18 
SUPPLEMENT. 
distance of the needle, it will act upon the opposite end of it 
with equal force, and will consequently keep it parallel to itself 
all round the equator; but in going from the equator north or 
south, the declination will be increased so as to be 180° on the 
little arcs or spaces of the meridian contained between the true 
and magnetic poles, which is the greatest possible declination, 
in all cases whatever. 
It is further to be observed, in this case, that the lines of no 
declination including these arcs of 180° form two great circles 
of the globe along the meridian and equator, crossing one an¬ 
other at right angles, and dividing the surface of the globe into 
four quarters, two in each hemisphere; the one hemisphere 
having west declination in the north, and east declination in the 
south half thereof. In the opposite hemisphere it is just the 
reverse, so that each of the arcs or semi-circles of no declination 
have east declination on one side of them, and west declination on 
the other. The small arcs of 180° declination, which are between 
the true and magnetic poles, we reckon in all cases as part of 
the lines of no declination, for there, indeed, the needle conforms 
itself to the meridian as well as in the other parts of the circle, 
though its ends be reversed. 
In short, as all the lines of declination or TIalleyan lines, as 
they are very properly called, do coincide and terminate in the 
magnetic and true poles; so these arcs of 180 are a kind of 
stop-gaps, making with each of these lines, as in the present 
case, a curvilinear figure returning into itself, which figures 
from 180° between the poles to 0° declination upon the equator, 
do each of them include a space larger than the other, till at 
last they fill up the whole quarter of the surface of the globe, 
and conform themselves as nearly as possible to the shape and 
figure thereof. As a variety of this case, we have just to men¬ 
tion, that the magnetic poles may be situated in the same meri¬ 
dian, but in parallels which are not opposite. In that case, the 
only alteration which could happen, is, that in the hemisphere in 
which the magnetic and the true poles are nearest to one an¬ 
other, the figures, formerly theHalleyan lines, become smaller, and 
the corresponding figures on the opposite hemisphere larger. 
