190 COSMOS. 



crust of the earth, and the system of the geo-isothermic 

 lines, not in too isolated a condition, but as a part of the 

 all-penetrating motion of heat, a truly cosmical activity. 



Instructive as are, in many respects, observations upon 

 the unequal diminution of temperature of springs which do 

 not vary with the seasons as the height of their point of 

 emergence increases, still the local law of such a diminish- 

 ing temperature of springs cannot be regarded, as is often 

 done, as a universal geothermic law. If we were certain 

 that waters flowed unmixed in a horizontal stratum of great 

 extent, we might certainly suppose that they have gradually 

 acquired the temperature of the solid ground, but in the 

 great network of fissures of elevated masses, this case can 

 rarely occur. Colder and more elevated waters mix with 

 the lower ones. Our mining operations, inconsiderable as 

 may be the depth to which they attain, are very instruc- 

 tive in this respect ; but we should only obtain a direct 

 knowledge of the isogeothermal lines, if thermometers were 

 buried, according to Boussingault's method, 36 to a depth below 

 that affected by the influences of the changes of temperature 

 of the neighbouring atmosphere, and at very different eleva- 

 tions above the sea. From the forty-fifth degree of latitude 

 to the parts of the tropical regions in the vicinity of the 

 equator, the depth at which the stratum of invariable tempe- 

 rature commences, diminishes from 60 to 1^ or 2 feet. Bury- 

 ing the geothermometer at a small depth in order to obtain a 

 knowledge of the average temperature of the earth, is there- 

 fore readily practicable only between the tropics or in the 

 subtropical zone. The excellent expedient of Artesian wells 

 which have indicated an increase of heat of 1 F. for every 

 54 to 58 feet in absolute depths of from 745 to 2345 

 feet has hitherto only been afforded to the physicist in 

 districts not much more than 1600 feet above the level of 

 the sea 37 I have visited silver-mines in the chain of the 

 Andes, 645' south of the equator at an elevation of nearly 

 13,200 feet and found the temperature of the water pene- 

 trating through the fissures of the limestone to be 52,3 F. 88 

 The waters which were heated in the baths of the Inca 



36 See Cosmos, vol. i, p. 218, and vol. v, p. 40, Bohn's edition. 



*7 See above, p. 37. 



88 Mina de Guadalupe, one of the Minas de Chota, I.e. sup. p. 43, 



