Hot and Thermal Springs. 147 



Geneva and Bex. London, situated in latitude 5\°A(y north, 

 lies 14 feet above the sea, and the mean temperature of the soil is 

 5I0.OI. The eighth gallery in the Churprinz-Friedriclc-Aiigust, 

 Erbstolle, near Freyberg, in nearly the same latitude, (50°.55'), 

 lies 41 feet above the sea, and 926 feet below the surface, and has 

 a mean temperature, according to the average of three years'" ob- 

 servations, of 60°.28*. Between these two points, therefore, 

 which are situated in nearly the same latitude and elevation 

 above the sea, there is a difference of temperature of 9°.^7. The 

 point in this Freyberg mine, at which the temperature of the 

 earth is the same as that of the soil at London, was found to be 

 731 feet above the sea. So that a chthonisothermal line of 

 5I0.OI, supposed to be drawn from London to the Erzgebirge, 

 would, at the former of these places, almost reach the level of the 

 sea, but in the latter it would rise to a height of 731 feet above it. 



Even many springs (Chap. XVIII) shew that the chthoniso- 

 thermal lines suifer an elevation in passing under elevated parts 

 of the earth's surface. Thus No. 1 and No. 8 shew nearly the 

 same temperature. And as the latter lies 134 feet higher than 

 the former, the chthonisothermal of 51°.35 must there rise 134 

 feet within a distance of about two leagues. 



The rising of the chthonisothermal lines under mountains is, 

 therefore, a very general phenomenon. Their inclination to the 

 horizon is determined by the inclination of the side of the moun- 

 tain, by the decrease of the temperature of the air or of the sur- 

 face of the earth with the increase of elevation, and by the in- 

 crease of temperature towards the centre of the earth. Let AB, 

 PI. II, Fig. 7, be the sloping side of a mountain, and let the 

 mean temperature at A and D = t°, and atB = t° — 1° : then 

 BD is the depth in which the temperature increases 1°, and 

 CB =: AE the height in which it decreases 1° ; we have there- 

 fore : 



DC 

 tang X = tang BAC- 



= tanjT BAC 



BC 



BC — BD 



BC 



When the depth below the surface, in which the temperature 



'■ Heidi., p. A7. 



K i2 



