On Underground Temperatures. 



55 



mentally ;* we may presume it to exist, although it may differ 

 materially in degree. 



Therefore, taking into consideration the probable limitation of the 

 percolation of water, and the possible diminution of conductivity with 

 increase of depth, if there should be any alteration in the thermo- 

 metric gradient, at great depths, it will <be more likely to be in the 

 direction influenced by these more or less certain factors, or in favour 

 of a decreased conductivity and a more rapid thermometric gradient, 

 rather than otherwise. 



I have made a few attempts to ascertain, with the data in our 

 possession, whether there exists any indication of such variation within 

 the limits of the depths reached, by comparing the gradients of the 

 upper with the lower portions of the mines, but without arriving 

 at any satisfactory result. f It is true that in the Coal Mines, taking 

 a depth below 1000 feet, the gradient, in all cases except two, shows, 

 with increased depth, an increased rapidity, but it is a question 

 whether this is not due to ventilation and convection currents causing 

 too low a reading of the gradients in the upper part of the mines, 

 and so throwing an apparent gain into the gradients in the deeper 

 parts of the mines. 



In the Mines other than Coal, some show at great depths a more 

 rapid, and others a slower gradient, but it has to be observed that 

 generally there is greater steadiness in the gradients at depths beneath 

 500 or 600 feet, than in those which are shallower. 



In Artesian Wells and bore-holes, on the contrary, the gradient is 

 often more rapid in the upper than in the lower section of the wells, 

 but this is clearly due to the action of convection currents ; while the 

 decrease in the diameter of the bore-hole with the increase of depth, 

 by unequally checking the flow of water, differently affects the tem- 

 perature of the water in the tubes as successive depths are reached. 



Looking, however, only at the more certain and determined causes 

 which have interfered with the value of even the best observations, 

 I believe that the effect of them has been to make the readings for 

 the Artesian Wells and bore-holes especially, as well as the Mines, too 

 low ; and it may be a question whether a general average gradient of 

 45 feet per degree would not be nearer the true normal than the one 

 of feet obtained by the foregoing investigation. 



* The large proportion of iron present in the deeper seated igneous rocks is an, 

 element to be considered. 



t See the figures between the brackets and in italics in Tables II, III, and IV. 



