INVARIABLE STRATUM. 41 



I believe that special attentioD 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 

 between 1 2,000 and 18,000 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 Micuipampa 42 lies, according to my astronomical 

 and hypsometrical observations, in the latitude 6 43' S., 

 and at an elevation of 1857 toises or 11,990 feet, at the 

 base of Cerro de Gualgayoc, celebrated for the richness of 

 its silver mines. The summit of this almost isolated 

 fortress-like and picturesquely situated mountain rises 240 

 toises or 1504 feet higher than the streets of Micuipampa ; 

 the external air at a distance from the mouth of the pit of 



de Physique, t. liii, 1833, pp. 225247. Objections have been advanced 

 by John Caldecott, the astronomer to the Rajah of Travaucore, 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 in, pp. 379 393), that the temperature of the 

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

 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. between 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 cer- 

 tain that the thermometer in this case was sufficiently covered to pro- 

 tect it from the influence of the sun's rays? Compare also Forbes, 

 E.cper. on the Temp, of the Earth at different depths, in the Edin. Tran- 

 sact, vol. xvi, part ii, p. 189. Colonel A. Costa, the admirable historian 

 of New Granada, has made a prolonged series of observations, which 

 fully confirm Boussingault's statement, and which were completed, 

 about a year ago, at Guadua, on the south-western side of the elevated 

 plateau of Bogota, where the mean annual temperature is 43.94 F. at 

 the depth of one foot, and at a carefully protected spot. Boussingault 

 thus refers to these experiments: "The observations of Colonel A. 

 Costa, whose extreme precision in everything which is connected with 

 meteorology is well known to you, prove that when fully sheltered from 

 all disturbing influences, the temperature within the tropics remains 

 constant at a very small depth below the surface." 



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

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



