116 COSMOS. 



to the Canary Islands, in the year 1776, which is preserved 

 at Paris in the Depot de la Marine, and which I have been 

 enabled to consult through the obliging courtesy of Admiral 

 Rosily, I have discovered that Borda was the first who made 

 an attempt to investigate the influence of a great elevation 

 on the inclination. He found that the inclination was 1 15' 

 greater at the summit of the Peak of Tenerifie than in the 

 harbour of Santa Cruz, owing undoubtedly to the local 

 attractions of the lava, as I have often observed on Vesuvius 

 and different American volcanoes. (Humboldt, Voy. aux 

 Regions Eqiiinox., t. i, pp. 116, 277, 288.) 



In order to try whether the deep interior portions of the 

 body of the earth influence magnetic inclination in the same 

 manner as elevations above the surface, I instituted an ex- 

 periment during my stay at Freiberg, in July 1828, with all 

 the care that I could bestow upon it, and with a constant 

 inversion of the poles ; when I found after very careful in- 

 vestigation that the neighbouring rock, which was composed 

 of gneiss, exerted no action on the magnetic needle. The 

 depth below the surface was 854 feet, and the difference 

 between the inclination of the subterranean parts of the 

 mine and those points which lay immediately above it, and 

 even with the surface, was only 2'. 06 ; but considering the 

 care with which my experiments were made, I am inclined 

 to think from the results given for each needle, as recorded in 

 the accompanying note,* 5 that the inclination is greater in 



45 In the Churprinz mine at Freiberg, in the mountains of Saxony, 

 the subterranean point was 133 fathoms deep, and was observed with 

 Freiesleben and Reich at 2 P.M. (temperature of the mine being 

 60.08 F.). The dipping needle A showed 67 37'.4, the needle B 

 67 32 / .7, the mean of both needles in the mine was 67 35'.05. In the 

 open air, at a point of the surface which lies immediately above the 

 point of subterranean observation, the needle A stood at 11 A.M. at 

 67 33'.87 and the needle B at 67 32'.12. The mean of both needles 

 in the upper station was 67 32'.99, the temperature of the air being 

 60.44 F., and the difference between the upper and lower result 

 2'.06. The needle A, which, as the stronger of the two, inspired me 

 with most confidence, gave even 3'. 53, whilst the influence of the depth 

 remained almost inappreciable when the needle B only was used (Hum- 

 boldt, in Poggend. Annal. Bd. xv, s. 326). I have already described in 

 detail, and elucidated by examples, in Asie Centr. t. iii, pp. 465 467, 

 the uniform method which I have always employed in reading the 

 azimuth circle in order to find the magnetic meridian by corresponding 



