MAGNETIC INTENSITY. 61 



restrial magnetism (1784 1788). Their ingenious physical 

 experiments embraced the magnetic attraction of all matter, 

 the local distribution of the force in a magnetic rod of a 

 given form, and the law of its action ar a distance. In order 

 to obtain accurate results, the vibrations of a horizontal 

 needle suspended by a thread, as well as deflections by a 

 torsion balance, were in turn employed. 



The knowledge of the difference of intensity of ter- 

 restrial magnetism at different points of the earth's surface 

 by the measurement of the vibrations of a vertical needle 

 ID. the magnetic meridian, is due solely to the ingenuity 

 of the Chevalier Borda ; ~Dt from any series of speciallv 

 successful experiments, but by a process of reasoning, and 

 by the decided influence which he exerted on those who 

 were equipping themselves for remote expeditions. Borda's 

 long cherished conjectures were first confirmed by means 

 of observations made from the year 1785 to 1787, by 

 i^amanon, the companion of La Perouse. These results 

 remained unknown, unheeded, and unpublished, although 

 they had been communicated as early as the summer 

 of the last-named year to Condorcet, the Secretary of the 

 Academic des Sciences. The first, and therefore cer- 

 tainly an imperfect knowledge of the important law of the 

 variability of intensity in accordance with the magnetic 

 latitude, belongs undoubtedly 67 to the unfortunate but scien- 

 tifically equipped expedition of La Perouse ; but the law 

 itself, as 1 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 Canary Islands, 

 the interior of tropical America both north and south of the 

 equator, and in the Atlantic and Pacific Oceans. The suc- 

 cessful expeditions of Le Gentil, Feuille"e, and Lacaille ; the 

 first attempt made by Wilke, in 1768, to construct an incli- 

 nation chart } the memorable circumnavigations of Bougain- 

 ville, Cook, and Vancouver, have all tended, although by the 



G " Cosmos, vol. i, pp. 179 181. The admirable construction of the 

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

 sibility 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, 



