Page:Popular Science Monthly Volume 17.djvu/547

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



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 triangulations. The directions and levels were thus ascertained. Observatories were then placed at the tunnel-mouths to serve as direction-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 The surveys, originally made by M. Gelpke, were verified by a second series of triangulations made in 1874 by another engineer, M. Koppe, on a different system.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 triangulations gave results agreeing within two seconds of direction with each other. M. Koppe also verified his survey practically by projecting 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 could 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 answer, then by means of lamps hung on the line of the axis. The direction 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