The compressed air and water being ready for their work, an iron frame containing the perforating needles moves along the rails and confronts the rock which is to be attacked in the gallery or heading. The frame is armed with nine or ten perforating machines arranged so that the greatest number of holes can be bored in the center of the opposing mass of rock. To each of these are attached flexible tubes, one containing the compressed air which drives the drills, and the other water, which is injected into the holes as they are bored. The machine consists of two parts; the one a cylinder for propelling the drill, by means of a piston, and the other a rotary apparatus for working the valve of the striking cylinder, and turning the drill on its axis at each successive stroke. To bore eight holes of the required depth, the piston rod gives 57,600 blows. The action of each machine is independent of the other, so that if one of them is broken, or gets out of order, that of the rest is not delayed. The drills act at different angles so as to pierce the rock in all directions, and when the requisite number of holes have been drilled, the iron frame is pushed back, and the central holes are charged and exploded. The smaller surrounding holes are then charged and fired. At each blast, a strong jet of compressed air is thrown into this advanced gallery to scatter the smoke and supply air for respiration. Wagons are next pushed forward and filled with the fragments of broken rock, which are conveyed to the mouth of the tunnel and dumped down the side of the mountain. After each blast a fresh relay of workmen come in, and the same operation is repeated night and day. One of the objections urged against the use of compressed air as a motive of force was, that if it were conveyed a long distance it would lose so much of its elasticity or expansive power, that it would be unavailable for any practical purpose. But this conjecture was confuted by facts. It was found that the loss of pressure at the ends of the conduit pipes where the air is applied, as compared with the pressure in the reservoir is only one sixteenth of the whole. M. Sommeiller calculates that in the center of the tunnel, a distance of three miles and three quarters from the reservoir, he will be able to apply the necessary pressure of six atmospheres. That M. Sommeiller is correct in this opinion appears to be conclusively proved by the latest accounts from Mt. Cenis, which state that the work is steadily progressing, that one half of the entire length would be excavated by the first of January 1866, and that at a distance of nearly two miles from the reservoir, the drills were operating with as much force as ever, and that there was no appreciable loss of motive power.