yard by yard, in the same proportion as it does when descending into the earth, the temperature in its centre should be about 90° higher than at its mouth. Although it is known that the rate of increase is very much less than this, the actual rate is not known. I believe it is correct to say that not a single observation has been made upon the natural temperature of the rock since the tunnel has been commenced. Four-fifths of it are now driven. The opportunity for observation has been lost; for, apart from the cooling which must inevitably have taken place, almost the whole of the tunnel has been lined with masonry, and it is not to be expected that any person, or any body of persons, will incur the expense, even if they were permitted, of removing this, and then making the necessary holes. It is to be hoped that some observations will be made on the remaining portion, for similar opportunities are not likely to occur very frequently.
About 2000 feet on the French side of the tunnel was undergoing the processes of enlargement and completion in the summer of 1869.[1] In some places portions of the advanced gallery remained untouched, and then one came to caverns, such as have been described above. This section was being completed faster than the advanced gallery was being driven. It was pleasant to get away from it farther into the bowels of the mountain; the heat became less, and the atmosphere more pure. The noise of the hammers died gradually away, and at last no sound whatever could be heard, except of our own footsteps and of water running in the subway. After a time the banging of the chisels could be distinguished which were at work on the front of the attack. Five hundred paces took us to them.[2] The ponderous frame, technically called "l'affût," sup-ported nine of the machines known as "perforatrices;" each perforative propelled a boring-rod, and each boring-rod was striking the rock at the rate of 200 strokes per minute, with a force of 200
