GEOLOGY. 269 



crease of temperature is by no means uniform. This is shown very 

 clearly in the results obtained by Mr. Astley. It is scarcely prob- 

 able, however, that the temperature in the mine-shaft influenced the 

 results, and we must therefore seek the cause of this irregularity in 

 the varying conducting power of the different strata, arising from 

 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 indi- 

 cated in these experiments is from 50 to 57|-, as the depth increases 

 from 5| yards to 231 yards, or an increase of 1 in 87 feet. But if 

 we take the results which are more reliable, namely, those between 



observed by other authorities Walferden and Arago found an in- 

 crease of 1 in 59 feet in the artesian well at Grenelle. At the salt 

 works at Rehme, where an artesian well penetrates to a depth of 760 

 yards, or rather more than the Dukinfield mine, the increase is 1 in 

 54.7 feet. MM. de la Rive and Marcet found an increase of 1 in 57 

 feet at Geneva. Other experiments have given an increase of 1 in 

 71 feet. In one respect the observations in the Dukinfield mine are 

 peculiarly interesting. As they give the temperature in various de- 

 scriptions of rock, they appear to prove what has hitherto been par- 

 tially 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 central heat, the temperature, so far 

 as can be ascertained, increasing in the simple ratio of the depth. I 

 do not, however, presume to offer an opinion as to whether this in- 

 crease continues to infinitely greater depths than we have yet pene- 

 trated, as observations upon this point are still imperfect. But 

 assuming as an hypothesis the law which prevails to a depth of seven 

 hundred 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, and at a d.epth 

 of forty miles a temperature of 3,000 Fahrenheit, which we may as- 

 sume to be sufficient to melt the most refractory rocks of which the 

 earth's crust is composed. If, therefore, no other circumstance mod- 

 ified 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 con- 

 ductivity 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 peculiar class of 

 rocks. Besides this, it has been ascertained by Mr. Hopkins's exper- 



