Page:Popular Science Monthly Volume 32.djvu/161

Rh their tunnel about five times as fast as was the progress in the Mont Cenis Tunnel in 1863 by hand-drilling. Now, in the years 1878 and 1879, by the general improvement of the whole administration in the St. Gothard Tunnel, they are making a progress of more than eight to one, or about thirteen feet per day, through granitic gneiss in a single heading. So that now, if they can excavate and remove rock from a tunnel eight times faster than they could fifteen years ago, you may readily see how much easier, how much more feasible, a tunnel has become to-day on an isthmus-canal route than at the time, in 1870, when the surveys of Selfridge were made. Indeed, these naval officers in their sphere of action were ignorant of and had not conceived as possible these strides of inventive and engineering skill.

A few weeks after the address of Major Shelbourne, a paper was read before the American Society of Civil Engineers by Walton W. Evans, in which he described the augmented power of the Sommeiller machine during the excavation of St. Gothard. He said:

"I was shown at the St. Gothard Tunnel steam-drills that by slow motion and high pressures would walk into granite as a knife would into cheese; there was nothing used on the Mont Cenis Tunnel to approach them in efficiency. I was shown air-compressors that kept their great reservoirs, night and day, under pressures of 110 pounds to the square inch, and without difficulty; it was with difficulty and uncertainty that the air-compressors of the Mont Cenis Tunnel could keep the pressure up to 60 pounds to the square inch." He adds: "We are clearly a progressive race, and it would be a wise brain that could predict with certainty what advance may be made by some live Yankee in tunneling machinery when we come to cut a ship-tunnel."

Mr. Evans was, We may remember, like Major Shelbourne, in favor of the San Blas route, and the tunnel which that route involves.

Certain details as to the machinery used in the Alps may be of interest. After hand-drilling was given up, all the drills used were run by compressed air, but the methods employed to compress it varied. At Mont Cenis hydraulic power exclusively was used. Sommeiller employed at first the fall of a column of water in the same way in which it is applied in the case of the water-ram. Afterward he substituted turbine-wheels. To compress the air at the St. Gothard, ordinary steam-power was at first used. Afterward the improved method of Sommeiller, the turbine-wheel, was substituted. It was found that the amount of water available at the southern terminus was not as large as at the northern; in the former case, accordingly, a higher fall of water was required. A useful or effective fall, as it is