484 Mr. D. T. Ansted on the Earth's interior Temperature, [May 25, 



mate of the distribution of the mountain mass seems to show that this is 

 somewhat in excess of the true difference, and that if the slope were per- 

 fectly even the difference would be reduced to about 5080 feet. 



It is necessary now to estimate as nearly as possible the mean annual 

 temperature of the air at the surface/and the depth and temperature of the 

 stratum of invariable temperature within the earth. None of these has as yet 

 been determined by experiment ; but it is found generally that the stratum 

 of invariable temperature is nearly 2° F. warmer than the mean temperature 

 of the air at the surface, and that the mean temperature of the air decreases 

 in ascending to the higher parts of the atmosphere at the rate of 1° F. in 

 317 feet. As, however, the mean temperature of the air at the mouth of 

 the tunnel is not known by observation, we must take Turin as the nearest 

 point of departure, this city being 820 feet above the sea, and its mean 

 annual temperature 54 0- 5 F. The difference of level between Turin and 

 the mountain-summit being 8710 feet, this, divided by 317, gives 27°*5 as 

 the amount to be deducted from 54°*5. Thus the calculated mean annual 

 temperature would be 27 0, 5 F. ; adding to this 2°, we have the calculated 

 temperature of the stratum of invariable temperature 29° # 5 F. 



As some check on this estimate, it may be worth while to refer to a some- 

 what analogous case determined by observation by Dolfuss-Ausset, in 

 1865-66, on the mountain of St. Theodule and in the valleys of Aosta and 

 the Vallais. In this case the mean temperature of the air was found to be 

 -o°-10 C, (22°-8 F.) at the height of 3333 m (10,936 feet). The excess 

 of elevation of this summit above the crest of the Alps over the tunnel 

 being 1406 feet, the latter should be (at the rate of 1° F. per 317 feet) 

 4°*5 F. warmer, or have a temperature of 27°'3 F., a result nearly in accord- 

 ance with the other calculation. 



Estimated in this way, the difference of temperature between the mean 

 temperature of the air on the assumed surface above the central point of the 

 tunnel would be (84°— 27 0, 5 = ) 56 0, 5 F., and the rate of increment (the 



difference of level being 5080 feet) 1° in =^ 90 feet nearly; or if 



we assume the stratum of invariable temperature to be 80 feet below the 

 surface, the rate will be 1° in ^ '5^.5 =^) 91 feet, showing, no doubt, a very 



considerable difference when compared with most other observations made 

 in Europe and elsewhere at various levels, but not altogether unparalleled in 

 special cases. It is, of course, possible that the difference may be due to 

 the topographical conditions and geological structure of the earth's crust 

 under the crest of a great mountain-chain of comparatively recent elevation. 



But there is an important fact to be observed not alluded to by Signor 

 Giordano, but bearing very strongly on the general question of the rate of 

 increment. The slope of the Alps in this part of the range above the 

 tunnel is nearly regular on the French side, but very sudden at first 

 towards Italy, after which there is a wide step or terrace, for a distance of 



