ON THE PHENOMENA OF TERRESTRIAL MAGNETISM. 71 
found near Behring’s Straits; whilst A, moving in a contrary 
direction, must be placed more to the east than by the later ob¬ 
servations 5 and thus the axis A B will appear to have had pre¬ 
dominant influence in the Pacific at that period. The easterly 
variation in the south part of the Atlantic would be chiefly due 
to the weaker pole a , then to the south-west of Terra del Fuego ; 
and the westerly variation in the Indian Ocean to thestrongerpole 
A, then south of Van Diemen’s Land ; whilst near Cape Lagul- 
lus the needle, equally attracted by both poles, would have, con¬ 
sequently, no variation. The westerly variation in Baffin’s Bay 
would be due to the stronger pole B, then in the North-west of 
America. The easterly variation in Europe and in a part of the 
Northern Atlantic, as well as the westerly variation at Nova 
Zembla, to the weaker pole b , then north-east of Spitzbergen. 
Why the needle showed no easterly variation at Spitzbergen 
will not be perfectly explained till the theory of the dip is exami¬ 
ned : had the two poles been of equal force, such a variation 
would have been found there. 
At an epoch antecedent to 1600, h must have been still more 
to the west, near, for example, the east coast of Greenland ; at 
which time we should infer that the easterly variation, which in 
1600 overspread Europe, prevailed much further to the west: 
and this is accordant with the evidence afforded by the earlier 
observations of the variation at Paris ; in 1541 the variation there 
was about 7° or 8 ° E.; in 1550 between 8 ° or 9° E.; about 1580 
the easterly variation reached a maximum, which appears to have 
been about 11 ° 30k From these facts we may conclude, that at 
a still earlier period than 1541, the easterly variation must have 
been less and less as we go further back, till about 1450, when 
it must have passed through zero into westerly variation. Turning 
now to the map of variation in 1600, we should say, in explana¬ 
tion of these phaenomena, that in 1450 the north-east branch of 
the line of no variation passed through Paris ; that the variation 
then became easterly, and increased till it reached a maximum 
in 1580, when it again decreased till 1666, in which year the 
north-west branch of the line of no variation passed through Pa¬ 
ris. The pole continuing to move to the east, was followed 
by the whole system of easterly variations then observed in Eu¬ 
rope, and which have now reached, and are found in the recent 
maps in, Siberia ; whilst B, in the mean time, slowly approaching 
Europe, has caused an increasing westerly variation, which will 
hereafter decrease as B moves to the eastward towards the me¬ 
ridians of Europe. In the United States of America the decrease 
of the westerly variation will precede that in Europe; before 
another half-century B will have passed to the eastward of the 
