88 



COSMOS. 



the earth's surface, by counting the number of oscillations in 

 equal times, was made by Mallet in 1769. He found, with 

 a very imperfect apparatus, that the number of the oscilla- 

 tions at St. Petersburg (59 56' N. lat.) and at Ponoi (67 

 4 X ) were precisely equal ;* and hence arose the erroneous 

 opinion, which was even transmitted to Cavendish, that the 

 intensity of the terrestrial force was the same under all lati- 

 tudes. Borda, as he has himself often told me, was prevent- 

 ed, on theoretical grounds, from falling into this error, and 

 the same had previously^been the case with Le Monnier ; but 

 the imperfection of the dipping-needle, the friction which ex- 

 isted between it and the pivot, prevented Borda (in his expe- 

 dition to the Canary Islands in 1776) from discovering any 

 difference in the magnetic force between Paris, Toulon, Santa 

 Cruz de Teneriffe, and Goree, in Senegambia, over a space 

 of 35 of latitude. (Voyage de La Perouse, t. i., p. 162.) 

 This difference was for the first time detected, with im- 

 proved instruments, in the disastrous expedition of La Pe- 

 rouse in the years 1785 and 1787, by Lamanon, who com- 

 municated it from Macao to the Secretary of the French 

 Academy. This communication, as I have already stated 

 (see p. 62), remained unheeded, and, like many others, lay 

 buried in the archives of the Academy. 



The first published observations of intensity, which, more- 

 over, were instituted at the suggestion of Borda, are those 

 which I made during my voyage to the tropical regions of 

 the New Continent between the years 1798 and 1804. The 

 results obtained at an earlier date (front 1791 to 1794), re- 

 garding the magnetic force, by my friend De Rossel, in the 

 Indian Ocean, were not printed till four years after my re- 

 turn from Mexico. In the year 1829 I enjoyed the advant- 

 age of being able to prosecute my observations of the mag- 

 netic intensity and inclination over a space of fully 188 of 

 longitude from the Pacific eastward as far as the Chinese 

 Dzungarei, two thirds of this portion of the earth's surface 

 being in the interior of continents. The differences in the 

 latitudes amounted to 72 (namely, from 60 N. to 12 S. 

 lat.). 



When we carefully follow the direction of the closed iso- 

 dynamic lines (curves of equal intensity), and pass from the 

 external and weaker to the interior and gradually stronger 



* Novi Comment. Acad. Scient. Petropol, t. xiv., pro anno 1769, pars 

 2, p. 33. See also Le Monnier, Lois du Magnetisme comparees aux 

 Observations, 1776, p. 60. 



