BRITISH ASSOCIATION MEETING. 



541 



increase of temperature In the earth's crust. The experiments hitherto made 

 give, unfortunately, somewhat conflicting results, and even in the same 

 mine the rate of increase of temperature is by no means uniform. This is 

 shown very clearly in the results obtained by Mr. Astle} r . It is scarcely pro- 

 bable, however, that the temperature in the mine-shaft influenced the results, 

 and we must therefore seek the cause of this irregularity in the varying con- 

 ducting power of the different strata, arising trom different density, and 

 different degrees of moisture of the strata. As to the rate of increase, they 

 appear to confirm previous experiments, in which it has been shown that the 

 temperature increases directly as the depth. The rate is at first rather less 

 than this, afterwards somewhat greater, and at last again less, but on the whole, 

 the straight line on which the temperature increases as the depths nearly ex. 

 presses the mean of the experiments. The amount of increase indicated in 

 these experiments is from 51 degs. to 57* degs., as the depth increases from 5f 

 yards to 231 yards, or an increase of 1 deg. in 99 feet. But if we take the results 

 which are more reliable, namely those between the depths of 231 and G85 yards, we 

 have an increase of temperature from 57t degs. to 75! degs., or 17£ degs. Fahren- 

 heit. That is a mean increase of 1 deg. in 7G'S feet. This rate of increase is not 

 widely different from that discovered by other authorities. Walferdin and Arago 

 found an increase of 1 deg. in 59 feet in the artesian well at Grenelle. At the salt- 

 works at Rheme, where an artesian well penetrates to a depth of 760 yards, or 

 rather more than the Dukinfield mine, the increase is 1 deg. in 51'7 feet. MM. de 

 la Hive and Marcet found an increase of 1 deg. in 71 feet. In one respect the 

 observations in the Dukinfield mine are peculiarly interesting. As they give 

 the temperature in various descriptions of rock, they appear to prove what has 

 hitherto been partially suspected, namely, that the conducting powers of the 

 rocks exercise a considerable influence on the temperature of the strata. If 

 we add to this the influence of the percolation of water, we shall probably have 

 a sufficient explanation of the irregularities observed in the experiments. 

 From the above observations we have evidence of the existence in the earth of 

 a central heat, the temperature, so far as can be ascertained, increasing in the 

 simple ratio of the depth. "VVe do not, however, presume to offer an opinion as 

 to whether this increase continues to infinitely greater depths than we have 

 yet penetrated, as observations upon this point are still imperfect. But, as- 

 suming as an hypothesis, that the law which prevails to a depth of 700 yards 

 continues to operate at still greater depths, we arrive at the conclusion that 

 at a depth of less than two and a half miles the temperature of boiling water 

 would be reached, andat a depth of 10 miles a temperature of 3,000 degs. Fahren- 

 heit, which we may assume to be sufficient to melt the most refractory rocks 

 of which the earth's crust is composed. If, therefore, no other circumstance 

 modified the conditions of liquefaction, all within a thin crust of this thickness 

 would be in a fluid state. This, however, is not the case. At these depths 

 the fusing point is modified by the pressure and conductivity of the rocks. 

 We know that in volcanic districts, where the great subterranean laboratory of 

 nature is partially opened for our inspection, the molten mass, relieved from 

 pressure, pours forth from volcanic craters currents of lava which form a pecu- 

 liar class of rocks. Besides this, it has been ascertained from Mr. Hopkins's 

 experiments on soft substances, such as spermaceti, wax, and sulphur, that 

 the temperature of fusion increases about 1*3 Farenheit for ever 5001b. pres- 

 sure per square inch, that is, in other words, that the temperature of fusion 

 under pressure is increased in that ratio. If we assume this to be the law for 

 the materials of the earth's crust, and correct our previous calculations in 

 accordance with it, we shall find that we have to go to a depth of G5 miles, 

 instead of merely 10, before the point of fusion of the rocks is reached. It 

 must, therefore, be observed that Mr. Hopkins's later experiments with tin 



