L. A. Bauwer— Variation of Terrestrial Magnetism. 325 
In conclusion a few words must be said with regard to the 
results obtained thus far. We have found that by resolving 
the observed magnetic field into two, one polar and symmetri- 
eal about the rotation axis, the other the residual field, that 
this latter is apparently directed approximately equatorially. 
It has been amply demonstrated, I believe, that this mode of 
resolution is a very convenient and promising one for the study 
of the complex phenomena of the distribution and the secular 
variation. This breaking up of the total field into two has 
thus far been wholly arbitrary, however. To assert the actual 
existence of the secondary equatorial field we must endeavor 
to deduce it theoretically in some manner. We must show 
that there is some cause for sucha polarization. For example, 
suppose we have given the primary polar tield, which in fact 
we know to exist, though in a more complex form, can we 
deduce from the motions of the earth a secondary equatorial 
polarization as the result of self-induction? If so, what condi- 
tions of the physical constitution of the earth must we presup- 
pose in order to get such an unsymmetrical equatorial polariza- 
tion as our residual field reveals? That is, why are the 
secondary magnetic dip poles but 90° different in longitude ?* 
It is evident that the solution of this question is likely to throw 
some light upon the secrets hidden in the bowels of the earth. 
It will be interesting to see whether we shall reach the same 
result obtained by Prof. Bigelow in a totally different manner, 
viz., that the earth is a magnetic shell of the thickness of about 
790 miles.+ 
Finally, can the secular variation be explained by the reflex 
action of the secondary field upon the primary? This is the 
nature of the questions that will serve as the subject of a 
systematic examination in subsequent numbers. 
Chicago, Aug. 15, 1895. 
* The two foci obtained by constructing “the isanomalous lines of geomagnetic 
potential” are according to Prof W. von Bezold about 180° apart in longitude. 
See foot-note page 200, of previous number. 
+ F. H. Bigelow: The Earth a Magnetic Shell, this Journal, Art. VIII, 
August, 1895. 
Am. Jour. Sci.—Tuirp SERIES, Vou. L, No. 298.—OcToBER, 1895. 
