148 Magnetism of the Earth. 



at which an equal intensity has been observed ; and those in dotted 

 lines exhibit the supposed completion of the curves, in parts of the 

 hemisphere where the intensity has not been as yet examined. The 

 portions which arrange themselves around the point in Hudson's Bay 

 are chiefly laid down from observations made by myself in two 

 voyages of north west discovery, those of 1818, and of 1819 — 1820; 

 in a voyage in 1822, to the equatorial shores of the Atlantic, and to 

 several of the Islands in the Atlantic and Caribbean seas ; and in a 

 fourth voyage, in 1823, to Greenland, Spitsbergen, and Norway. 

 Their prolongations around the point in Siberia, are from the recent 

 obs;ervations of M. Hansteen, and the gentlemen who accompany 

 him. A brief notice of each of the curves in succession, will enable 

 me to point out generally the places which furnished their respective 

 authorities. 



Commencing with the intensities of the highest order, the curve 

 drawn through the countries surrounding Hudson's Bay, is laid down 

 from observations made at occasional intervals from Regent's Inlet in 

 the north west quarter, by Baffin's Bay in the north, to Davis' Straits 

 in the north east ; and again at New York in the south. In places 

 situated under this curve, a needle freely suspended, which required 

 three hundred seconds to perform one hundred vibrations in London, 

 would perform the same number of vibrations (in integer numbers) 

 in two hundred sixty nine seconds. In the space included by this 

 curve, in which no observations have hitherto been made, it may be 

 presumed that the intensity progressively increases to a central point 

 of maximum ; for the observations made in receding from the curve 

 in different directions, namely in Melville Island, in Greenland, and 

 to the southward of New York, all manifest an opposite tendency. 



The observations of M. Hansteen have made known the reappear- 

 ance in Siberia of an equal intensity to that beneath the curve which 

 has been just described ; forming a curve, probably similar in figure 

 but of smaller dimensions, around a point of maximum intensity, situ- 

 ated in long. 102° east of Greenwich, (which is as nearly as can be 

 judged 180° from the present position of the corresponding point in 

 Hudson's Bay,) and in latitude apparently somewhat to the north of 

 60°, but which will be more particularly determined in the present 

 summer. M. Hansteen h^s traced the southern bend of tliis curve 

 below the GOth parallel, from the Jenisei River on the west, to the 

 longitude of 115° E. (25° east of the Jenisei,) and to the latitude of 

 01°, where it lins already gained a direction nearly north and south. 



