119 



the slow changes of the variation in different 

 meridians, would still throw much doubt over 

 the real position of the curves, which are at- 

 tempted to be traced on variation charts *. 



On comparing the observations made during 

 several voyages, it appears, that we had cut the 

 curve of no variation in latitude 13° north, and 

 longitude 53° and 65° west : this curve is now 

 prolonged toward Cape Hatteras, and toward a 

 point in Canada, in 33° 27' north latitude, and 

 70° 44' west longitude. Before the first voyage 

 of Christopher Columbus, in 1460, the variation 

 was nought near the island of Corvo , but the 

 progress of the curve of no variation toward the 

 west is not the same in all it's parts, and it is 

 sometimes retarded by the local influence of the 

 continents and islands, which form so many par- 

 ticular systems of magnetic forces. Thus it 

 seemed stopped for some time by the southern 

 extremity of New Holland ; and at Jamaica and 

 Barbadoes the variation has not undergone any 



* The uncertainty of the observations of the magnetic 

 needle made at sea does not arise solely from the rolling and 

 pitching of the vessel, or the imperfection of the azimuth 

 compasses ; it is in a great measure caused by the masses of 

 iron spread through the vessel, and acting unequally, accord- 

 ing to the direction in which it is steered. Loewenoern, in 

 the Nye Samling of Danske Vid. Selsk. Skr. t. iii, p. 117, and 

 t. v, p. 299. Zach. Man. Cor. 1800, p. 529. Flinders, in the 

 Phil. Trans. 1805, p. 187. 



