MAGNETISM. 



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the earliest iacts yielded by observation is, that the intensity 

 of the total force increases from the equator towards the pole.* 

 The knowledge which we possess of the quantity of this 

 increase, and of all the numerical relations of the law of 

 intensity affecting the whole Earth, is especially due, since 



* The following is the history of the discovery of the law that the 

 intensity of the force increases (in general) with the magnetic latitude. 

 AVhen I was anxious to attach myself in 1798 to the expedition of 

 Captain Baudin, who intended to circumnavigate the globe, I was re- 

 quested by Borda, who took a warm interest in the success of my project, 

 to examine the oscillations of a vertical needle in the magnetic meridian 

 in different latitudes in each hemisphere, in order to determine whether 

 the intensity of the force was the same, or whether it varied in different 

 places. During my travels in the tropical regions of America, I paid 

 much attention to this subject. I observed that the same needle which in 

 the space of ten minutes made 245 oscillations in Paris, 246 in the Ha- 

 vanna, and 242 in Mexico, performed only 216 oscillations during the 

 same period at St. Carlos del Rio Negro (1 53' north lat. and 80 40' west 

 long, from Paris), on the magnetic equator, i. e. the line in which the in- 

 clination = 0; in Peru (7 1' south lat. and 80 40' west long, from Paris) 

 only 211 ; while at Lima (12 2' south lat.) the number rose to 219. I found, 

 in the years intervening between 1799 and 1803, that the whole force, if 

 we assume it at TOOOO on the magnetic equator in the Peruvian Andes, 

 between Micuipampa and Caxamarca, may be expressed at Paris by 

 1-3482, in Mexico by T3155, in San Carlos del Eio Negro by 1-0480, 

 and in Lima by TO 773. When I developed this law of the variable 

 intensity of terrestrial magnetic force, and supported it by the numerical 

 value of observations instituted in 104 different places, in a Memoir 

 read before the Paris Institute, on the 26th Frimaire, An XIII. (of 

 which the mathematical portion \*as contributed by M. Biot), the facts 

 were regarded as altogether new. It was only after the reading of the 

 paper, as Biot expressly states (Lametherie, Journal de Physique, t. lix. 

 p. 446, note 2), and as I have repeated in the Relation liistorique, t. i. 

 p. 262, note 1, that M. de Rossel communicated to Biot his oscillation- 

 experiments made six years earlier (between 1791 and 1794) in Van 

 Diemen's Land, in Java, and in Amboyna. These experiments gave 

 evidence of the same law of decreasing force in the Indian Archipelago. 

 It must, I think, be supposed that this excellent man, when he wrote 

 his work, was not aware of the regularity of the augmentation and dimi- 

 nution of the intensity, as before the reading of my paper he never 

 mentioned this (certainly not unimportant) physical law to any of our 

 mutual friends, La Place, Delambre, Prony, or Biot. It was not till 

 1808, four years after my return from America, that the observations 

 made by M. de Rossel were published in the Voyage de I' Entrecasteaux, 

 t. ii. pp. 287, 291, 321, 480, and 644. Up to the present day it is still 

 usual, in all the tables of magnetic intensity which have been published 

 in Germany (Hansteen, Magnet, der Erde^ 1819, s. 71; Gauss, eob. 



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