186 COSMOS. 



The knowledge which we possess of the quantity of" this in 

 crease, and of" all the numerical relations of the law of in- 



may be expressed at Paris by 1-3482, in Mexico by 1-3155, in San Carlos 

 del Rio Negro by 1-0480, and in Lima by 1-0773. When I developed 

 this law of the variable intensity of terrestrial magnetic force, and sup- 

 ported it by the numerical value of observations instituted in 104 dif- 

 ferent places, in a Memoir read before the Paris Institute on the 2Gth 

 Frimaire, An. XIII. (of which the mathematical portion was 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 Historique, t. i., p. 262, note 1, that M. de Rossel commu- 

 nicated to Biot his oscillation experiments made six years earlier (be- 

 tween 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 ex- 

 cellent man, when he wrote his work, v^^as not awai'e of the regularity 

 of the augmentation and diminution of the intensity, as before the read- 

 ing 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 retui-n from America, 

 that the observations made by M. de Rossel were published in the Voy' 

 age de V Entrecasteaux, t. ii., p. 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 iti Germany (Hausteen, Magnet, der Erde, 1819, 

 s. 71; Gauss, Beob. des Magnet. Vereins, 1838, s. 36-39 ; Erman, Phy- 

 eikal. Beoh., 1841, s. 529-579), iu England (Sabine, Report on Magnet. 

 Intensity. 1838, p. 43-62 ; Contributions to Terrestrial Magnetism, 1843), 

 and in France (Becquerel, Traits de Electr. et de 3Iagnet., t. vii., p. 

 354-367), to reduce the oscillations observed in any part of the Earth 

 to the standard of force which I found on the magnetic equator iu 

 Northern Peru, so that, according to the unit thus arbitranly assumed, 

 the intensity of the magnetic force at Paris is put down as 1*348. Tht 

 observations made by Lamanoii in the unfortunate expedition of La 

 Perouse, during the stay at TenerifFe (1785), and on the voiyage to 

 Macao (1787), are still older than those of Admiral Rossel. They were 

 sent to the Academy of Sciences, and it is known that they were in the 



Eossession of Condorcet in the July of 1787 (Becquerel, t. vii., p. 320) ; 

 ut, notwithstanding the most careful search, they are not now to be 

 found. From a copy of a very important letter of Lamanon, now in the 

 possession of Captain Duperrey, which was addressed to the then per- 

 petual secretary of the Academy of Sciences, but was omitted in the 

 narrative of the Voyage de La Pe^-ouse, it is stated " that the attractive 

 force of the magnet is less in the tropics than when we approach the 

 poles, and that the magnetic intensity deduced from the number of os- 

 cillations of the needle of the inclination-compass varies and increases 

 with the latitude." If the Academicians, while they continued to ex- 

 pect the return of the unfortunate La Perouse, had felt themselves justi- 

 fied, in the course of 1787, in publishing a truth which had been inde- 

 pendently discovered by no less than three different travelers, the theory 

 of terrestrial magnetism would have been extended by the knowledge 

 of a new class of observations, dating eighteen years earlier than they 

 now do. This simple statement of facts may probably justify the ol> 

 Bervations contained in the third volume of my Relation Historique (p 



