ON UNDERGROUND TEMPERATURE. 15 



to the hundredth of a degree, from 90° to 110° F. ; and on the 7th 

 November the Secretary wrote to Mr. Bryham requesting him to commence 

 operations. Unfortunately, during this brief interval, circumstances had 

 changed. In a neighbouring pit, where the workings were in the same seam of 

 coal as at Eosebridge, though less deep by 200 yards, a considerable quantity 

 of water was found in sinking into the strata underlying this seam. This 

 was a very unexpected circumstance ; and as any irruption of water at the 

 bottom of Eosebridge pit, which is now quite dry, would be a most serious 

 affair, Mr, Bryham was afraid to risk the experiment of boring do^vn. Sub- 

 sequent reflection has only confirmed him in the opinion that such a step 

 would bo hazardous, and the Committee have accordingly been most reluc- 

 tantly compelled to renounce the plan. Mr. Bryham's final refusal was 

 received on the 28th February. 



Professor Ansted read a paper last year, in the Geological Section of the 

 Association, upon the Alpine tunnel, commonly called the Mont-Cenis tuii- 

 nel, and in that paper some interesting statements were made regarding its 

 temperature. Since that time, Professor Ansted has interchanged very 

 numerous letters with the Secretary, and has furnished mxxch valuable in- 

 formation, gathered from Prof. Sismonda, of Turin, and from M. Borelli, the 

 resident engineer of the tunnel. Observations which appear to be reliable 

 have been made in bore-holes in the sides of the tunnel, and the tempera- 

 tures thus observed have been compared with the estimated mean tempera- 

 ture at the surface overhead, which in the highest part is a mile above the tun- 

 nel, or 2905 metres above sea-level. It is directly under this highest part that 

 the highest temperature is foimd in the walls of the tunnel, namely 29°-5 C, 

 or 85°"1 F., which is 9° F. lower than the temperature found at the bottom 

 of the Eosebridge shaft at the depth of only 815 yards. But though the 

 tunnel is at more than double this depth fi'om the crest of the mountain 

 over it, we must bear in mind that the surface-temperatures are very dif- 

 ferent. In a paper pubKshed by the engineer of the tunnel, M. F. Giordano, 

 the mean temperature of the air at the crest of the mountain (Mont Frejus) 

 is calculated to be — 2°-6 C, or 27°'3 F. Assuming this estimate to be 

 correct, we have a difference of 57°-8 F. between the deepest part of the tun- 

 nel and the air at the surface vertically over it ; assuming further, as we did 

 in the case of Eosebridge in last year's Eeport, that the surface of the hill itself 

 has a mean temperature 1° F. lower than that of the air above it, we have a 

 difference of 56°-8 F., and the thickness of rock between is 1610 metres, or 

 5280 feet (exactly a mile). This gives, by simple division, a rate of increase of 

 1° F. for 93 feet ; but a very large correction must be applied for the con- 

 vexity of the ground ; for it is evident that a point in the ground vertically 

 under a steep crest is more exposed to the cooling influence of the air than 

 a point at the same depth beneath an extensive level surface. No correction 

 for convexity would be needed if the temperature of the air decreased up- 

 wards as fast as the temperature of the internal rock ; but this is very far 

 from being the case, the decrease being about 3| times more rapid in the 

 rock than in the air. To form an approximate notion of the amount of this 

 correction, we must determine, as well as we can, the forms of the succes- 

 sive isothermal surfaces in the interior of the mountain. The tendency is 

 for all corners and bends to be eased off as we descend, so that each suc- 

 ceeding isothermal surface is flatter than the one above it. Accordingly, if 

 we have a mountain rising out of a plain, without any change of material, the 

 isothermals will be further apart in a vertical through the crest of the moun- 

 .tain than under the plain on either side ; they will also be further apart 



