Hot and Thermal Springs. 1 45 



equator, or 81°.50, they will touch the surface at the equator, 

 but will diverge from it as the latitude becomes higher, and at 

 the poles they will dip deepest into the earth.* If, for ex- 

 ample, we draw such a line in the meridian of Cornwall, it will 

 there dip to a depth of 144 fathoms ; for at this depth, accord- 

 ing to Fox, the waters in the mines have the mean temperature 

 of the equator. All these curves in the different meridians form 

 a spheroid, which is somewhat more flattened than the surface 

 of the earth. We may denominate these curves of equal tem- 

 perature subterranean Isothermal or clithonisothermal lineSy 

 from ;^5wv, the earth. It is easy to be perceived, that these 

 curves will bend sometimes upwards and sometimes downwards. 

 Supposing the earth once to have had a higher temperature, equal 

 throughout its surface, whether on mountains or in valleys ; 

 then the increase of temperature towards the centre, all other 

 circumstances being the same, and supposing the atmosphere 

 at any place to have had the ' same temperature at all heights, 

 would, after the cooling down of the earth from the surface 

 towards the centre, have followed the same progression from all 

 points of its surface. But as the temperature of the air de- 

 creases in proportion to the height above the surface of the 

 earth, the different points where the increase of temperature is 

 the same, cannot, even within a small space, lie equally distant 

 from the curved surface of the earth. If we apply this to an 

 uneven portion of the surface of the globe, of so small an ex- 

 tent, that the decrease of temperature caused by the difference 

 of latitude is not perceptible ; then the curve ABC, Fig. 6. 

 Plate II, parallel to the surface of the earth, will not have an 

 equal temperature at every point. For, as the temperature of 

 the air is lower at D than at E, the curve of equal tempera- 

 tures must be different from ABC, and must pass through a 

 point F, lying deeper, or further from the surface than B.t 



* Ifthernean temperature at the North Pole is, as Von Buch assumes, 

 = 19°.85, and if the temperature there increases at the same rate as in the 

 temperate zone, the curve of 81°.50 would fall at the North Pole to a depth 

 of .3151 feet. 



+ In former ohservations on the temperature in mines, only the vertical 

 depth in which a certain increase of temperature has been discovered, i.s 

 given, without taking into consideration, that observat'ons from £ to A must 



vol.. WiV. NO. XI.VII. JANUAKY 1838. K 



