. 
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 ina 
fourth voyage, in 1823, to Greenland, Spitsbergen, and Norway. 
Their prolongations around the point in Siberia, are from the recent 
observations 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 whieh 
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 has traced the southern bend of this curve 
below the 60th parallel, from the Jenisei River on the west, to the 
longitude of 115° E. (25° east of the Jenisei,) and to the latitude of 
61°, where it has already gained a direction nearly north and south. 
