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POPULAR SCIENCE REVIEW. 
liad to be surveyed and triangulated, bases measured, and the 
exact length and levels of the tunnel determined with the 
greatest care and accuracy; for errors in the angles and 
reduced calculations would have led to the most disastrous 
consequences in a tunnel of this enormous length. The diffi- 
culty of tracing the axis, and setting out a corresponding line 
over the summit of an Alpine range, over rocks, ravines, and 
precipices ; of continually climbing, ascending, and descending 
the immense heights, from point to point and from station to 
station, may be readily appreciated by those accustomed to 
ramble among lofty mountain ranges. 
Great as the labour and difficulty of climbing and scaling 
these rugged heights may have been, they were as nothing 
compared to the engineering difficulties created by the incle- 
mencies of the weather. These were the greatest stumbling- 
blocks of all, from the impossibility of continuous work with 
levels and theodolites in such a wild and exposed place. The 
engineers were exposed to all the tempests common to these 
Alpine regions — rain, mist, sleet, drifting snow, and their more 
troublesome and ever-constant enemy, the howling wind, ever 
constant in its wild and fierce inconstancy. It was impossible 
to work with delicate instruments when exposed to storms and 
hurricanes at these lofty elevations ; hence, it frequently hap- 
pened for days together that not a single observation or an 
angle could be taken. However, after a long, laborious, and 
patient work, this all-important and interesting triangulation, 
with all its troublesome details, was brought to a happy issue 
and triumphant conclusion ; and the final direction and exact 
length of the axis of the tunnel accurately determined. 
The total length of the tunnel was determined to be 12,220 
metres, or about 7 miles, 4 furlongs, and 164 yards ; and, 
notwithstanding all the difficulties of the various operations 
and ceaseless hindrances, in spite of Alpine seasons, so accu- 
rately was the triangulation executed, that the maximum 
deviation from exactitude was confined within the limit of 
11*417 inches in the whole distance of 7J miles, or about 
inch per mile, which, of course, can have no practical 
effect in a work of this magnitude. 
The mountain through which the tunnel is to be pierced is 
exceedingly rough and irregular in outline, and, as might have 
been expected, very steep and rugged. The lowest point is at 
Fourneau, close by the main route over Mont Cenis. From this 
valley, which is at an elevation of about 3,600 feet above the 
level of the sea, the mountain ascends rapidly 348 feet to the 
northern entrance to the tunnel, which is in a picturesque gorge 
in the valley. It then rises rapidly 1,140 feet more, to Los 
Aysards, a cluster of chalets in the slope of the mountain, 
where is placed the first signal station. The line then runs 
