352 
rorULAR SCIENCE REVIEW. 
degree in a hundred feet. It was observed with some astonish- 
ment by the resident engineer, that for a long distance there 
appeared no change of temperature, insomuch that he almost 
ceased to take interest in the observations. During this time, 
however, the work was progressing under a plateau varying in 
height from two to three thousand feet above the tunnel and 
extending for a distance of 13,000 yards from the entrance 
without much change in elevation.* Shortly afterwards, when 
the plateau had been passed and when the mountain side rises 
rapidly, the observations showed a gradual rise, but they have 
not been reduced and are not at present accessible. It will be 
important and interesting to know if they show a rise in direct 
proportion to the increase of height above the tunnel. On the 
French side the largest spring was tapped at a temperature of 
about 62° with a superincumbent mass of about 2,000 feet 
of rock. 
A good series of temperature observations on the northern 
side of the tunnel would have had extreme interest and great 
value if they had been carried on during the construction with 
good instruments and due precautions. It is to be feared that 
this enquiry was to some extent neglected. The holes were 
bored, and still remain ; but it is believed that the tempe- 
rature of the rock may be to some degree affected by the 
altered temperature of the tunnel when the works are com- 
pleted and ventilation is ensured from one end to the other. 
There is a difference of level between the north and south 
ends of the tunnel, the total difference being 435 feet. The 
Italian end is nearly level, the slope being chiefly on the other 
side. The contour of the mountain will be understood by 
reference to the annexed section, but it would be difficult for 
anyone who had walked over the ground to admit that the 
grand abrupt mountain pass from Modana and Bardonneche 
could be so apparently tame and regular as is there represented. 
The reader will, however, see that it fully exemplifies the above 
remarks. 
The perforation of the Alps under Mont Frejus has been 
throughout a remarkably simple and easy operation. There 
have been no drawbacks of the smallest importance in an en- 
gineering sense, and the work has been carried on steadily 
from the commencement. There has been but a small propor- 
tion of hard, tough rock, no loose, treacherous shales, no influx 
of water, no crushing in of the roof, and no rise of the floor. It is 
not easy to imagine a more complete instance of plain sailing, 
or a great work less interrupted by natural or unexpected 
difficulties. The mere magnitude and novelty of the under- 
* This will be at once seen by reference to the section in Plate LXIV., 
which is drawn to the same scale for vertical and horizontal distances. 
