THERMAL SPRINGS. 183 



according to Boussingault's method,* to a depth below that 

 affected by the influences of the changes of temperature of 

 the neighboring atmosphere, and at very different elevations 

 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 temperature 

 commences diminishes from 60 to 1^ or 2 feet. Burying 

 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 

 sub-tropical 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 dis- 

 tricts not much more than 1600 feet above the level of the 

 sea.f I have visited silver mines in the chain of the Andes, 

 6° 45^ south of the equator, at an elevation of nearly 13,200 

 feet, and found the temperature of the water penetrating 

 through the fissures of the limestone to be 52°.*3 F.j: The 

 waters which were heated in the baths of the Inca Tupac 

 Yupanqui, upon the ridge of the Andes {Paso del Assiiay), 

 probably come from springs of the Ladera de Cadlud, where 

 I have traced their course, near which the old Peruvian 

 causeway also ran, barometrically to an elevation of 15,526 

 feet (almost that of Mont Blanc).§ These are the highest 

 points at which I could observe spring water in South Amer- 

 ica. In Europe the brothers Schlagintweit have found gal- 

 lery-water in the gold mine in the Eastern Alps at a height 

 of 9442 feet, and fbund that the temperature of small springs 

 near the opening of the gallery is only 33°*4 F.,|| at a dis- 

 tance from any snow or glacier ice. The highest limits of 

 springs are very different according to geographical latitude, 

 the elevation of the snow line and the relation of the highest 

 peaks to the mountain ridges and plateaux. 



If the radius of our planet were to be increased by the 

 height of the Himalaya at the Kintschindjunga, and therefore 

 uniformly over the whole surface by 28,175 feet (4-34 En- 

 glish miles), with this small increase of only ^-ou^^ ^^ *^^® 



* See Cosmos, vol. i., p. 221, and vol. v., p. 42. 



f See above, p. 39. 



j Mina de Gaudalupe, one of the Minas de Chota, I. c. sup., p. 41. 



§ Humboldt, Views of Nature, p. 393. 



11 Mine on the Great Fleuss in the Moll Valley of the Tauern ; see 

 Hermann and Adolph Schlagintweit, Untersuchungen uber die phjsika- 

 lische Geographie der Alpen, 1850, s. 242-273. 



