Hot and Thermal Springs. 339 



But the deeper the clefts, the nearer will the temperature of 

 the water coincide with that of the rock. And lastly, if the wa- 

 ters reach the regions where the increase of temperature towards 

 the interior of the earth becomes sensible, the springs resulting 

 from them must be thermal. This, as already mentioned, is the 

 case with the most springs rising in the Teutoburger Wald. 

 The influence of the external temperature, then, diminishes in 

 proportion as the thermal springs exceed the mean temperature 

 of the air. It is however easy to conceive, that water, sinking 

 through the wide channels in the dislocated chalk rocks, into the 

 regions where the increase of temperature towards the centre of 

 earth begins, should reappear, in a lower situation, as a thermal 

 spring; but should, nevertheless, shew the variations of the ex- 

 ternal temperature, as seems to be the case with the salt-springs 

 of Werl^ already alluded to more than once. 



Chap. VIII. — To what depth in thecru^t of the earth does the influ- 

 ence of the external temperature continue to be felt 9 



The depth to which the external temperature continues to ex- 

 ert its influence upon the crust of our globe cannot be the same 

 at all points. It depends, in the first place, upon the extent of 

 variation of temperature of the air, for the less such variation is, 

 the less considerable will be that depth, and vice versa. There- 

 fore the depth will vary in the same place, the extent of annual 

 variation of temperature of the air being in the various years 

 also various. It depends, further, on the conductibility of heat 

 of the earthy and rocky strata composing the crust of the earth. 

 The former of these circumstances varies according to the geo- 

 graphical latitude of the place and its elevation above the sea, for 

 the nearer to the equator, and the more elevated above the sea, the 

 less, in general, are the variations of temperature. The latter is 

 naturally dependent solely upon local geognostical circumstances. 



Between the tropics, from 11° N. Lat. to 5° S. Lat. the depth to 

 which the external temperature continues to be felt, is scarcely 

 so much as 1 foot ; for, according to Boussingault's * observa- 



*'Annal. de Chim. el de Phys. vol. liii. p. 225, and following. In the same 

 manner Sir Thomas Brisbane determined the temperature of the soil at I'ara- 

 matta (New South Wales). See Edinb. Phil. Journal, x. 270. 



vol-. XXIII. NO. XLVI. OCTOBF.R 1837. Z 



