190 KEACTION OF THE INTERIOR OF THE EARTH 



to about 2 feet. It is therefore only within the tropics, 

 or at no great distance beyond them, that this method 

 of arriving at the knowledge of the mean temperature 

 of the ground is easily practicable. The valuable re- 

 source offered by Artesian wells (which, with absolute 

 depths of from 750 to 2340 feet in round numbers, 

 have shown an increase of temperature of 1 Fahr. for 

 a mean descent of 56 feet,) has as yet only been 

 available to physical inquirers in districts not more 

 than 1600 feet above the level of the sea. ( 261 ) I have 

 visited excavations made in search of silver ores in the 

 Andes 6 45' south of the equator, at an elevation of 

 13,200 feet, and I there found the temperature of the 

 waters oozing out of the fissures in the limestone rock 

 52-3 Fahr.( 262 ) The waters which on the ridge of the 

 Andes, at the Paso del Assuay, were warmed for the 

 baths of the Inca Tupac Yupanqui, came probably from 

 the springs of the Ladera de Cadlud ; where, by baro- 

 metric measurement, I found the elevation of the path 

 (near which ran also the ancient skilfully constructed 

 Peruvian road,) 15,525 feet above the sea, or nearly 

 equal to the height of Mont Blanc.( 263 ) These are the 

 highest points at which I was ever able to observe 

 spring water in South America. In Europe, in the 

 Eastern Alps, the brothers Schlagintweit, at the height 

 of 9440 feet, found the temperature of the water in the 

 gallery of a gold mine (the " Groldzeche "), and of small 

 springs near the mouth of the gallery, only 33*4 Fahr.,( 264 ) 

 at a distance from either snow or glacier ice. The 

 limits of elevation at which springs are met with differ 

 with differences of latitude and of the height of the 

 line of perpetual snow ; and also with the different pro- 



