NOTES. li x 



Prony, and Biot, before the reading of my Memoir. It was not until 1808, 

 four years after my return from America, that his observations were published 

 in the Voyage d'Entrecasteaux, T. ii. pp. 287, 291, 321, 480, and 644. 

 The custom of expressing all observations of the magnetic force made in any 

 part of the globe in terms of the force found by me on the magnetic equator 

 in the North of Peru, in which arbitrary scale the force at Paris is taken ;is 

 = 1'348, has been continued, even up to the present time, in all the tables 

 of magnetic intensity which have been published in Germany (Hansteen's 

 Magnetismus der Erde, 1819, S. 71 ; Gauss, Beob, des Magnet. Vereins, 1838, 

 S.36 39 ; Eruiau, Physikal. Beob. 1841,8.529 579): m England, (Sabine's 

 Report on Magnetic Intensity, 1838, pp. 4362 ; and Contributions to Ter- 

 restrial Magnetism, 1840, etseq.) : and in France, (BecquereljTraited'Electricite 

 et de Magnetisme, T. vii. pp. 354 367.) Still earlier than Admiral Rossel's 

 observations were those made by Lamanon, in the unfortunate expedition of 

 La Perouse, from its stay at Tenerifle, 1785, to its arrival at Macao, 1787, 

 and which were sent to the Academic des Sciences. It is known, with certainty, 

 (Becquerel, T. vii. p. 320), that they had reached the hands of Condorcet in 

 July, 1787 , but all the attempts which have been hitherto made to discover 

 them have proved fruitless. Captain Duperrey is in possession of a copy of a 

 very important letter, written by Lamanon to the then perpetual secretary of 

 the Academy, and which was omitted to be printed in the " Voyage de La 

 Perouse." It is expressly said in this letter, " Que la force attractive de 

 1'aiinant est moindre dans les tro^iques qu'en avancant vers les poles, et que 

 1'intensite magnetique deduite du nonibre des oscillations de 1'aiguille de la 

 boussole d'inclinaison change et augmcnte avec la latitude." If the Academy 

 had at that time thought itself justified iu anticipating the then hoped-for 

 return of La Perouse. and in making known a truth which was eventuallf 

 discovered independently by three travellers unknown to each other, Lamanok. 

 De Rossel, and myself, the theory oi % terrestrial magnetism would have been 

 advanced eighteen years earlier by the knowledge ot a new class ot phenomena. 

 This simple narrative of facts may, perhaps, be held to justify the statement con- 

 tained in the following passage of my " Relation historique," Vol. iii. p. 615 -. 

 " Les observations sur les variations du magnetisme terrestre auxquelles jc me 

 suis livre pendant 32 aiis, au moyen d'instrumens comparables entre eux en 

 Ame'rique, en Europe et en Asie, embrassent, dans les deux hemispheres, 

 dcpuis les frontieres de la Dzoungarie chinoise jusque vers 1'ouest a la Mer da 

 Sud qui baigne les cotes du Mexique et du Pe'rou, un espace de 188 de 

 longitude, depuis les 60 de latitude nord jusqu'aux 12 de latitude sud. J'ai 

 VOL. I. r< 



