1871]. as indicated hj the Alpine-Tunnel Observations, 485 



about two miles. It results that the depth from the summit of the ridge to 

 the tuunel being 5280 feet, the depth from the surface to the tunnel at the 

 Torrent of Merdovine, where the step or terrace commences, and which is 

 8240 feet in horizontal distance from the summit, is only 1700 feet, show- 

 ing a diminution of 3580 feet of elevation in 8240 feet of distance, or, 

 allowing for a sudden rise near the summit, a slope of 1 in 2|, or an angle 

 of 21°. On the other hand, the depth from the surface to the tunnel be- 

 tween the Torrent of Merdovine and a point about 3000 feet from the 

 entrance (a horizontal distance of 10,000 feet) shows hardly any appre- 

 ciable difference, varying only a few hundred feet from one point in the 

 profile to another. Beyond this point there is another rapid descent to the 

 valley of Bardonneche, where the mouth of the tunnel is situated. 



Temperature observations were taken (1) at a point under the summit 

 of the crest, (2) at a point under the Torrent of Merdovine, and (3) under 

 the last point mentioned, 3000 feet from the entrance. These show re- 

 spectively 85°* 1 F., 74°*5 F., and 62 0, 6 F. In each case the observations 

 were made from borings, and they may all be regarded as good and fairly 

 to be compared with each other. Making the corrections already ex- 

 plained, the result would be, first, that borings of 1700 feet made in dif- 

 ferent parts of the terrace below the steep mountain face for a breadth of 

 about two miles might show a rate varying from 1° of increase of tempera- 

 ture in 43 feet of depth to 1° in 63 feet ; whilst on the steep slope the rate 

 at the lowest part of the slope being 1° in 63 feet, the rate at the summit is 

 1° in 91 feet, the horizontal distance between the two places of observation 

 being 8240 feet, or about a mile and a half. In all these cases there is no 

 difference in the nature of the rock. 



There are two intermediate observations, numbered (10) and (1 1) in the 

 Table, which correspond to depths below the surface of 3000 and 4500 

 feet respectively. Estimated as before, the rate of increment is in the 

 former case 1° in 65 feet, in the latter 1° in 84 feet. 



One observation (No. 14) was made 1800 feet beyond the centre of the 

 tunnel towards the north, at a depth which may be fairly estimated at 

 4750 feet. The temperature observed was 80 o, 6 F., which, reduced as 

 before, shows a rate equivalent to 1° in 93*4 feet. 



These results tabulated will appear as follows : — 



Distance Depth from 

 from S. entrance, surface. 



Temp. 



o 



Rate of increment. 



Observation No. 3 . 



. 3281 



1700 



62-6 F. 



1° F. 



in 43 feet. 



5 . 



. 6562 



>y 



67-0 



)> 



50 „ 



8 . 



; 9843 



S) 



73 



>) 



61 „ 



9 . 



.13124 





74-5 



)) 



63 „ 



10 . 



.16405 



3000 



81-5 





65 „ 



11 . 



.19686 



4500 



84-0 



>) 



84 „ 



12 . , 



,21156 



5280 



85-1 



jj 



91 „ 



From N. end. 











14 ., 



18345 



4750 



80-6 



>) 



93-4,, 



