98 COSMOS. 



which rises immediately over the town of Bogota, upon the 

 declivity of a steep wall of limestone rock, with a difference 

 of elevation amounting to upwards of 2000 feet ; and the 

 volcano of Purace, which rises 8740 feet above the Plaza 

 Mayor of the town of Popayan. Kupffer in the Caucasus, 13 

 Forbes in many parts of Europe, Laugier and Mauvais on 

 the Canigou, Bravais and Martins on the Faulhorn, and 

 during their very adventurous sojourn in the immediate 

 vicinity of the summit of Mont Blanc, have certainly observed 

 that the intensity of the magnetic force diminished with the 

 height, and this decrease appeared from Bravais's general 

 consideration of the subject to be more rapid in the Pyrenees 

 than in the chain of the Alps. 13 



Quetelet's entirely opposite results, obtained in an excur- 

 sion from Geneva to the Col de Balme and the Great St. 

 Bernard, make it doubly desirable for the final and decisive 

 settlement of so important a question, that observations 

 should be made at some distance from the surface of the 

 earth ; and these observations can only be carried on by 

 means of balloon ascents, such as were employed in 1804, by 

 Gay-Lussac, first in association with Biot, on the 24th of 

 August, and subsequently alone on the 16th of September. 

 Oscillations measured at elevations of 19,000 feet, can how- 

 ever only afford us certain information regarding the trans- 

 mission of the terrestrial force in the free atmosphere, when 



12 Kupffer' s observations do not refer to the summit of the Elbruz, 

 but to the difference of height (4796 feet) between two stations, viz. the 

 bridge of Malya, and the mountain declivity of Kharbis, which unfor 

 tunately differ considerably in longitude and latitude. Regarding the 

 doubts which Necker and Forbes have advanced in relation to this result 

 see Transact, of the Royal Soc. of Edin. vol. xiv, 1840, pp. 2325. 



13 Compare Laugier and Mauvais, in the Comptes rcndi*$, t. xvi, 1843, 

 p. 1175; and Bravais, Observ. deVIntensite du Magnet isme Terrestre en 

 France, en Suisse, et en Saroie, in the Annales de Chemie et de Phys 

 Seme Sfirie, t. xviii. 1846, p. 214; Kreil, Einfluss der Alpen auf die. 

 Intensitat, in the Denkschriften der Wiener Akad. der Wiss. Mathem. 

 Naturwiss. Cl. Bd. i, 1850, s. 265, 279, 290. It is very remarkable that 

 BO accurate an observer as Quetelet should have found, in a tour which 

 he made in the year 1830, that the horizontal intensity increased with 

 the height, in ascending from Geneva (where it was 1.080), to the 

 Col de Balne (where it was 1.091), and to the Hospice of St. Bernard 



where it was as high as 1.096). See Sir David Brewster, Treatise on 

 Uagn. p. 275. 



