MAGNETIC OBSERVATIONS. 97 



difference of elevation amounting to upward of 2000 feet; 

 and the volcano of Purace, which rises 8740 feet above the 

 Plaza Mayor of the town of Popayan. Kupffer in the Cau- 

 casus,* Forbes in many parts of Europe, Laugier and Mau- 

 vais 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 ob- 

 served 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.f 



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- 



J DO 



mission of the terrestrial force in the free atmosphere when 

 care is taken to obtain corrections for temperature in the 

 needles that are employed both before and after the ascent. 

 The neglect of such a correction has led to the erroneous 

 result deducible from Gay-Bussac's experiments, that the 

 magnetic force remains the same to an elevation of roOre 



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

 but to the difference of height (1796 feet) between two stations, v'z., 

 the bridge of Malva and the mountain declivity of Kharbis, which un- 

 fortunately 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. ofEdin., vol. xiv., 1810, p. 

 23-25. 



t Compare Laugier and Mauvais, in the Comptes rendus, t. xvi., 

 1813, p. 1175 ; and Bravais, Observ. de ITntensite du Magneiisme Ter- 

 restre en France, en Suisse, et en Savoie, in the Annales de Chemie et de 

 Phys., 3eme Serie, t. xviii., 1816, p. 211; Kreil, Einfluss der Alpen 

 auf die Intensitdt, in the Denkschri/ten der Wiener A/cad. der Wiss. 

 Mathem. Naturwiss. Classe, bd. i., 1850, s. 265, 279, 290. It is very 

 remarkable that so 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 Brew- 

 ster, Treatise on Magrt., p. 275. 



Vol. V.— E 



