'LINES OF NO VARIATION. 141 



presents lines which exhibit a western variation of from 6 

 to 29 ; for, according to Purchas, the Atlantic line of no 

 variation left the southern point of Africa (the Cape of Good 

 Hope) in the year 1605, inclining farther from east to west. 

 The possibility that we may discover in some part of Cen- 

 tral Africa an oval group of concentric lines of variation de- 

 creasing to 0, and which is similar to that of the Pacific, 

 can neither be asserted nor denied on any sure grounds. 



The Atlantic portion of the American curve of no varia- 

 tion was accurately determined in both hemispheres for the 

 year 1840, by the admirable investigations of General Sa- 

 bine, who employed 1480 observations, and duly took into 

 account the secular changes. It passes in the meridian of 

 70 S. lat,, and about 19 W. long.,* in a NN.W. direction, 

 to about 3 east of Cook's Sandwich Land, and to about 9 

 30' east of South Georgia ; it then approaches the Brazilian 

 coast, which it enters at Cape Frio 2 east of Rio Janeiro, 

 and traverses the southern part of the New Continent no 

 farther than 36 X S. lat., where it again leaves it some- 

 what to the east of Gran Para, near. Cape Tigioca, on the 

 Eio do Para, one of the secondary outlets of the Amazon, 

 crossing the geographical equator in 47 44' W. long., then 

 skirting along the coast of Guiana at a, distance of eighty- 

 eight geographical miles as far as 5 N. lat., and afterward 

 following the arc of the small Antilles as far as the parallel 

 of 18, and, finally, touching the shore of North Carolina 

 near Cape Lookout, southeast of Cape Hatteras, in 34 50' 

 N. lat., 74 8 / W. long. In the interior of North America, 

 the curve follows a northwestern direction as far as 41 30' 

 N. lat., 77 38' W. long., toward Pittsburgh, Meadville, and 

 Lake Erie. We may conjecture that it has advanced very 

 nearly half a degree farther west since 1840. 



The Australo- Asiatic curve of no variation (if, according 

 to Erman, we consider the part which rises suddenly from 

 Ivasan to Archangel and Russian Lapland as identical with 



Ocean, straight across Africa, on to Newfoundland. The very com- 

 prehensive plan of the African expedition, conducted by Richardson, 

 Barth, and Overweg, under the orders of the British government, may 

 probably lead to the solution of such magnetic problems. 



* Sir James Ross intersected the curve of no variation in 61 30' S. 

 lat. and 27 10' W. long. (Voyage to the Southern Seas, vol. ii., p. 

 357). Captain Crozier found the variation in March, ^843, 1 38' in 

 70 43' S. lat. and 21 28' W. long., and he was therefore very near 

 the line of no variation. See Sabine, On the Mngn. Declination in the 

 Atlantic Ocean for 1840, in the Phil. Transact, for 1849, pt. ii., p. 233. 



