150 LIGHT SCIENCE FOR LEISURE HOURS. 



pierce. And the work bold and daring as it seemed- 

 is three parts finished. (See date of article.) 



The Mont Cenis tunnel was sanctioned by the Sar- 

 dinian Government in 1857, and arrangements were 

 made for fixing the perforating machinery in the years 

 1858 and 1859. But the work was not actually com- 

 menced until November 1860. The tunnel which 

 will be fully seven and a half miles in length was to 

 be completed in twenty-five years. The entrance to 

 the tunnel on the side of France is near the little village 

 of Fourneau, and lies 3,946 feet above the level of the 

 sea. The entrance on the side of Italy is in a deep 

 valley at Bardoneche, and lies 4,380 feet above the sea 

 level. Thus there is a difference of level of 434 feet. 

 But the tunnel will actually rise 445 feet above the 

 level of the French end, attaining this height at a dis- 

 tance of about four miles from that extremity ; in the 

 remaining three and three-quarter miles there will be 

 a fall of only ten feet, so that this part of the line will 

 be practically level. 



The rocks through which the excavations have been 

 made have been for the most part very difficult to work. 

 Those who imagine that the great mass of our moun- 

 tain ranges consists of such granite as is made use of in 

 our buildings, and is uniform in texture and hardness,, 

 greatly underrate the difficulties with which the engi- 

 neers of this gigantic work have had to contend. A 

 large part of the rock consists of a crystallised calcareous 

 schist, much broken and contorted ; and through this 

 rock run in every direction large masses of pure quartz.. 



