MAGNETIC OBSERVATIONS. 97 



the sea to an elevation of 15,944 feet, and therefore to the very 

 limits of perpetual snow, but the greatest heights did not 

 afford me the most reliable results. The most satisfactory 

 were obtained on the steep declivity of the Silla de Caracas 

 (8638 feet), which inclines towards the neighbouring coasts of 

 La Guayra ; the Santuario de Nbstra Safiora de Guadalupe, 



observations from the comparisons of the Silla de Caracas (8638 feet 

 above the sea, intensity 1.188), with the harbour of Guayra (height 

 feet, intensity 1.262), and the town of Caracas (height 2648 feet, 

 intensity 1.209) ; from a comparison of the town of Santa Fe* de Bogota 

 (elevation 8735 feet, intensity 1.147), with the chapel of Neustra 

 Senora da Guadalupe (elevation 10,794 feet, intensity 1.127), which 

 seems to hang over the town like a swallow's nest, perched upon a steep 

 ledge of rock ; from a comparison of the volcano of Purace (elevation 

 14,548 feet, intensity 1.077), with the mountain village of Purace 

 (elevation 8671 feet, intensity 1.087), and with the neighbouring 

 town of Popayan (elevation 5825 feet, intensity 1.117) ; from a com- 

 parison of the town of Quito (elevation 9541 feet, intensity 1.067), 

 with the village of San Antonio de Lulumbamba (elevation 8131 

 feet, intensity 1.087) lying in a neighbouring rocky fissure directly 

 under the geographical equator. The oscillation experiments, which I 

 made at the highest point at which I ever instituted observations of the 

 kind, namely, at an elevation of 15,944 feet on the declivity of the long 

 since extinct volcano of Antisana, opposite the Chussulongo, were quite 

 at variance with this result. It was necessary to make this observation 

 in a large cavern, and the great increase in the intensity was no doubt 

 the consequence of a magnetic local attraction of the trachytic rock, as 

 has been shown by the experiments which I made with Gay-Lussac 

 within, and on the margin of, the crater of Vesuvius. I found the 

 intensity in the Cave of Antisana increased to 1.188, while in the neigh- 

 bouring lower plateau it was scarcely 1.068. The intensity at the 

 Hospice of St. Gotthard (1.313) was greater than that at Airolo (1.309), 

 but less than that at Altorf (1.322). Airolo, on the other hand, exceeded 

 the intensity of the Ursern Lake (1.307). In the same manner Gay- 

 Lussac and myself found that the intensity was 1.344 at the Hospice of 

 Mont Cenis, whilst at the foot of the same mountain, at Lans le Bourg, 

 it was 1.323, and at Turin 1.336. The greatest contradictions were 

 necessarily presented by the burning volcano of Vesuvius, as we have 

 already remarked. Whilst in 1805 the terrestrial force at Naples was 

 1,274, and at Portici 1.288, it rose in the Monastery of St. Salvador to 

 1.302, whilst it fell in the crater of Vesuvius lower than anywhere else 

 throughout the whole district, namely, to 1.193. The iron contained in 

 the lava, the vicinity of magnetic poles, and the heat of the soil, which 

 probably has the effect of diminishing this force, combined to produce 

 the most opposite local disturbances. See my Voyage aux Regions 

 Equinoxiales, t. iii, pp 619 626, and Mvm. de la Societe d'Arcueil, t. i, 

 1807, pp. 1719. 



VOL. V. H 



