PROGRESS IN SCIENCE IN TERRESTRIAL MAGNETISM. 329 
for by considering the earth to be a magnetic body. Among 
his predecessors, Cortesius, for example, spoke of a magnetic 
force beyond the farthest heavens. Marsidius of Fincio, 
found the cause of the variation in a star of Ursa; and 
Petrus Peregrinus referred it to the pole of the world. 
It had thus become known through Gilbert, a century 
before Halley’s time, that the earth acts upon the magnetic 
needle somewhat as a bar magnet does, and that it has 
definite poles of magnetic strength and a magnetic field 
surrounding it which may be represented in general by lines 
of magnetic intensity issuing from one pole and passing 
to the other by curved paths to which a freely suspended 
magnetic needle will everywhere set itself tangent. 
For generations it has been customary among geomag- 
neticians to represent the elements of the direction and in¬ 
tensity of the Earth’s magnetism, as manifested at its sur¬ 
face, by lines conceived to be drawn upon the surface of the 
globe. The lines passing through all places where the angle 
between the plane of the astronomical meridian and the ver¬ 
tical plane passing through a freely suspended magnetic 
needle is the same are called lines of equal magnetic de¬ 
clination, or isogonic lines. These lines issue from one mag¬ 
netic pole and pass by curved paths to the other and through 
the geographical poles of the earth. The lines which are 
conceived to be drawn through all places where the angle 
between the direction of a freely suspended needle and the 
plane of the horizon is the same are called lines of equal 
magnetic inclination, or isoclinic lines. They gird the earth 
in circumferences approximately parallel to the magnetic 
equator, somewhat similar to the arrangement of the paral¬ 
lels of latitude with reference to the geographical equator. 
The magnetic equator is the locus of the places at which the 
freely suspended needle lies in a horizontal plane. As we 
travel from the magnetic equator toward the northern mag¬ 
netic pole, the north-seeking end of the needle inclines more 
and more below the horizontal plane until it assumes a ver¬ 
tical direction when the magnetic pole is reached; and as we 
