TERRESTRIAL MAGNETISM. 103 



ground, never can reach the sea-level. Thus the works in 

 the Joachimsthal, in Bohemia, have reached 2000 feet of 

 absolute descent, but the lowest part is still 250 feet above 

 the sea.( 110 ) Aerostatic ascents offer very different and far 

 more favourable conditions. Gay-Lussac ascended to 21600 

 French feet above Paris, a height eleven times greater than 

 the greatest depth attained in Europe, (speaking even merely 

 of depth beneath the surface of the ground, without 

 reference to the level of the sea). The results of my own 

 mountain observations between the years 1799 and 1806 

 led me on the whole to regard a decrease of force in ascend- 

 ing as probable, although, (perhaps from the perturbing 

 causes above alluded to), several amongst them gave indica- 

 tions of a contrary character. I have placed together in a 

 note,( m ) data obtained by me in the course of 125 determi- 

 nations of magnetic force in the chain of the Andes, in the 

 Alps, and in Italy and Germany. My observations extended 

 from the level of the sea to the limits of perpetual snow, and 

 to a height of 14960 French, or 15944 English, feet; but 

 the greatest elevations did not give the most assured results. 

 The most satisfactory among them are afforded by the steep 

 declivity of the Silla de Caracas, 8100 French feet above 

 the closely-adjacent coast of La Guayra; the Santuario de 

 N tra S ra de Guadelupe, on the summit of a limestone cliff 

 rising immediately above the town of Bogota, giving a 

 difference of elevation of 2000 feet ; and the volcano of 

 Purace, 8200 French feet above the Plaza Mayor of the 

 town of Popayan. Kupffer( 112 ) in the Caucasus, Forbes in 

 many parts of Europe, Laugier and Mauvais on the Canigou, 

 Bravais and Martins on the Faulhorn, and in their adven- 

 turous sojourn very near the summit of Mont Blanc, have 



