For this purpose a signal was erected on the mountain above the culminating point or the centre of the tunnel; and opposite to each entrance, in the same longitudinal axis, a large theodolite pointed to the above-mentioned signal and towards a light in the interior of the tunnel, so that the slightest deviation from the requisite direction was rendered mathematically impossible.
The machines used in the drivage of the tunnel were worked by means of compressed air. The comminuted rock produced during the boring was continuously removed by a jet of water squirted into the hole by the machine—a precaution which was found to be of great economy in the wear of the tools. Under ordinary conditions the working speed was about 200 strokes per minute, at which rate from eight to ten holes, of 1·6 inches diameter and 35 inches deep, could be bored in the shift of six hours. The tunnel is of an
section, 13 feet broad and 9¾ feet high; eight machines were employed at one time; they were mounted on a wrought-iron frame, which travelled on a railway, and could be removed to a safe distance when the blasting took place. From 65 to 70 holes were bored in the face of the rock, and then loaded with uniform charges, made up into cartridges, and primed with fuses of different lengths, so as to explode in groups at definite intervals. A horizontal series at about mid height was first fired, then followed two vertical side rows, then a group near the crown of the arch, and finally a horizontal series near the floor. In this way the rock was blown down in nearly uniform fragments of from six to eight inches in the side. At first, owing to the hardness of the quartz-slate which had to be pierced, the work went on very slowly; but at a greater depth, the mountain was found to consist of limestone strata, so that at length it advanced about thirteen feet per day.
The compressed air which set the borers in motion served also for the ventilation of the tunnel, as every stroke of the piston liberated sufficient pure air to drive back to a considerable distance from the workmen the suffocating gases, produced by the blasting, which otherwise would render breathing impossible. Air-pumps placed before the entrance finally conducted the fumes of the gunpowder or dynamite into the open air. It is evident that the difference in height of the two openings, and the gradual ascent towards the centre, will insure a thorough ventilation. In the beginning of September 1869 the Italian side of the tunnel, which is 5,913 metres long, was completed, while on the French side only 4,222 metres were accomplished and 2056 remained to be bored, and it was calculated that this gigantic enterprise would most probably be terminated by the beginning of 1871. It was, however, completed a few days earlier.
When Louis Quatorze, that model of regal pomposity, succeeded in placing the crown of Spain on the head of his grandson Philip of Anjou, he bid him grandiloquently farewell with the words: ‘Go, my son! the Pyrenees no longer exist.’ With more truth is our age able now to declare that the Alps have ceased to be a barrier between nations.
There can be no doubt that the Mont Cenis tunnel will soon be followed by similar undertakings, some probably on a still grander scale. The governments of Italy, Switzerland, and Germany have already decided upon piercing a tunnel through the Mount St. Gothard. In winter the traveller will no longer be obliged to ascend the solitudes of the Schöllenen, to cross the Devil’s Bridge, and then, after traversing the valley of Urseren, to pursue his perilous way up to the summit of the pass, before a zigzag descent, along yawning precipices, leads him into the sunny vale of the Ticino; but at Göschenen on the north he will plunge at once into the bowels of the mountain, and in an hour will suddenly emerge at Airilo into the genial south. In early spring the change of climate will be almost miraculous; on one side winter still reigning supreme, the earth covered with deep snow and the cold wind moaning through the leafless trees—and then, after a short interval of darkness, the rich vegetation of an Italian vale bursting forth in all its first luxuriance, birds singing in the chestnut groves, and the green meads enamelled with myriads of flowers.
It is curious to compare these stupendous works with the most celebrated tunnel of ancient times, the Grotto of the Pausilippo, near Naples, which serves as a passage for the travellers who intend visiting Puzzuoli, Baiæ, or Cumæ, without being obliged to ascend the mountain or to cross the bay. It is about a mile long, from thirty to eighty feet high and twenty-eight feet broad, so as to allow three carriages to pass abreast. Twice a year, in February and October, the last rays of the sun dart for a few minutes through the whole length of the grotto; at all other times a speck of gray uncertain light terminates the long and solemn perspective. The origin of this celebrated grotto is unknown. Some old chroniclers have imagined it as ancient as the Trojan war; others attribute it to the mysterious race of the Cimmerians.
The Neapolitans attribute a more modern, though not more certain, origin to their famous cavern, and most piously believe it to have been formed by the enchantments of Virgil, who, as Addison very justly observes, is better known at Naples in his magical character than as the author of the ‘Æneid.’ This strange infatuation most probably arose from the fact that the tomb, in which his ashes are supposed to have been deposited, is situated above the grotto, and that, according to popular tradition, it was guarded by those very spirits who assisted in constructing the cave. King Robert, a wise, though far from poetical, monarch, conducted his friend Petrarch with great solemnity to the spot, and, pointing to the entrance of the grotto, very gravely asked him whether he did not adopt the general belief, and conclude that this stupendous passage derived its origin from Virgil’s powerful incantations.
‘When I had sat for some time,’ says Beckford,[[34]] ‘on a loose stone immediately beneath the first gloomy arch of the grotto, contemplating the dusky avenue, and trying to persuade myself that it was hewn by the Cimmerians, I retreated, without proceeding any further, and followed a narrow path which led me, after some windings and turnings, along the brink of the precipice, across a vineyard, to that retired nook of the rocks which shelters Virgil’s tomb, most venerably mossed over and more than half concealed by bushes and vegetation. The clown who conducted me remained aloof at awful distance whilst I sat communing with the manes of my beloved poet, or straggled about the shrubbery which hangs directly above the mouth of the grot.