ON THE KATE OF INCnEASB OP UNDERGROUND TEMPERATURE. 89 



Sfc. Gotharrl tunnel and for the numerous deep mines of the East Man- 

 Chester coalfield, will have only a trifling effect on the rates of increase 

 assigned to these localities. 



To obtain an approximation to the rate at which heat escapes annually 

 from the earth, we will first reduce the above rate of increase "01566 to 

 Centigrade degrees per centimetre of depth. For this purpose we must 

 multiply by -0182, giving -000285. 



To calculate the rate of escape of heat, this must be multiplied by the 

 conductivity. 



The most certain determinations yet made of the conductivity of a 

 portion of the earth's substance are those deduced by Sir William 

 Thomson by an indirect method, involving observations of underground 

 thermometers at three stations at Edinburgh, combined with laboratory 

 measurements of the specific heats and densities of the rocks in which 

 the thermometers were planted. The specific heats were determined by 

 Regnault, and the densities by Forbes. Specific heats and densities can 

 be determined with great accuracy in the laboratory, but the direct 

 determination of conductivity in the laboratory is exceedingly difficult, it 

 being almost impossible to avoid sources of error which make the con- 

 ductivity appear less than it really is. 



Professor Herschel, in conjunction with a Committee of the British 

 Association, has made a very extensive and valuable series of direct 

 measurements of the conductivities of a great variety of rocks, and 

 has given additional certainty to his results by selecting as two of the 

 subjects of his experiments the Calton Hill trap and Craigleith sand- 

 stone, to which Sir "William Thomson's determinations apply. Comparison 

 shows that Professor Herschel's results for these two substances, as given 

 in the fifth Report of the Conductivity Committee (1878), must be multi- 

 plied by about 1"4 to make them agree with Sir William Thomson's.' 



The following list, condensed from that Report, will be useful for our 

 purpose : — 



Mean Conductivities in C.G.S. measure, from Professor HersclieVs Determinations. 



Eock salt 



Quartz 



Sandstone 



Flagstone 



Slate 



Granite 



Limestone 



Serpentine 



Trap . 



•0027 

 •0023 

 •0026 

 ■0021 

 •0019 

 •0009 

 •0070 

 •0008 

 •0006 



The mean of these 18 values is -00413, which, if multiplied by the 

 correcting factor 1-4 above mentioned, gives •0058. 



Sir William Thomson's three determinations were : — 



Trap rock of Calton Hill -00415 



Sand of Experimental Garden -00262 



Sandstone of Craigleith Quarry •OlOeS 



These give a mean of "00582, which is sensibly the same as the above. 



' The sixth Report of the Conductivity Committee states that a mistake was made 

 in the factor employed for reducing the results to C. G. S., and that the results as 

 given in the fifth Report require to be increased by one-eighth part of their 

 respective amounts. 



