years at each end; and eight hundred metres in four years gives us two hundred metres per year, or just one-half excavated by the machine in the same period. In using the machines, up to the present time, a perfect ventilation of the tunnel has been secured by the compressed air escaping from the exhaust of the boring-engines; or by jets of air expressly impinged into the lower end of the gallery to clear out rapidly the smoke and vapor formed by the explosion of the mine. It should be remembered, moreover, that in working a gallery of this kind, where vertical shafts are impossible, by manual labor, a powerful and costly air-compressing apparatus would have been necessary for the ventilation of the tunnel alone, so that the economy of the system, as applied at the Mont Cenis over the general system of tunnelling in hard rock, is evident. I propose, in the second portion of this article, to give a short description of the machinery employed and the system of working adopted, both at the South and North ends of the Mont Cenis gallery. II. Travellers who are given to pedestrian exercises may easily visit the works being carried on for the perforation of the tunnel through the Alps, both at Bardonnêche and at Modane, passing from one mouth of the tunnel to the other by the Colle di Frejus; and in fine weather, the tourist would not repent the eight hours spent in walking from Bardonnêche to Susa--a distance of about twenty-five miles. The road descends the valley of the Dora Ripari, and abounds in beautiful scenery. The railway to be constructed along this narrow defile will be found to tax the skill of the engineer as much as any road yet attempted. Its total length, from the terminus at Susa to the mouth of the Mont Cenis tunnel, will be forty kilometres, {66} or about twenty-four miles; and the difference of level between these two points is about two thousand five hundred feet, the line having a maximum gradient of one in forty, and a minimum of one in eighty-four. There will be three tunnels of importance, having a total length of about ten thousand feet; three others of lesser dimensions, having a total length of five thousand five hundred feet; and twelve other small tunnels, of lengths varying from two hundred and twenty to eight hundred and fifty feet, their total length being five thousand four hundred feet. Thus, the total length of tunnel on these twenty-four miles of railway will be nearly twenty-one thousand feet, or about four miles--just one-sixth of the whole line. There will also be several examples of bridges and retaining walls of unusual dimensions.