94 TERRESTRIAL MAGNETISM. 



vibrations of his dipping-needle, with the view of examining 

 whether they were constant, ( 92 ) and of ascertaining the 

 ratio of the directive force to the force of gravity. The 

 first attempt to determine, by the number of vibrations 

 performed in equal times, the comparative force of magne- 

 tism at points of the earth distant from each other, was 

 made by Mallet, in 1769. "With very imperfect instru- 

 ments, he found no difference in the number of vibrations 

 performed in the same interval of time at St. Petersburg 

 in lat. 59 56', and at Ponoi, in lat. 67 4',p 3 ) whence 

 arose the erroneous opinion, which continued to the time of 

 Cavendish, that the intensity of the earth's magnetic force 

 is the same in all latitudes. Borda, indeed, as he has often 

 related to me, on theoretical grounds never shared in this 

 error, nor did Le Monnier ; but Borda himself, in his expe- 

 dition to the Canaries in 1776, was prevented by the 

 imperfection of his dipping-needle (friction on its pivots) 

 from discovering any differences of magnetic force at places 

 distributed over an interval of 35 of latitude, Paris, 

 Toulon, Santa Cruz in Teneriffe, and Goree (Voyage de la 

 Perouse, Yol. i. p. 162). By the aid of improved instru- 

 ments, these differences were found for the first time in the 

 ill-fated expedition of La Perouse, in the years 1785 and 

 1787, by Lamanon, and were communicated by him from 

 Macao to the Secretary of the Paris Academy ; but, as I have 

 already remarked (p. 63), they remained till a much later 

 period, unnoticed, and buried in the Archives of the 

 Academy. 



The first published observations of the variation of the 

 force (which were also begun at Borda' s request) were my 

 own, made in my travels to the equinoctial regions of 



