14 KEMARKS ON TEMPEHATURE OF THE EARTH IX MIXES. 



his " Conversations on Mines," gives 50 feet to 1 degree in the 

 first 100 fathoms, 70 feet in the second, and 85 feet in the 

 third. This diminishing ratio of increment certainly not 

 strengthening the internal heat theory. 



But if a similar increase of temperature takes place in our 

 mines, even with a decreasing ratio of increment, we will have 

 at the depth of 3,000 feet a permanent addition of 40 degs. to 

 the normal heat at the plane of invariable temperature, or 

 the annual mean average temperature at the surface ; which, 

 taken at '-'08 degs., will give 108 degs. as the coolest possible 

 state of the mine at that depth. But there are many additional 

 sources of heat — adventitious or extraneous to the natural 

 source — such as the pressure and friction brought about by 

 the partial working away of the natural supports of the 

 superincumbent rocks producing chemical decomposition, 

 particularly in coal mines where the strata are much more 

 pliable than those of other more crystalline formations, and these 

 therefore subject the stratum of coal, where partial excavation 

 has taken place, to severe pressure. There is also the heat 

 produced by the men, the horses, the lamps or candles ; and 

 the heat generated by the decomposition of pyrites and other 

 refuse of the mine. So that, according to this theory, it will be 

 quite impossible to Avork our mines to anything approaching 

 the depths mentioned. 



There are, however, examples both of coal and metalliferous 

 mines being worked on the continent of Europe to much 

 greater depths than that mentioned, but they have the advantage 

 •of a lower average annual mean temperature than we have here. 



I have taken the examples above as representing the two 

 great divisions of the mining industry. I^ut even in these there 

 are circumstances other than the internal heat theory which 

 account for the increase of temperature. In the coal mines 

 it may be due to the various extraneous sources mentioned 

 .above, and which, although likely to be somewhat of a general 



* Gympie is more than a degree north of Brisbane. Hence, 1 degree 

 higher temperature is assumed for it ; and all our other mining fields are 

 in warmer latitudes. 



