202 L. A. Bauer— Variation of Terrestrial Magnetism. 
first being the primary polar field, the other the secondary 
equatorial. Were only the polar field present, then would the 
compass everywhere point due north and south. Were but 
the eguatorial field present, the needle would everywhere 
point east and west. Under the combined action of the two 
fields, the needle takes up an intermediate position. As we 
proceed along the equator the north end of the compass alter- 
nately points E. or W. of N., the points of change from E. to 
W. or vice versa being over the equatorial poles. 
i Thus, in the figure, let Pp represent 
Pp the north magnetic pole of the polar 
field, Pe the same for the equatorial. 
At A under the combined influence of 
Pe A Pp and Pe the north end of the com- 
Ww E pass points W. of N. As we proceed 
from A towards Pe, the westerly decli- 
nation diminishes until over Pe it 
vanishes entirely, the component of 
horizontal intensity of the secondary 
field vanishing over Pe. As we leave Pe and proceed towards 
B the needle points E. of N. That is, over the equa- 
torial poles the declination is zero, to the right of the 
equatorial magnetic north pole, the declination is westerly, to 
the left, easterly ; for the equatorial south magnetic pole, a 
precisely reverse condition of things obtains. 
Hence, if two such systems are present on the earth the 
actually observed declination ought to be westerly between the 
two isapoclinic foci, and beyond, easterly. Now as a matter of 
fact over the region between the agonics given on the diagram, 
the declination zs westerly and beyond them easterly. It will 
be seen that this region is nearly identical with that prescribed 
by the foci! In other words, without the knowledge of a 
single declination observation we might have delineated roughly 
e) 
the region where the declination is westerly or easterly with — 
the aid of the isapoclinics. 
It seems to me that the proof is overwhelming that these 
isapoclinic foci are magnetic dip poles just as truly as the one sup- 
posed north of Hudson’s Bay. Ido not see how we can escape 
the fact that we have secondary magnetic dip poles situated 
approximately in the equator. The field that we actually 
observe is the resultant of the two systems—the polar and the 
equatorial. 
Hence, perhaps the prime reason why a declination needle 
does not point truly north is because of a secondary polari- 
zation approximately equatorial in direction. 
To give further proof that. the isapoclinics are not random 
lines but the isoclinics of a definite magnetic system and hence 
