62 cosmos. 



made from the year 1785 to 1787, by Lamanon, the com- 

 panion of La Perouse. These results remained unknown, 

 unheeded, and unpublished, although they had been commu- 

 nicated as early as the summer of the last-named year to 

 Condorcet, the Secretary of the Academie des Sciences. The 

 first, and therefore certainly an imperfect knowledge of the 

 important law of the variability of intensity in accordance 

 with the magnetic latitude, belongs undoubtedly* to the un- 

 fortunate but scientifically equipped expedition of La Perouse ; 

 but the law itself, as I rejoice to think, was first incorporated 

 in science by the publication of my observations, made from 

 1798 to 1804, in the south of France, in Spain, the Canaiy 

 Islands, the interior of tropical America both north and 

 south of the equator, and in the Atlantic and Pacific oceans. 

 The successful expeditions of Le Gentil, Feuillee, and La- 

 caille; the first attempt made by Wilke, in 1768, to con- 

 struct an inclination chart ; the memorable circumnaviga- 

 tions of Bougainville, Cook, and Vancouver, have all tended, 

 although by the help of instruments possessing very unequal 

 degrees of exactness, to establish the previously neglected but 

 very important element of inclination at various intervals of 

 time, and at many different points — the observations being 

 made more at sea, and in the immediate vicinity of the ocean, 

 than in the interior of continents. Toward the close of the 

 18th century, the stationary observations of declination which 

 were made by Cassini, Gilpin, and Beaufoy (from 1784 to 

 1790), with more perfect instruments, showed definitely that 

 there is a periodical influence at different hours of the day, 

 no less than at different seasons of the year — a discovery 

 which imparted a new stimulus to magnetic investigations. 



In the 19th century, half of which has now expired, this 

 increased activity has assumed a special character differing 

 from any that has preceded it. We refer to the almost si- 

 multaneous advance that has been made in all branches of 

 the theory of terrestrial magnetism, comprising the numeric- 

 al determination of the intensity, inclination, and variation 

 of the force ; in physical discoveries in respect to the excita- 

 tion and the amount of the distribution of magnetism ; and 



* Cosmos, vol. i., p. 185-187. The admirable construction of the 

 inclination compass made by Lenoir, according to Borda's plan, the 

 possibility of having long and free oscillations of the needle, the much 

 diminished friction of the pivots, and the correct adjustment of instru- 

 ments provided with scales, have been the means of enabling us accu- 

 rately- to measure the amount of the terrestrial force in different zones. 



