INVARIABLE STRATUM. 43 



remained perfectly uniform, whether the thermometric sound- 

 ings (of less than one foot in depth) were made on the torrid 

 shores of Guayaquil and Payta, on the Pacific, or in an 

 Indian village on the side of the volcano of Purace, which I 

 found from my barometrical measurements to be situated at 

 an elevation of 1356 toises, or 8671 feet above the sea. The 

 mean temperatures differed by fully 25 F. at these different 

 stations.* 



I believe that special attention is due to two observations 

 which I made on the mountains of Peru and Mexico, in 

 mines which lie at a greater elevation than the summit of 

 the Peak of Teneriffe, and are therefore the highest in which 

 a thermometer has ever been placed. At a height of be- 

 tween 12,000 and 13,000 feet above the level of the sea I 

 found the subterranean air 25 F. warmer than the external 

 atmosphere. Thus, for instance, the little Peruvian town of 

 Micuipampaf lies, according to my astronomical and hypso- 



* Boussingault, Sur la profondeur a laquelle on trouve dans la zone 

 torride la couche de temperature invariable, in the Annales de Chimie et 

 de Physique, t. liii., 1833, p. 225-247. Objections have been advanced 

 by John Caldecott, the astronomer to the Eajah of Travancore, and by 

 Captain Newbold, in India, against the method recommended in this 

 memoir, although it has been employed in South America in many 

 very accurate experiments. Caldecott found at Trevandrum (Edin. 

 Transact., vol. xvi., part iii., p. 379-393) that the temperature of the 

 soil, at a depth of three feet and more below the surface (and there- 

 fore deeper than Boussingault's calculation), was 85 and 86 F., while 

 the mean temperature of the air was 80 '02. Newbold's experiments 

 (Philos. Transact for the Year 1845, pt. i., p. 133), which were made 

 at Bellary, lat. 15 5', showed an increase of temperature of 4 F. be- 

 tween sunrise and 2 P.M. for one foot of depth ; but at Cassargode, lat. 

 12 29', there was only an increase of 1'30 F., under a cloudy sky. 

 Is it quite certain that the thermometer in this case was sufficiently 

 covered to protect it from the influence of the sun's rays ? Compare 

 also Forbes, Exper. on the Temp, of the. Earth at different Depths, in the 

 Edin. Transact., vol. xvi., part ii., p. 189. Colonel A. Costa, the ad- 

 mirable historian of New Granada, has made a prolonged series of ob- 

 servations, which fully confirm Boussingault's statement, and which 

 were completed, about a year ago, at Guadua, on the southwestern 

 side of the elevated plateau of Bogota, where the mean annual tem- 

 perature is 43 '94 F. at the depth of one foot, and at a carefully pro- 

 tected spot. Boussingault thus refers to these experiments: "The 

 observations of Colonel A. Costa, whose extreme precision in every 

 thing which is connected with meteorology is well known to you, prove 

 that, \\henfully sheltered from all disturbing influences, the temperature 

 within the tropics remains constant at a very small depth below the 

 surface." 



f In reference to Gualgayoc (or Minas de Chota) and Micuipampa, 

 see Humboldt, Recueil d'Observ. Astron., vol. i., p. 324. 



