'JO TUNNELING 



floor 5 more holes were bored for the fourth round, by which 

 the floor of the drift was reached. The total number of the 

 holes bored at the front of the drift varied from 70 to 80; their 

 depth was 3 ft. Three holes in the middle of those of the 

 first round were made deeper so as to loosen the rock a little to 

 facilitate the blasting of the succeeding rounds. Gunpowder 

 was the only explosive used in the excavation of the Mont 

 Cenis tunnel, both when the work was done by hand labor and 

 by machines. 



The time required for boring the holes of the drift varied 

 between and 8 hours. From 1 to 2 hours were required for 

 filling in the holes with explosives, and from 3 to 5 hours in 

 removing the blasted rock, so that in 24 hours no more than 

 two blasts were made at the front of the drift. The different 

 excavations were made by various gangs following each other 

 at an average distance of 900 ft. 



Power Plant. The mechanical installation consisted of 

 the Sommeilier air compressors built near the portals. The 

 Soinmeilier compressors, Mr. W. L. Saunders says, were oper- 

 ated as a ram, utilizing a natural head of water to force air at 

 80 Ibs. pressure into a receiver. The column of water con- 

 tained in the long pipe on the side of the hill was started and 

 stopped automatically by valves controlled by engines. The 

 weight and momentum of the water forced a volume of air with 

 such a shock against the discharge valve that it was opened, 

 and the air was discharged into the tank ; the valve was then 

 closed, the water checked ; a portion of it was allowed to dis- 

 charge, and the space was filled with air, which was in turn 

 forced into the tank. Only 73 % of the power of the water was 

 available, 27 % being lost by the friction of the water in the 

 pipes, valves, bends, etc. Of the 73 % of net work, 49.4 was 

 consumed in the perforators, and 23.6 in a dummy engine 

 for working the valves of the compressors and for special 

 ventilation. 



The compressed air was conveyed from each end through a 



