200 PHYSIOGRAPHY. [ch. xrr. 



the locality, the constant temperature being nearly the mean 

 temperature of the surface. 



On going still deeper, the heat is found to increase; and, 

 at the bottom of a deep mine, it is generally so warm that 

 the miners are glad to discard most of their clothing. At 

 present, the deepest mine in this country is the Rosebridge 

 Colliery, at Ince, near Wigan, which has reached a depth of 

 2,445 f^^t. Experiments on the temperature at different 

 depths, while sinking this pit, showed that the average 

 increase is about i° Fahr. for every fifty-four feet. In other 

 sinkings, somewhat different results have been obtained, the 

 rate of augmentation being affected by the character of 

 the rocks bored through and by the position which the 

 strata occupy ; whether, for example, they are inclined or 

 horizontal. Thus at the Astley pit at Dunken field in 

 Cheshire the rate was found to be i° for every seventy-seven 

 feet, but this appears to be unusually slow. Perhaps it will 

 not be far wrong to assume that the average increase is i° 

 for every sixty feet : such at least is the rate which was 

 adopted a few years ago by the Royal Coal Commission in 

 their calculations. 



Even the deep sinking at the Rosebridge Colliery is but 

 the veriest dent in the earth's surface compared with the 

 actual radius of the globe. It gives therefore but scant 

 information respecting the temperature of the deep-seated 

 portions of the interior ; but, assuming such a rate of 

 increase to continue, it is evident that at the depth of only 

 a few miles the heat would be sufficient to fuse any known 

 rock. It is true that the melting-point of a solid body may 

 be greatly modified by pressure ; and it is obvious that, at 

 great depths, the pressure must be prodigious. Nevertheless, 

 the eruption of lava from volcanic vents sufficiently shows 



