From Chambers's Journal.
THE MONT CENIS TUNNEL.
The tunnel through the Alps at present being pierced to connect the railway system of France and Italy, has acquired the title of the "Mont Cenis Tunnel;" but its real position and direction have very little in common with that well-known Alpine pass. On examining a chart of the district which has been selected for this important undertaking, we shall observe that the main chain of the Cottian Alps extends in a direction very nearly East and West, and that this portion of it is bounded on either side by two roughly parallel valleys. On the North we have the valley of the Arc, and on the South the valley of the Dora Ripari, or, more strictly speaking, the valley of Rochemolles, a branch of the Dora. The Arc, flowing from East to West, descends from Lanslebourg to Modane, and from thence, after joining the Isere, empties itself into the Rhone above Valence. The torrent Rochemolles, on the other hand, flowing from West to East, unites itself with the Dora Ripari at Oulx, descends through a narrow and winding valley to Susa, and thence along the plain to Turin. The postal road, leaving St. Michel, mounts the valley of the Arc as far as Lanslebourg, then turns suddenly to the South, passes the heights of the Mont Cenis, and reaches Susa by a very steep descent. On mounting the valley of the Arc, and stopping about eighteen miles West of Mont Cenis, and a mile and a half below the Alpine village of Modane, we arrive at a place called Fourneaux. Here, at about three hundred feet above the level of the main road, is the Northern entrance of the tunnel; the Southern entrance is at the picturesque village of Bardonnêche, situated at about twenty miles West of Susa, in the valley of Rochemolles.
The considerations which decided the Italian engineers upon selecting this position for the contemplated tunnel, were principally the following: first, it was the shortest route that could be found; secondly, the difference of level between the two extremities was not too great; and, thirdly, the construction of the connecting lines of railway—on the North, from St. Michel to Fourneaux, and on the South, from Susa to Bardonnêche were, as mountain railways go, practicable, if not easy. The idea of a tunnel through the Alps had long occupied the minds of engineers and of statesmen both in France and Italy; but it is to the latter country that we must give the credit of having worked the idea into a practical shape, and of having inaugurated one of the most stupendous works ever undertaken by any people. To pierce a tunnel seven and a half English miles long, by ordinary means, through a hard rock, in a position where vertical shafts were impossible, would be an exceedingly difficult, if not, in a practical point of view, an impossible undertaking, not only on account of the difficulties of ventilation, but also on account of the immense time and consequent expense which it would entail. It was evident, [{61}] then, that if the project of a tunnel through the Alps was ever to be realized, some extraordinary and completely new system of mining must be adopted, by means of which not only a rapid and perfect system of ventilation could be insured, enabling the miners to resume, without danger, their labors immediately after an explosion, but which would treble, or at least double, the amount of work usually performed in any given time by the system hitherto adopted in tunnelling through hard rock. To three Piedmontese engineers, Messrs. Grandis, Grattoni, and Sommeiller, is due the merit of having solved this most difficult problem; for whether the opening of the Alpine tunnel take place in ten or twenty years, its ultimate success is now completely assured.
A short review of the history of this undertaking, and a summary of the progress made, together with a description of the works as they are conducted at the present time, derived from personal observation, cannot fail to be interesting to English readers.
Early in 1857, at St. Pier d'Arena, near Genoa, a series of experiments was undertaken before a select government commission, to examine into the practicability of a project for a mechanical perforating-engine, proposed by Messrs. Grandis, Grattoni, and Sommeiller, for the more rapid tunnelling through hard rock, and with a view to its employment in driving the proposed shaft through the Alps. This machine was to be worked by means of air, highly compressed by hydraulic or other economical means; which compressed air, after performing its work in the perforating or boring machines, would be an available and powerful source of ventilation in the tunnel. These experiments placed so completely beyond any doubt the practicability of the proposed system, that, so soon as August of the same year, the law permitting the construction of the tunnel was promulgated.
At this time, absolutely nothing had been prepared, with the exception of a very general project presented by the proposers, and the model of the machinery with which the experiments had been made before the government commission; we cannot, therefore, be much surprised on finding that some considerable time elapsed before the new machinery came into successful operation, the more particularly when we consider the entire novelty of the system, and the unusual difficulties naturally attending the first starting of such large works, in districts so wild and uncongenial as those of Fourneaux and Bardonnêche. Fourneaux was but a collection of mountain-huts, containing about four hundred inhabitants, entirely deprived of every means of supporting the wants of any increase of population, and where outside-work could not be carried on for more than six months in the year, owing to its ungenial climate. Nor was the case very different at Bardonnêche, a small Alpine village, situated at more than thirteen hundred metres (4,225 feet) above the level of the sea, and populated by about one thousand inhabitants, who lived upon the produce of their small patches of earth, and the rearing of sheep and goats, and with their only road of communication with the outer world in a most wretched and deplorable condition. Under these circumstances, we can imagine that the task of bringing together large numbers of workmen, and their competent directing staff, must have been by no means easy; and that the first work of the direction, although of a nature really most arduous and tedious (requiring, above all, time and patience), was also of a nature that could scarcely render its effects very apparent to the world at large for some considerable time. Again, it was necessary in this time to make the detailed studies not only of the tunnel itself, but of the compressing and perforating machinery on the large scale proposed to be used. This machinery had to be made and transported through a country abounding in difficulties. Then, as might be [{62}] expected, actual trials showed serious defects in the new machines for the compression of air; and, in perfecting the mechanical perforators, unexpected difficulties were encountered, which often threatened to prove insurmountable. The total inexperience and unskilfulness of the workmen, and the necessity of giving to them the most tedious instruction; accidents of most disheartening and discouraging kinds—all tended to delay the successful application of the new system.
