Whatever may be the nature of the rock, if it is very hard, the depth of the holes is reduced; that is, the perforation is only continued for a certain given time--about six and a half hours--which, for the eighty holes with ten perforaters, gives us about three-quarters of an hour for each hole. The rock is generally of calcareous schist, crystallized, and exceedingly hard, traversed by thick veins of quartz, which often break the points of the boring-tools after a few blows. Each jumper gives about three blows per second, and makes one-eighteenth of a revolution on its axis at each blow, or one complete revolution every six seconds. Thus, in the three-quarters of an hour necessary to drive a single hole to the depth of twenty-seven inches, we have four hundred and fifty revolutions of the bar, and eighteen hundred violent blows given by the point against the hard rock, and that under an impulse of about one hundred and eighty pounds. These figures will give us some idea of the wear and tear of the perforating-machines. It is calculated that on an average one perforating-machine is worn out for every six metres of gallery, so that more than two thousand will be consumed before the completion of the tunnel. The total length completed at the Bardonnêche side at the present time is just two thousand three hundred metres, or nearly a mile and a half. At the north or Modane end, the mechanical perforators are precisely the same as at Bardonnêche, as also is the system of working in the gallery. The machinery for the compression of air, however, is very different, more simple, and in every way an improvement upon that at the South end. Not finding any convenient means of obtaining a head of eighty-four feet of water sufficient in quantity for working a series of compressors, as at Bardonnêche, there has been established at Modane a system of direct compression, the necessary force for which is derived from the current of the Arc. Six large water-wheels moved by this current give a reciprocating motion to a piston contained in a large horizontal cylinder of cast iron. This piston, having a column of water on each side of it, raises and lowers alternately these two columns, in two vertical tubes about ten feet high, compressing the air in each tube alternately, and forcing a certain quantity, at each upward stroke of the water, to enter into a cylindrical recipient. There is very little loss of water in this machine, which in its action is very like a large double-barreled common air-pump. It is a question open to science whether the employment of compressed air for driving the perforating engines in a work such as is in operation at the Mont Cenis, could not be advantageously and economically exchanged for the employment of a direct hydraulic motive force, the ventilation of the tunnel being provided for by other means. The system, however, employed at Modane has many advantages, which it is impossible to