46 ANNUAL OF SCIENTIFIC DISCOVERT. 



either horizontally, vertically, or obl : quely ; as many as eighty are 

 sometimes driven in the face ; but they are not all charged with pow- 

 der, the object in making most of them being merely to facilitate the 

 breaking up of the rock when the powder explodes in the others. 

 When the holes are bored the machine is drawn back upon the rails 

 to a distance of about fifty yards ; great wooden doors are then shut 

 to prevent the blasting from doing any injury to the machinery ; and 

 the holes are filled with powder, the fuse being used to ignite it. The 

 blasting takes place twice in twenty-four hours. The machine is re- 

 moved the fifty yards and returned to its place in ten minutes. 



It requires a long time to get rid of the foul gases produced by the 

 explosion of the powder : it is found that from one to four hours and a 

 half are necessary to remove the vapors and the debris. From this 

 cause not more than about five feet of the heading can be done per 

 day on each side, at which rate the tunnel would not be done for ten 

 years ; but there are machines being made now which it is calculated 

 will effect a saving in time of 30 per cent. ; so that, if nothing unfore- 

 seen happen, the whole tunnel can be finished in about six years 

 more ; or, if new improvements are successfully introduced, it may be 

 completed even in a less time still ; and this will very probably be the 

 case, for there are means of using the machines more expeditiously, 

 and experiments are being made to see how far they can be taken ad- 

 vantage of. 



However that may be, the great work is now assured, and it is only 

 a question of time ; the machinery is so perfect that everything goes like 

 clockwork ; and the great desideratum of making a long tunnel with- 

 out shafts having been obtained, there need now be no further doubt 

 about the success of such undertakings. 



The sides of the Mont Cenis Tunnel are lined with the stone exca- 

 vated in the tunnel itself, nicely worked, while the roof is lined with 

 four rings of brickwork. It should be also stated that there is a main 

 tube which passes over the ten iron reservoirs, and this is in communi- 

 cation with them by means of short pipes ; this tube is continued to the 

 far end of the tunnel, and conveys the compressed air to the boring- 

 machines, into which the air enters by means of small pipes of vulcan- 

 ized India-rubber. 



The tunnel has vertical walls and a semicircular roof; the lining is 

 carried about a foot below the roadway, and makes a mitre joint with 

 the rock, so as to convert the substratum into a natural invert. The 

 tunnel itself is 26 feet 3 inches wide in the clear, and its height in the 

 centre is 24 feet 7 inches. 



The total length of the heading on the Italian side is 3,530 feet, 

 while that on the Savoy side is 2,620 feet ; and the length of tunnel 

 taken out to its full dimensions, lined and finished, is about 6,000 

 lineal feet altogether. 



In the headings there are constantly employed at each face thirty 

 men to direct and attend to the working machinery ; six men and 

 three boys are kept for the purpose of blasting the rock ; and six 

 additional men are occupied, when it is necessary, to remove the 

 debris after the blasting has taken place ; besides, there are numer- 

 ous workmen working the tunnel out to its full dimensions, lining it, 

 and so on. The tunnel is at present in one straight line ; but, in 



