But the perforation of the rock is not the only serious impediment to progress. All things that live and breathe, miners among the number, require air for their sustenance; and, in order to supply it in sufficient quantities for the support of the human moles within the interior of the tunnel, it has been necessary to resort to special appliances for this purpose. Immense machinery, moved by water-power of an aggregate force for each end of about 400 horses, erected at both entrances of the tunnel, works not only the boring machines, but, at the same time, furnishes the miners with the necessary ventilation. The air is compressed to five atmospheres by means of the water-power just referred to; and the double application of the air is the ingenious contrivance of Messrs. Someilier, Grandis and Grattoni, the distinguished Italian engineers, under whom the works are conducted. Until recently the ventilation, although indifferent except at the site of the boring machines, was excellent in their vicinity; but with each metre that the works progress farther into the mountain, the difficulties of ventilation are added to; especially so on the French side, and for a reason that will at once, on a moment’s explanation, be evident to the reader. When the boring machines have made the usual holes in the rock about three feet deep, they are filled with gunpowder, and exploded. Now, if the excavation of the mountain had been made partly by adit and partly by shaft, as soon as one of the latter had been made, the smoke from each explosion beyond it would have found vent through it to the outer air, and in a few minutes the forward face of the tunnel would be ready for the fresh action of the boring machines. But as there is no shaft in the Cenis Tunnel, the whole of the smoke created by each explosion has only one means of exit—that is, through the tunnel’s mouth. Smoke, in its escape, wants to ascend (as we know by our everyday experience), but it cannot do so for the want of a shaft; it cannot get out even on the level, for the gradient in the tunnel on the French side is, on an average, 1 in 45, or 117 feet 4 inches per mile. As the perforation had, on the 1st November last, been accomplished to the extent of 3,205 metres, the face of the tunnel was about 226 feet higher than the entrance,—consequently, the smoke of each explosion at the present time is not only not able to come out on the level, but it has actually to descend the number of feet just stated before it can commingle with the outer air; and the number of these feet of elevation wall continually increase, until, at the end of the French perforation, there will be 429. We believe it is a matter of fact, that the ventilation is every day becoming worse, and the means of supplying the requisite air are becoming more difficult with each metre of advance made, and so it must continue until the final perforation has been (for no doubt it will be) accomplished.[138] On the Italian half, the rock is throughout, schist, and the gradient, being only one in 2,000, or 2-8/25 per mile, all the elements for progress will continue to be, comparatively, as favourable as they have hitherto been; and even with a much less rapid rate of advance than on the French side, it requires no great gift of prophesy to “forecaste” that the boring machines from Italy will be ready to embrace those from France long before the latter shall have arrived at the midway point. But let it be understood that the works from the Italian side cannot go on a yard beyond midway,—first, on account of the gradient, which would dam up all the water percolating into the tunnel; second, because it would be impossible to keep the line of the tunnel, owing to the absence of all the external landmarks required for ensuring its correct direction. The embrace we have just referred to will take place 4,138 feet above the level of the sea, 1,645 above the level of St. Michel, 429 above the Modane entrance of the tunnel, but only 2-8/25 above that at Bardoneche, 2,360 above the level of Susa, about 5,360 feet _below_ the highest point of the Great Vallon Mountain, and 2,520 feet below the summit of the Mont Cenis Railway.