On Underground Temperatures. 



9 



are the geological conditions very dissimilar, but also that the dis- 

 turbing causes are of a different order. In Mines, the latter are 

 attributable to — 



1st. Air currents established for ventilation, and by convection ; 



2nd. The circulation of underground waters ; 



3rd. Chemical reactions ; 



4th. The working operations. 

 And in Artesian Wells, to — 



1st. The pressure of the column of water on the thermometer ; 



2nd. Convection currents in the column of water. 

 While a general cause affecting each group is conductivity, which 

 variably influences all the rocks, and which is itself liable to be in- 

 fluenced by a number of causes to which we shall refer separately. 



We shall be able therefore to deal more readily with the subject, if, 

 as Professor Everett has done, we divide the observations in Table I 

 into groups in accordance with these considerations, and take each 

 separately in the following order : — 



I. Coal Mines. 

 IT. Mines other than coal. 



III. Artesian Wells and Bore-holes. 



IV. Tunnels. 



I. Coal Mines. {Table II, p. 88.) 



Considering the general uniformity of geological structure of the 

 Coal-measures, the temperature observations in coal mines are more 

 discordant than might be expected. This depends upon various causes, 

 of which ventilation is seemingly the principal, while in some cases 

 differences of conductivity may possibly have influenced the results. 

 It is true that in most cases the sources of error have been carefully 

 guarded against, but it is doubtful whether in others sufficient allow- 

 ance has been made for them, and whether in some they have not 

 altogether been overlooked. The main disturbing causes are as 

 follows : — 



Loss of Heat through Exposed Surfaces. — To guard against the 

 cooling of the coal or rock produced by ventilation, it has been 

 customary to place the thermometer in holes 2 to 3 feet deep, but it 

 is a question whether this is sufficient, for we are rarely informed, as 

 is essential, of the precise time that the face of the coal or rock has 

 been exposed. Even in the best observations we are only told that 

 they have been made in "freshly exposed faces," which may or may 

 not be sufficiently soon. For on these working faces the ventilation is 

 necessarily well maintained, and consequently the difference between 

 the temperature of the air and of the strata is there often most con- 

 siderable, and the cooling of the rock very rapid. 



On the surface of the ground the diurnal variation of temperature 



