26 
SUPPLEMENT. 
arrives at the region of the greatest cold, which is supposed to 
be at some degrees distant from the natural pole of the earth. 
In this manner the variation of the magnetic needle in the 
northern hemisphere may be accounted for. But with respect to 
the southern hemisphere, it must be recollected that though the 
lines of demarcation in the northern hemisphere have constantly 
moved from west to east, yet in the southern hemisphere, it is 
equally certain that they have moved from east to west, ever 
since any observation has been made on the subject.* It is, 
therefore, scarcely within the range of possibility, that the mag¬ 
netic pole in the southern hemisphere can move from east to 
west, whilst that in the northern hemisphere moves from west 
to east. On considering the matter, however, more attentively, 
it must in the first place be observed, that in speaking of the 
declination or variation of the magnetic needle, we always refer 
to the north end of the needle only. Thus, when the north end 
of the needle points to the west of the meridian, we say it has so 
many degrees 'of west variation, though the south end thereof 
points as many degrees to the eastward. Again, when the north 
end of the needle points to the eastward of the meridian, we 
say it has east variation, though the south end points to the west¬ 
ward thereof. And the same language is used in the southern 
as in the northern hemisphere, so that if the south magnetic ole, 
which governs the needle in that hemisphere, move to the east¬ 
ward, occasions, as we say, the needle to have west variation, 
and on the contrary, if it move to the westward, it makes what 
we term east variation. This, therefore, is the cause, on account 
of which the lines of magnetic declination appear to have a con¬ 
trary motion in the southern hemisphere, to what they have in 
the northern, though both the magnetic poles of the earth move 
in the same direction, that is from west to east. 
In the northern hemisphere there was a line of no variation, 
which had east variation on its eastern side, and west variation 
on its western side. This line evidently moved from west to 
east, during the last two centuries; the lines of east variation 
* See Cavallo’s Treatise on Magnetism. 
