32 REPORT— 1870. 



" 3. Between 309 and 419 yards, the increase was at the rate of 1° for 

 60 feet. 



" 4. Between 419 and 613 yards, the increase was at the rate of 1° for 

 86-91 feet. 



" 5. Between 613 and 685 yards, the increase was at the rate of 1° for 

 65-6 feet 



" . . . . The result of the whole series of observations gives an increase 

 of 1° for every 8:'.-2 feet " 



Mr. Fairbairn's own summary is as follows : — " The amount of increase 

 indicated in these experiments is from 51° to 57|° as the depth increases 

 from 5| yards to 231 yards, or an increase of 1° in 99 feet. But if we take 

 the results which are more rehable, namely those between the depths of 231 

 and 685 yards, we have an increase of temperature from 57|° to 75 j°, or 17|° 

 Fahrenheit — that is, a mean increase of 1" in 76-8 feet." 



Mr. Fairbaii-n here by implication throws doubt on the alleged invariable 

 temperature of 51° at the depth of 17 feet, a determination which iu itself 

 appears highly improbable, seeing that, at Greenwich, the thermometer whose 

 bulb is buried at a depth of 25-6 feet, exhibits an annual range' of 3°"2, 

 while that at the depth of 12-8 feet exhibits a range of 9°. But even if we 

 assume the mean surface-temperatui'e to be 49°, we have still upou the whole 

 depth an increase at the rate of 1° in 80 feet, as against 1° in 54*3 feet at 

 Eose Bridge. 



Mr. Fairbairn's paper gives also the results obtained at a second pit at 

 Dukenfield, which agree with those in the first in showing an exceptionally 

 slow rate of increase downwards. The temperatures at the depths of 167^ 

 yards and 467 yards were respectively 58° and 66^°, showing a difference of 

 81° in 2991 yards, which is at the rate of 1° in 106 feet. The increase from 

 the surface down to 167^ yards, assuming the surface-temperature as 49°, 

 would be 9°, or 1° in 56 feet ; and the mean rate of increase from the surface 

 to the bottom would be 1° iu 80 feet, the same as iu the first pit. 



A tabular list of the strata at Eose Bridge is appended to this Report. A 

 full account of the strata at Dukenfield is given in Mr. Fairbairn's paper 

 (British Association Report, 1861). 



With strata so nearly similar, and in two neighbouring counties, we should 

 scarcely have expected so much difference in the mean rates of increase 

 downwards. In this respect Rose Bridge agrees well -svith the average of 

 results obtained elsewhere. Dukenfield far surpasses all other deep mines or 

 wells, so far as our present records extend, in slowness of increase. 



This implies one of two things — either that the strata at Dukenfield afford 

 unusual facilities for the transmission of heat, or that the isothermal surfaces 

 at still greater depths dip down iu the vicinity of Dukenfield. 



Mr. HuU has called attention to a circumstance which favours the first of 

 these explanations — the steepness of inclination of the Dukenfield strata. He 

 argues, with much appearance of probability, that beds of very various cha- 

 racter (sandstones, shales, clays, and coal), alternating with each other, must 

 offer more resistance to the transmission of heat across than parallel to their 

 plaues of bedding, as Mr. Hopkins has shown that every sudden change of 

 material is equivalent to au increase of resistance ; and it is obvious that 

 highly inchued strata furnish a path by which heat can travel obhquely up- 

 wards without being interrupted by these breaches of continuity. 



To this suggestion of Mr. HuU's it may be added that inclined strata 

 furnish great facilities for the convection of heat by the flow of water along 

 the planes of junction. It appears likely that surface-water, by soaking 



