Magnetism of the Earth. 147 
dynamic lines would form parallel circles, arialogous to those of geo- 
graphical latitude, progressively diminishing in intensity from the two 
points of maximum or poles, ‘to the boundary circle of the two 
hemispheres, which, following the same analogy, might receive the 
appellation of the Magnetic Equator. | Such was in fact the system, 
which, until the decisive discoveries which M. Hansteen has now 
made, appeared sufficiently conformable to the existing observations; 
to receive their countenance and support. It had so happened that 
the previous observations, although extending widely over the mag- 
netic parallels in the northern hemisphere, namely, from the least al- 
most to the greatest intensity, were confined, in respect to longitude, 
to a space, little more than the quarter of an hemisphere ; and to 
that quarter which is immediately opposite to the countries visited by 
M. Hansteen. Within the space that had been thus examined, the 
isodynamic curves appeared to arrange themselves, with comparative- 
ly insignificant deviations, in parallel circles around a point situated 
in the north-eastern part of Hudson’s Bay, and as nearly as could be 
judged, about the intersection of the 60th degree of geographical 
latitude, with the meridian of 80° west of Greenwich. That a sys- 
tem apparently so simple, so like the arrangement of induced mag- 
netism in a sphere of iron, and corroborated by the approximation of 
results observed over a fourth part of an hemisphere, should have 
been viewed as likely to prove eventually the general system of the 
globe, is not surprising. It is the peculiar distinction of M. Han- 
steen, to have been led by a more careful consideration of the slight 
apparent deviations which have been noticed, and of the general dis- 
position on the globe of the lines of dip and variation, to infer the 
existence of a second point of principal magnetic action in the north- 
ern hemisphere ; a fact which, by his recent observations, must now 
be regarded as fully established ; the isodynamic curves being found 
to arrange themselves systematically around the two points, in Hud- 
son’s Bay, and in Siberia, and to be governed in the courses which 
they follow, partly by their distances respectively from those points, 
and partly by a disparity in the absolute attractive force of the two 
points, the maximum intensity in Siberia appearing to be greater thar 
the maximum in Hudson’s Bay. 7 
The accompanying sketch of the northern hemisphere, may en~ 
able me to convey a more distinct notion of the arrangement of the 
isodynamie curves, than could be done by description alone: the 
portions traced in unbroken lines mark the connection between places 
