1870.] Observations on Underground Temperature. 209 



appears to have been tolerably uniform, giving the impression that 

 there were no disturbing causes at work. The resulting propor- 

 tionate increase was found to be 1° for every 54*57 feet ; the " inva- 

 riable stratum" being assumed at 50° of temperature, and at a depth 

 of 50 feet from the surface. The results of the observations at 

 Kose Bridge as compared with those at Dukinfield show a remark- 

 able dissimilarity. The rate of increase in the former case being 

 much more rapid than in the latter : for assuming the same stan- 

 dard of departure in both cases, it will be found that in the case of 

 Rose Bridge the rate of increase is 1° for about 47*2 feet as against 

 1° for about 83 '2 feet at Dukinfield; an amount of discordance 

 which I am satisfied is to be explained on other grounds than those 

 of error in the observations themselves. 



Such a diversity of results cannot be attributed to differences in 

 the conducting powers of the rocks in each locality : for although 

 sections of moderate depth taken at two different parts of the same 

 coal-field would undoubtedly present different proportions of the 

 sandstones, clays, and shales, &c, arranged in a varying manner to 

 one another, which combine to form what is termed the " Coal- 

 measures ; " yet if observations in each case be extended down- 

 wards to considerable depths, such as those in the present instance 

 which exceed 2000 feet, the varieties of strata will tend to balance 

 each other, and all cause of discrepancy in the rate of increase of 

 temperature will probably disappear, or be reduced to such a degree 

 as to be immaterial. 



Neither is this diversity to be attributed to the presence or flow 

 of water in the strata ; for, as I observed on a former occasion,* the 

 percolation of water decreases with the depth, and at variable dis- 

 tances from the surface altogether ceases ; or is limited to the very 

 small quantity which appears in some cases to have been locked up 

 in the strata for ages, and which of course takes the temperature of 

 the surrounding strata themselves. We must, therefore, seek for 

 other causes to explain the dissimilarity of results in question. 



The amount of inclination in the strata, is a cause of variation 

 in the rate of increase which has not as yet been sufficiently taken 

 into account, but is one which I think ought not to be lost sight of, 

 especially in comparing results obtained at moderate depths. In the 

 case of strata of uniform composition throughout, as the Chalk, or 

 New Eed Sandstone formations, the amount of inclination is of com- 

 paratively little importance ; but where we have to deal with a for- 

 mation such as the Coal-measures, composed of variable strata 

 alternating with each other, and possessing varying degrees of 

 thermal conductivity, the inclination, or amount of dip of the beds, 

 becomes an element of much importance when the rate of increase 



* "Experiments for Ascertaining the Temperature of the Earth's Crust." 

 ' Quart. Journal of Science,' vol. v., p. 18. 



