it was dangerous to work for fear the mass would fall and bury the workmen, and cut off their retreat. Communication was opened between the two galleries on the 29th of February, 1880. The chief miner on the southern (Airolo) side had pierced an horizontal hole about ten feet long, and had caused the attacks from the side of Goeschenen to be suspended on his penetrating to the northern gallery. Proceeding with a boring of moderate depth, he reduced the thickness of the remaining mass to about four feet. Preparations were made for the final attack by piercing four holes in the center of the boring, and eleven other holes at equal intervals around it and not very far distant from it. The explosion opened a passage of a little more than thirty inches in diameter, through which the engineers and some of the workmen were able to go over from one gallery to the other. When the communication was first opened, at eleven o'clock in the morning, the barometer stood .156 of an inch higher at Goeschenen than at the southern end of the tunnel. A current of air was immediately produced in the gallery, which blew at the opening at the rate of a metre and a half (nearly five feet) a second. A few hours later the relative pressure was reversed, and the barometer stood .039 of an inch lower at Goeschenen than at Airolo. The direction of the current of air was consequently changed; it blew from south to north, but at the rate of only about a foot in a second. The actual length of the tunnel was about twenty-five feet shorter than the calculated length. The difference in the level of the two galleries was not more than four inches, and their lateral deviation was not more than eight inches. According to a statement made by M. Colladon to the Academy of Sciences at Paris, the most efficacious means adopted to speed the work of excavation were the diking of the torrents and the application of water collected in aqueducts as a moving power to turbine-wheels requiring high falls, the adoption of air-compressors which worked with great rapidity, the cooling of the air in the compressors, at the moment of compression, by the injection of water in a fine spray, the use of dynamite, and the determination which was adopted from the beginning to carry on the excavations from the top of the tunnel. By the aid of these improved methods the advance through the hard rocks was made with double the speed that the engineers in charge had been able to attain in boring the Mont Cenis Tunnel. It is estimated that, notwithstanding its greater length, the tunnel of St. Gothard when completed will have cost from twenty-five to thirty per cent, less than that of Mont Cenis.
The provisions for conducting compressed air into the galleries, involving a system of pipes upward of sixteen thousand feet long in each gallery, afforded excellent opportunities for studying the flow of compressed air through metallic conduits. The loss of air in passing through the pipes was an important factor. The absolute pressure of the air, which was equivalent to six and a half atmospheres at the
