122 



calm weather. It would be above all important 

 to know the magnetic force, where the dip is the 

 smallest." The imperfection of the compasses 

 put on board Mr. La Perouse's vessels no doubt 

 prevented the astronomers in this expedition 

 from paying attention to investigations of this 

 kind, and the wishes of the academy of sciences 

 were fulfilled only in the voyage of d'Entrecas- 

 teaux, and in that of which this work gives the 

 narrative. Among the great number of valuable 

 observations for which we are indebted to Mr. 

 Rossel, there are five on the oscillations of the 

 magnetic needle * made at Brest, at TenerifFe, 

 at Van Diemen's Land, at Amboyna, and at 

 Java. For my part, I have determined, jointly 

 with Messrs. Gay-Lussac and Bonpland, from 

 5° of magnetic latitude south, to 60° magnetic 

 latitude north, the intensity of the forces in a 

 hundred and eighty places belonging to two sys- 

 tems of particular attractions ^. In the space 



* These observations were published only in 1808 (Voy. 

 de d'Entrecasteaux, t. ii, p. 287, 291, 321, 480, and 644) ; 

 but they were made eight years before my journey to the 

 Oroonoko. I was acquainted with them as early as the year 

 1805, immediately after having communicated to tbe first 

 class of the Institute the general results of the progressive 

 decrement of the magnetic forces from Paris to the magnetic 

 equator. See the memoir, which I published at that time, 

 conjointly with Mr. Biot, in the sixtieth vol. of the Journ. de 

 Physique. 



f On account of the inflexions of the magnetic equator, we 



