192 L. A. Bauer— Variation of Terrestrial Magnetism. 
and parcel of our earth and endeavor to find out the function 
it plays in the economy of nature’s household. 
To-day the magnetic earth is rotating about an eccentric 
axis.*¥ What has caused this eccentricity? The relevancy 
and importance of this interesting question can best be seen 
from the following extract taken from Prof. Rees’s paper on 
“ Variation of Latitude.” + 
‘Tn an address made before Section A, of the British Asso- 
ciation, in 1892, Professor Schuster stated that he believed the 
evidence at hand was in favor of the view that there was 
sufficient matter in interplanetary space to make it a conductor 
of electricity. This conductivity, however, must be small, for 
if it were not, he said, the earth would gradually set itself to 
revolve about its magnetic poles. However, changes im the 
position of the magnetic poles would tend to prevent this 
result.t Perhaps the investigator in the near future, work- 
ing on the suggestion of Prof. Schuster, may find some con- 
nection between the earth’s magnetism, rotation-time and 
position of rotation-axis.” 
Let us assume that the earth’s field was originally uniform 
and symmetrical about the geographical axis. The magnetic 
poles (whether defined as the points of vertical dip or of maxi- 
mum or minimum potential) fall together with the rotation 
poles. At every point on the earth the compass points truly 
north and south. In every parallel of latitude, the potential, 
the intensity, and the inclination are constant, though varying 
according to definite laws from parallel to parallel. The line 
of zero equipotential, the line of no dip and the line of maxi- 
mum horizontal intensity fall together with the geographical 
equator. Let some cause operate now to deflect the magnetic 
axis or to tilt the primary uniform field until at some given 
date the magnetic axis is observed to make a definite angle 
with the rotation axis. We will suppose, for the present, that 
the field is still uniform about the new magnetic axis. If we 
were now to determine the difference of inclination, for 
example, at various points along a parallel of latitude caused 
by the change in the magnetic axis, and imagine perpen- 
diculars erected at the points of observation proportional to the 
observed inclination differences, having due regard to sign, then 
would the surface passing through the ends of the perpendicu- 
*The term “magnetic axis” is used throughout in the Gaussian sense, i. e., 
that straight line with reference to which the earth’s magnetic moment is a 
maximum. This is the only precise definition that can be given this term in the 
case of such a heterogeneously magnetized body as the earth. According to the 
latest computation (Neumayer-Petersen’s), the magnetic axis makes an angle of 
11°-7 with the geographical axis. 
t+ ‘‘Science,” May 24, 1895, pp. 563-564. 
t The italics are my own. 
