Iviii NOTES. 



hemispheres ; and it is not always easy to discriminate, without much patient 

 consideration, to which hemisphere a particular effect is due. EDITOR.] 



( 159 ) p. 174. The following is the historical account of the discovery of an 

 important law in terrestrial magnetism, that of the general increase of the 

 intensity of the force with the increase of magnetic latitude. When, in 

 1798, I was about to join the expedition of Captain Baudin, on a voyage of 

 circumnavigation, Borda, who took a warm interest in my intended proceed- 

 ings, proposed to me to observe, in different latitudes and both hemispheres, 

 the oscillations in a vertical plane of a needle moving freely in the magnetic 

 meridian, for the purpose of examining whether the magnetic force varied, o 

 was every where the same. In my subsequent voyage to the equinoctial 

 regions, this investigation formed one of the principal objects which I had in 

 view. I observed the same needle perform in ten minutes, at Paris, 245 oscilla- 

 tions ; at Havanna, 246 ; at Mexico, 242 ; at San Carlos del Rio Negro (lat. 1 

 53' N., long. 80 40' W. from Paris) 216 ; on the magnetic equator, or on the 

 line where the inclination = 0, in Peru (in 7 1' S. lat., 80 40' W. long, from 

 Paris), only 211 ; and in Lima (12 2' S. lat.) again 219 oscillations. I thus 

 found that at that time, 1799 1803, if the intensity of the total magneti* 

 force were taken as = 1,0000 on the magnetic equator, in the Peruvian chain 

 of the Andes between Micuipampa and Caxamarca, it should be expressed, in 

 Paris, by 1,3482; in Mexico, by 1,3155; at San Carlos del Rio Negro, by 

 1,0480; and at Lima, by 1,0773. When, in a Memoir, the mathematical 

 portion of which belonged to M. Biot, I developed to the French Insti- 

 tute in the Seance du 26 Frimaire, An. xiii. de la Republique, this law 

 of the variable intensity of the terrestrial magnetic force, supporting it by 

 numerical values obtained by observations at 104 different points of the 

 Earth's surface, the fact was regarded as perfectly new. It was not until 

 after the reading of this Memoir, as Biot has said in it most distinctly, 

 (Lametherie, Journal de Physique, T. lix. p. 446, note 2,) and as I have re- 

 peated in my " Relation historique," T. i. p. 262, note 1, that M. de Rossel 

 communicated to Biot his six previous observations of the oscillations of a needl* 

 made between 1791 and 1794 in Van Diemen's Land, Java, and Amboina. 

 These observations shewed the law of decreasing force to exist also in the 

 Indian Archipelago. It seems reasonable to conjecture that this excellent 

 man had not recognised in his observations the regularity of the increase and 

 decrease of the magnetic intensity, since he had never spoken of this surely 

 not unimportant physical law to our common friends, Laplace, 



