340 
rorULAR SCIENCE REVIEW. 
immediately on leaving St. Michel, where the mountain rail- 
road comes into operation, and where the carriages of the 
French railway are exchanged for the very uncomfortable 
omnibuses of the American or Fell line. The map and section 
in the annexed Plate will show the order of the rocks and the 
way in which they succeed each other. The direction of the 
valley is at first nearly east, it turns after a certain distance to 
the south-east, and then towards the north-east, which direction 
it retains as far as Lanslebourg. The rocks have a general 
north-westerly dip, averaging about 50° in the tract of country 
between the Arc and Dora valleys which is the geographical 
axis of the Alps in this part. The line of railroad rises pretty 
steadily after leaving the valley of the Ehone at Culoz, at first 
slightly, afterwards, when the valley of the Isere is reached, more 
rapidly, the level of the valley bottom near St. Michel being 
nearly 3,500 feet above the sea. 
The project of a railway tunnel to be cut through the crest 
of the Alps for the purpose of affording permanent and rapid 
intercourse between France and Italy at all seasons and with- 
out change of rail, was due to an Italian engineer, and has 
been carried on throughout under Italian superintendence. 
The first suggestion of such a tunnel is said to be due to 
Signor Medail of Bardonneche, who put it forward in a pamphlet 
published at Lyons in 1841. This pamphlet coming under 
the notice of the king (Charles Albert), the Minister of the 
Interior was ordered to make further investigations. The 
enquiry was referred to Professor Sismonda and a Belgian 
engineer (the Cav. Maus), who was at that time in Turin con- 
structing the Turin and Genoa railway. After careful con- 
sideration it was agreed that the scheme was practicable, and 
that the position suggested was the best in that part of the 
Alps; but it was estimated that the tunnel could not be con- 
structed in less than thirty-five years by the ordinary means 
then available, even making no allowance for drawbacks and 
accidents. M. Maus, noticing the large quantity of water- 
power on both flanks of the mountain, conceived the possibility 
of utilising this, and invented machinery adapted for the pur- 
pose ; but though a model was prepared the machine was not 
constructed. At a later period the present perforating machine 
was perfected under the superintendence of the engineers, 
Messrs. Grattoni, Grandis, and Sommeiller. It is quite distinct 
from that proposed by M. Maus, and being worked by com- 
pressed air (compressed by water-power to a pressure of between 
six and seven atmospheres), the air is available for ventilation. 
The exact site selected was so near the ordinary line of road 
crossing the Mont Cenis, that the tunnel has always been called 
by that name, but it really cuts through the Alps under the 
