FT PTH report-1835. 
E F is a magnetic diameter of the earth passing through y ; 
E is the point on the earth’s surface most distant, and F the 
Fig. 1. 
point least distant, from the centre of the magnetic chord; or 
the apocentric and pericentric points. 
Every plane section of the earth passing through the mag¬ 
netic chord is a magnetic meridian ; all of which are small 
circles except the first, E B F A, which passes through the apo¬ 
centric and pericentric points. A e B is a magnetic meridian, 
and e its point of intersection with the magnetic equator. The 
first meridian passes through the ends of the magnetic chord, 
the poles of the magnetic equator, and the apocentric and peri¬ 
centric points. When the chord has no eccentricity there is no 
first meridian determined by nature. 
Every plane passing through the axis of the magnetic equator 
is a magnetic vertical circle : every place has its own, and all 
are great circles. Were there no eccentricity in the magnetic 
axis, every magnetic meridian would be a magnetic vertical 
circle. 
The magnetic polar colure is a great circle passing through 
the poles of the earth and those of the magnetic equator. The 
diametral colure is a great circle passing through the poles of 
the earth and the apocentric and pericentric points. Thus in 