The first important work to be undertaken was the tracing or setting out of the centre line of the proposed tunnel. It was necessary first to fix on the summit of the mountain a number of points, in a direct line, which should pass through the two points chosen, or rather necessitated by the conditions of the locality, for the two ends of the tunnel in the respective valleys of the Arc and of Rochemolles; secondly, to determine the exact distance between these two ends; and thirdly, to know the precise difference of level between the same points. These operations commenced toward the end of August, 1857. Starting from the Northern entrance at Fourneaux, a line was set out roughly in the direction of Bardonnêche, which line was found to cut the valley of Rochemolles at a point considerably above the proposed Southern entrance of the tunnel. On measuring this distance, however, a second and corrected line could be traced, which was found to be very nearly correct. Correcting this second line in the same manner, always departing from the North end, a third line was found to pass exactly through the two proposed and given points. The highest point of this line was found to be very nearly at an equal distance from each end of the tunnel, and at but a short distance below the true summit of the mountain-point, called the "Grand Vallon." The line thus approximately determined, it was necessary to fix definitely and exactly three principal stations or observatories—one on the highest or culminating point of the mountain, perpendicularly over the axis of the tunnel; and the other two in a line with each entrance, in such a manner that, from the centre observatory, both the others could be observed. At the Southern end, owing to the convenient conformation of the mountain, the observatory could be established at a point not very far from the mouth of the tunnel; but toward the North, several projecting points or counterforts on the mountain necessitated the carrying of the Northern observatory to a very considerable distance beyond the entrance of the gallery—not, however, so far as not to be discerned clearly and distinctly, and without oscillation, by the very powerful and excellent instrument employed. These three points permanently established, remain as a check for those intervening, and serve as the base of the operations for the periodical testing of the accuracy of the line of excavation.
The first rough tracing out of the line was completed before the winter of the year 1857, and it was considered sufficiently correct to permit the commencement of the tunnel at each end by the ordinary means—manual labor. In the autumn of 1858, the corrected line was traced, and the observatories definitely fixed, and all other necessary geodetic operations completed. Contemporaneously was undertaken a careful levelling between the two ends, taken along the narrow path of the Colle di Frejus, and bench-marks were established at intervals along the whole line. All the data necessary for an exact profile of the work were now obtained. The exact length of the future tunnel was found to be twelve thousand two hundred and twenty metres, or about seven and a half English miles; and the difference of level between the two mouths was ascertained to be two hundred and forty metres, or seven hundred and eighty feet, the Southern or Bardonnêche end being the highest. Under these circumstances, it would have been easy to have established a [{63}] single gradient from Bardonnêche down to Fourneaux of about two centimètres per mètre—that is, of about one in fifty. But a little reflection will show, that in working both ends of the gallery at once, in order to effect the proper drainage of the tunnel, it would be necessary to establish two gradients, each inclining toward the respective mouths, and meeting in some point in the middle. This, in fact, has been done, and the two hundred and forty metres' difference of level has been distributed in the following manner: From Bardonnêche, the gradient mounts at the rate of 0.50 per one thousand mètres—that is, one in two thousand as far as the middle of the gallery; here it descends toward Fourneaux with a gradient of 22.20 mètres per one thousand, or about one in forty-five. The highest point of the Grand Vallon perpendicularly over the axis of the tunnel is 1615.8 mètres, or 5251.31 feet.
The difficulties encountered in the carrying out of these various geodetic operations can scarcely be exaggerated. It is true that nothing is more easy than to picket out a straight line on the ground, or to measure an angle correctly with a theodolite; but if we consider the aspect of the locality in which these operations had to be conducted, repeated over and over again, and tested in every available manner with the most minute accuracy, we shall be quite ready to accord our share of praise and admiration to the perseverance which successfully carried out the undertaking. In these regions, the sun, fogs, snow, and terrific winds succeed each other with truly marvellous rapidity, the distant points become obscured by clouds, perhaps at the very moment when an important sight is to be taken, causing most vexatious delays, and often necessitating a recommencement of the whole operation. These delays may in some cases extend for days, and even weeks. To these inconveniences add the necessity of mounting and descending daily with delicate instruments from three thousand to four thousand feet over rocks and rugged mountain-paths, the time occupied in sending from one point to another, and the difficulty of planting pickets on elevated positions often almost inaccessible. All these inconveniences considered, and we must admit the unusual difficulties of a series of operations which, under other circumstances, would have offered nothing peculiarly remarkable.