The most remarkable tunnel yet executed is the Mont Cenis* tunnel. This tunnel connects the railways of France and Italy, and is on the direct railway route from Paris to Turin. The length of this tunnel is 7 in. 4-11 furlongs. It is 434 ft. higher at Bar domteche• on the Italian side, than at Modane, on the French side. On this account, it is on a gradient of 1 in 451 from Modane to the middle, and thence it falls 1 in 2.000 to Bardonneche, this latter fall being sufficient to run off the water. The dimensions at Nodane are 25 ft. 31 in, wide at base, 26 ft. 2/ in. at widest part, and 24 ft. 7 in, high, the arch being nearly semi-circular. At Bardonni:ehe it is 11/. in. higher. It is all lined with stonemasonry, except at the Bardonveche end, where the arch is of brick.
The work was begun in 1857, and was at first done in the usual way by hand; but in 1861 the perforating machines described below were introduced on the Italian side, and two years later (1893) on the French side. On June 80, 1863, the tunnel bad been driven 2,S00 yds., and the rate of advancement was 91 ft. per day. All the efforts of the engineers to accelerate the work were for several years unavailing; and in Oct., 1866, just one-half the distance, or 6,680 yds., had been pierced, showing the same constant rate of 91 ft. per day. At this rate, the tunnel would not have been completed till 1872. Owing, however, to improved modes of working, and to a favorable change in the nature of the rock, the rate of advancement became greater toward the end, and the two parties met on Dec. 25, 1870. The tunnel was formally opened in Sep., 1871. A premium was to be paid by the French government to the Italian government, who did-the work, for each year by which a term of 25 years, counting from 1862, was reduced. Tne French government were also to pay 1:L287,000 for the construction of one-half the tunnel when completed.
This great work, which appeared almost impracticable to ordinary methods of tunnel ing by manual labor, was rendered practicable by machinery introduced by the engineers, Messrs Souuneiller, Grandis, and Grattoni. The great difficulty lay in the fact that, from the great height of the mountain, shafts were impracticable. and progress could only be made from each end. The ventilation also presented serious difficulties. M.
Sommeifier perfected a small machine, weighing 6 cwt., which bored a hole in. diameter and 3 ft. deep in twenty minutes: the time taken by two millers working by the ordinary method being two hours, Eleven of these machines were placed on a movable support, and were capable of working at almost any angle. Three or four large holes were bored in the center of the heading, and round these other holes of the ordinary size, in all 80 holes. The large holes were not fired, but were for the purpose of weakening the rock. The others were then fired in succession and in detachments, beginning with those nearest the center. and working outward. The machines were worked by compressed air acting, like high-pressure steam, on a piston in a cylinder; this air being compressed outside the tunnel by water-power acting on the hydraulic-rain principle, mid also by an air-pump; it was used at a pressure of five atmospheres above the atmospheric preasure. and was conveyed to the workings by a pipe 71 in. diameter. After it had expended itself in working the borers, it escaped into the tunnel, and so ventilated the workings. The advanced heading was the dilly place where these machines were used; the enlargement of the tunnel to the full size, the building, etc., were all performed by manual labor. It was calculated that when the tunnel was com pleted there would be a constant current of air from the n. to the s. end, as the latter is the higher end, and in a situation more exposed to the heat of the sun. In view of the great importance of the Mont Cenis route, and the uncertainty of the time of com pletion of the tunnel. a locomotive railway was constructed in the mean time over the top of thepass. The mils were laid on the existing road, and ascended the hill in zigzag lines. The steepest gradient wits 1 in 12, and on this gradient and down to 1 in 20. a third rail was laid in the center of the way, raised about 9 in. above the other rails. The engines were provided with two pairs of horizontal wheels, which being made to press against the center rail, provided the adhesion necessary for aseendingand descend ing these steep inclines. A. similar enterprise to the piercing of Mont Cenis has been begun in connection with the new railway by the St. Gotthard.