398 



The magnetic observations made on the banks 

 of the Rio Negro are, of all those we know in 

 the interior of a great continent, the nearest to 

 the magnetic equator. They have consequently 

 served to determine* the position of this equa- 

 tor, which I crossed more to the west on the 

 ridge of the Andes, between Micuipampa and 

 Caxamarca, in the seventh degree of south lati- 

 tude. The magnetic parallel of San Carlos (that 

 of 22 6° cent.) passes through Popayan, and in 

 the South Sea through a point (at 3° 12' north 

 lat., and 89° 36' west long.), where I was for- 

 tunate enough to have an opportunity of mak- 

 ing observations in very calm weathers- 

 eight times ; that of Mr, de Rossel, two hundred and eighty- 

 eight times. The first therefore would have made two hun- 

 dred and forty-five oscillations at Brest, reducing it to the 

 observations of Mr. de Rossel. This is exactly the number, 

 which it gave at Paris, and this number confirms the exact- 

 ness of the comparison. (Hanstem, p. 70 and 72.) 



* Mr. Hansteen finds, according to my observations, the 

 magnetic equator in the longitude of San Carlos del Rio 

 Negro (69° 58' west of Paris) in the latitude of nine 

 degrees and a half south. Mr. Orlet, in a valuable paper 

 presented lately to the Academy of Sciences, makes the line 

 of no dip pass through 7° 44' of south latitude . M. Biot gives 

 San Carlos 10° 1# 14' of magnetic latitude. 



f Popayan (lat. 2° 26' 1.7 " north ; long. 78<> 69'). Dip 

 23'05° cent. South Sea (the spot mentioned in the text). 

 Dip. 22'8° cent. But the isodynamic parallel of San Carlos, 

 that is to say, the line of equal intensity, passes to the south 

 of these two places. 



