54 



Prof. J. Prestwich. 



Taking these three classes of observations we obtain the following 

 values for their several gradients : — 



Thermometrie gradient 

 per 1° Fahr. 



Coal mines 49*5 feet 



Mines other than coal 43'2 „ 



Artesian wells 50*0 „ 



The mean of the three thus gives a general thermometrie gradient 

 of 47'5 feet per degree. 



I do not, however, by any means consider this more than an ap- 

 proximation to the true normal gradient. In Coal Mines the effects of 

 ventilation, and in other Mines the effects of chemical action and the 

 circulation of water, have yet to be more accurately determined ; while 

 in the case of Artesian Wells, I believe the gradient of 49*5 feet may 

 be too low in consequence of the unequal velocity of the water in deep 

 overflowing wells, and of the uncertain measure of convection currents 

 in those which do not overflow. 



Admitting, however, these determinations to be approximately 

 correct, they show that different geological areas have, in all pro- 

 bability, different gradients, and indicate possible inequalities in the 

 underground isothermals, unless the altered conditions which come 

 into play at greater depths tend to reduce and level them. 



There is reason also to believe that the conductivity of the rocks at 

 great depths may be affected by their hydrometric state and tempera- 

 ture. The descent of the surface water may ultimately be retarded or 

 stayed by friction and heat. Faults, although they may stay its 

 descent, leave untouched the water originally inclosed or imbibed. 

 M. Delesse, who made some calculations on the probable depth to 

 which water descends, concluded that water might circulate to the 

 depth of about 8 miles before this limit was reached. 



Further the experiments of Regnault determined the expansive force 

 of the vapour of water up to a temperature of 239° C, the pressure then 

 being equal to 27^ atmospheres. Beyond this, it has only been carried 

 by empirical formulae, but both experiment and calculation indicate 

 that, with the increase of temperature, the increase of force is extremely 

 rapid, and there is in all probability a point at which the vapour- 

 tension of the heated water will equilibrate the hydrostatic pres- 

 sure. 



With respect to the possibility of change in the thermometrie 

 gradient at depths, it is known that the conductivity of wrought 

 iron diminishes as the temperature increases, and at a rate agreeing 

 very elosely with the empirical law that the conducting power of 

 iron for heat is inversely as the absolute temperature. What the 

 variation in rocks may be has yet to be determined experi- 



