113 



for the use of repeating circles, has also furnish- 

 ed travellers with the means of making accurate 

 observations on the magnetic dip. Borda's 

 compass was successfully employed in the voy- 

 age of Viceadmiral d'Entrecasteaux, in that of 

 Captain Baudin, and in the excursions of Mr- 

 Nouet in Egypt. If we add the results obtained 

 in these different voyages to those I have col- 

 lected during seven years in the two divisions of 

 America, in Spain, France, Italy, Switzerland, 

 and Germany, we shall have a great mass of 

 observations, comparable * with each other, 



* The observations of the clipping needle made by deRos- 

 sel, Freycinet, Nouet, Gay-Lussac, and myself, are so much 

 the more adapted to be compared with each other, as they 

 embrace but a very short portion of time. Le Monnier {Lois 

 du Magne'ti$me> p. 57) and Lord Mulgrave {Voyage to the 

 North Poky p. 68) still admitted the invariability of the mag- 

 netic dip : but Messrs. Gilpin and Cavendish proved, in 1806, 

 by direct experiments, that the dip of the needle is subject, 

 like the variation, to annual oscillations, though extremely 

 slow. The cities of London and Paris are hitherto the only 

 places, where the extent of these oscillations is known. The 

 dip at London, in 1775, was 72° 30' and in 1805, 70° 21> 

 (Phil. Trans., vol. Ixvi, pi. 1, p. 401). We cannot admit* 

 with P. Cotte (Journ. de Physique, t. Ixvi, p. 277), that, 

 before the year 1808, the dip of the magnetic needle was 

 not known with precision at Paris. I had determined it 

 with a great deal of care, conjointly with Mr. de Borda, in 

 1798, before my departure for Spain. It was then 69o 51'. 

 Mr.- Gay-Lussac found it in 1806, 69° 12'. On the 7th of 

 October, 1810, the dip at Paris was 68° 50/ ; and on the 10th 



