ox the phenomena of terrestrial magnetism. 7 ( J 
fig. 2, in which P p are the poles of the earth, P N M is the 
polar colure, and P F p the diametral colure. A B and a b are 
Fig. 2. 
P 
as in fig. 1, and the great circle passing through them is the first 
magnetic meridian. Q F R is the magnetic equator cutting the 
geographical equator M N in E, the pole of the polar colure ; 
PByj and PAjo are geographical meridians passing through 
the ends of the magnetic chord. 
If in fig. 1. L be a place on the earth's surface of known geo¬ 
graphical position, L y is its magnetic radius, or a line drawn 
from the place to the centre of the magnetic chord ; Ly e is its 
true magnetic latitude, or the angle formed by its magnetic ra¬ 
dius and the magnetic equator; L y B is its true magnetic polar 
distance ; E y e is its true magnetic longitude, or the angle be¬ 
tween the magnetic meridian of L and the first magnetic meridian. 
BLe A being its magnetic meridian, a perpendicular C c to the 
magnetic equator from C gives c the centre of the meridian, Cc 
its eccentricity, and L c its radius. L c is then the eccentric 
magnetic latitude measured at c, the eccentric centre. 
In fig. 3. h L Q is a magnetic vertical circle through L, cutting 
the magnetic equator in Q C ; L R is the intersection of b L Q 
with the magnetic meridian of L ; the arc L Q, is the apparent- 
magnetic latitude of L intercepted on the vertical circle between 
L and the magnetic equator, or it is the angle L C Q; E Q is 
the apparent magnetic longitude, or the arc of the magnetic 
equator intercepted between E, the apocentric point, and the 
vertical magnetic circle passing through L; or it is the spheric 
angle E b L, or the plane angle ECQ. 
