THE TUNNEL THROUGH MONT CENIS. 151 



It will be conceived how difficult the work has been of 

 piercing through so diversified a substance as this. The 

 perforating machines are calculated to work best when 

 the resistance is uniform ; and it has often happened 

 that the unequal resistance offered to the perforators 

 has resulted in injury to the chisels. But before the 

 work of perforating began, enormous difficulties had to 

 be contended with. It will be understood that, in a 

 tunnel of such vast length, it was absolutely necessary 

 that the perforating processes carried on from the two 

 ends should be directed with the most perfect accuracy. 

 It has often happened in short tunnels that a want of 

 perfect coincidence has existed between the two halves 

 of the work, and the tunnellers from one end have 

 sometimes altogether failed to meet those from the 

 other. In a short tunnel this want of coincidence 

 is not very important, because the two interior ends of 

 the tunnellings cannot in any case be far removed from 

 each other. But in the case of the Mont Cenis tunnel 

 any inaccuracy in the direction of the two tunnellings 

 would have been fatal to the success of the work, since 

 when the two ought to meet it might be found that they 

 were laterally separated by two or three hundred yards. 

 Hence it was necessary before the work began to sur- 

 vey the intermediate country, so as to ascertain with 

 the most perfect accuracy the bearings of one end of 

 the tunnel from the other. * It was necessary,' says 

 the narrative of these initial labours, ' to prepare accu- 

 rate plans and sections for the determination of the 

 levels, to fix the axis of the tunnel, and to " set it out " 



