11^ 



Part of these observations served as a basis to 

 the theories and calculations of Messrs. Loewen- 

 oern, Biot, and Kraft # ; they give the direction 

 of the equator, or of the magnetic parallels, with 

 so much the more precision, as I took the same 

 pains in examining the dip, as in ascertaining 

 the geographical position of the vessel. The 

 most accurate observations on the variation of 

 the needle, on it's dip, and on the intensity of 

 the magnetic forces, would be of little value, if 

 the traveller were not provided with the instru- 

 ments necessary to ascertain astronomically the 

 latitude and longitude of the place where the 

 different phenomena of terrestrial magnetism 

 were observed. 



I shall not speak of the attempt I made during 

 the passage, to determine the curves of the mag- 

 netic variation. The results obtained at sea by 

 the best azimuth compasses are so uncertain, 

 that, according to the testimony of the most ex- 

 perienced navigators -jf, errors of two or three 

 degrees often occur. Even supposing them 

 only one degree, this uncertainty, augmented by 



* Danske Vid. Selskabs Scrivter, 1802, p. 295. Journ. 

 de Plrys. t. lix, p. 287. Mem. de Petersbourg, 1809, t. i, 

 p. 248. See also Mollweide's Attempt to generalize the The- 

 ories of Euler and Mayer, in Gilbert's Annalen, t, xxix, p. 1 

 and 251. 



+ Voy. de Vancouver, t. i, p. 40 and 99. De Rossel, in 

 xhe Voy. de d'Entrecasteaux, t. ii, p. 172. Cooke's second 

 Voyage, vol. i, p. 24. 



