MAGNETISM. 1 77 



change of configuration, have been made an object of careful 

 investigation in modern times. According to the admirable 

 work of Duperrey,* who crossed the magnetic equator six times 

 between 1822 and 1825, the nodes of the two equators, that 

 is to say, the two points at which the line without inclination 

 intersects the terrestrial equator, and consequently passes from 

 one hemisphere into the other, are so unequally placed, that in 

 1825 the node near the island of St. Thomas, on the western 

 coast of Africa, was 188-^ distant from the node in the South 

 Sea, close to the little islands of Gilbert, nearly in the meri- 

 dian of the Viti group. In the beginning of the present cen- 

 tury, at an elevation of 11,936 feet above the level of the sea, 

 I made an astronomical determination of the point (7 1' south 

 lat., 48 40' west long, from Paris), where, in the interior of 

 the New Continent, the chain of the Andes is intersected by 

 the magnetic equator between Quito and Lima. To the west of 

 this point, the magnetic equator continues to traverse the 

 South Sea in the southern hemisphere, at the same time slowly 

 drawing near the terrestrial equator. It first passes into the 

 northern hemisphere a little before it approaches the Indian 

 Archipelago, just touches the southern points of Asia, and 

 enters the African continent to the west of Socotora, almost in 

 the Straits of Bab-el-Mandeb, where it is most distant from 

 the terrestrial equator. After intersecting the unknown regions 

 of the interior of Africa in a south-west direction, the 

 magnetic equator re-enters the south tropical zone, in the Gulf 

 of Guinea, and retreats so far from the terrestrial equator, that 

 it touches the Brazilian coast near Os Ilheos, north of Porto 

 Seguro, in 15 south lat. From thence to the elevated plateaux 

 of the Cordilleras, between the silver mines of Micuipampa 

 and Caxamarca, the ancient seat of the Incas, where I observed 

 the inclination, the line traverses the whole of South America, 

 which in these latitudes is as much a magnetic terra incognita 

 as the interior of Africa. 



The recent observations of Sabinef have shown that the 



* Dnperrey, De la Configuration de VEquateiLr Magnetique, in the 

 Annales de Chimie, t. xlv. pp. 371 and 379. (See also Morlet, in the 

 Memoires presentes par divert Savans a VAcad. Eoy. des Sciences, 

 t. iii. p. 132.) 



t See the remarkable chart of isoclinic lines in the Atlantic Ocean for 

 the years 1825 and 1837, in Sabine's Contributions to Terrestrial Mag- 

 netism, 1840, p. 134. 



