even 

 without examination made on the spot, that the tunnel 

 might be straight, and that its length would be about 

 1 5,000 metres. Afterwards it was thought preferable to 

 complete it at the southern end by a slight curve, to give 

 more facilities for building the Airolo station, but the 

 tunnel throughout was opened in a straight line, and it 

 was not till later that a curved bifurcation of 125 metres 

 was made near the mouth. 

 Vot. xxi. — No. 547 



In St. Gothard there were not the same topographical 

 advantages as in Mont Cenis. In the operations made 

 to trace the tunnel under the latter, there was at the 

 highest part of the ground a starting-point from which 

 could be seen, if not the two openings, at least points in 

 their neighbourhood placed on the continuation of the 

 very axis of the tunnel. They were able accordingly to 

 build at a high elevation an observatory supplied with a 

 field-glass rotating in a vertical plane passing through 

 the middle of the tunnel. From that observatory two 

 other observatories were determined in the same ver- 

 tical plane towards the mouths of the tunnel, and the 

 field-glasses of the two new observatories very easily 

 supplied, when necessary, the direction of the tunnel 

 axis. 



On St. Gothard it was entirely different. The moun- 

 tain presents several ridges over the tunnel. At no point 

 can the places near both openings be seen at once, and 

 some of the summits are so steep and high that it is 

 impossible to take observations, and any direct tracing of 

 the line over the mountain is impracticable. It was 

 therefore necessary to make an indirect tracing, that is, 

 to connect the extremities of the line of direction by 

 means of a chain of triangles, calculate the relative posi- 

 tion of the two openings, and thence deduce the angle 

 formed by the tunnel axis either with the sides of the 

 triangles of which Goeschenen and Airolo are the ver- 

 tices, or with the meridian. This operation was facili- 

 tated by the great triangulation of the map of Switzerland. 

 five vertical points of which are near the tunnel and could 

 be utilised, but only as checks, since the entire work was 

 done afresh, and it was even necessary to reconstruct the 

 points of observation, many having disappeared, and 

 nearly all of them being on peaks placed above the 

 perpetual snow-level. M. Gelpke, the engineer who had 

 charge of the triangulation, measured a base line of 1,450 

 metres in the Urseren Valley, and then measured all the 

 angles not only of the eleven triangles connecting the 

 two mouths, but also of several triangles used for verifi- 

 cation. It was necessary to work with extreme care, as 

 a mistake in direction of only five seconds would produce 

 a deviation of 40 centimetres in the middle of the tunnel. 

 Besides the verification indicated, another was made by a 

 system of arrows continued from the point where the 

 tunnel-axis was calculated to intersect the observer's base 

 line in the direction of the line forming with the base the 

 angle given by calculation. This measurement by arrows 

 was continued as far as Goeschenen, and led precisely to 

 the mouth of the tunnel. 



When measuring the angles of the triangles, M. Gelpke 

 also took the vertical angles, and was able to calculate the 

 difference in altitude of the tunnel-mouths. The leve) 

 moreover was ascertained directly, and connected with 

 the great European triangulation of precision, which 

 passes directly over the Col of St. Gothard. 



The direction and level having been thus obtained, 

 observatories were placed near the tunnel-mouths, to serve 

 as direction-points for the miners. At Airolo there was 

 no difficulty in fixing the observatory, the site being 

 suitable ; but at Goeschenen, where the direction of the 

 tunnel forms with the narrow and deep valley a very 

 acute angle, there was not a sufficient line of sight, and it 

 was necessary, in order to have one, to traverse two 



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