102 COSMOS. 



made by Sabine* at the beginning of his pendulum expedi- 

 tion in 1822. Subsequently, in 1840, the same learned ob- 

 server noted down the results obtained by Duperrey, Allen, 

 Dunlop, and Sulivan, and constructed a chart of the magnet- 

 ic equator^* from the west coast of Africa at Biafra (4° N. 

 lat., 9° 30^ E. long.), through the Atlantic Ocean, and Bra- 

 zil (16° S. lat., between Porto Seguro and Rio Grande), to 

 the point where, upon the Cordilleras, in the neighborhood 

 of the Pacific, I saw the northern inclination assume a south- 

 ern direction. The African node, as the point of intersection 

 of both equators, was situated, in 1837, in 3°£. long., while 

 in 1825 it had been in 6° 57^ E. long. The secular motion 

 of the node, turning from the basaltic island of St. Thomas, 

 which rises to an elevation of more than 7000 feet, was, 

 therefore, somewhat less than half a degree westward in the 

 course of a year ; after which the line of no inclination tu'ned 

 toward the north on the African coast, while oii the Brazil- 

 ian coast it is inclined southward. The convexity of the 

 magnetic equatorial curve is persistently turned toward the 

 south pole, while in the Atlantic Ocean it passes at a dis- 

 tance of about 16° from the geographical equator. For the 

 interior of South America, the terra incognita of Matto Grosso 

 between the large rivers of Xingu, Madera, and Ucayle, we 

 have no observations of the dip until we reach the chain of 

 the Andes, where, 68 geographical miles east of the shores 

 of the Pacific, between Montan, Micuipampa, and Caxa- 

 marca, I determined astronomically the position of the mag- 

 netic equator, which rises toward the northwest (7° 2^ S. 

 lat., and 78° 46^ W. long.).}: 



* Sabine, Pendul. Exper., 1825, p. 476. 



t Sabine, in the Phil. Transact, for 1840, pt. i., p. 136, 139, 146. 

 I follow, for the progression of the African node, the map which is 

 appended to this treatise. 



X I here give, in accordance with my usual practice, the elements 

 of this not wholly unimportant determination : Micuipampa, a Peru- 

 vian mountain town at the foot of Cerro de Guelgayoc, celebrated for 

 its rich silver mines, 6° 44' 25'' S. lat., 78° 33' 3 " W. long., elevation 

 above the Pacific 11,872 feet, magnetic inclination 0°'42 north (ac- 

 cording to the centesimal division of the circle) ; Caxamarca, a town 

 situated on a plateau at an elevation of Q362 feet, 7° 8' 38" S. lat, 

 5h. 23' 42" long., inclination 0*15 south; Montan, a farm-house (or 

 hacienda), surrounded by Llama flocks, situated in the midst of mount- 

 ains, 6° 33' 9" S. lat., 5h. 26' 51" W. long., elevation 8571 feet, in- 

 clination 0-70 north; Tomependa, on the mouth of the Chinchipe, on 

 the River Amazon, in the province of Jaen de Bracamoros, 5° 31' 28" 

 S. lat., 78° 37' 30" W. long., elevation 1324 feet, inclination 3°-55 

 north; Truxillo, a Peruvian town on the Pacific, 8° 5' 40" S. lat., 



