[Illustration: BORING MACHINE IN THE SECOND WORKING GALLERY, MONT CENIS TUNNEL.]
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 [18] 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.