200 L. A. Bauer— Variation of Terrestrial Magnetism. 
It will be noticed that the zero isapoclinics, or secondary 
magnetic equators as they might be termed, lie midway between 
the secondary magnetic poles. 
It is certainly surprising that these two secondary poles 
which represent the centers of the forces producing the asym- 
metrical distribution of the earth’s magnetism should be on 
the same side of the equator and so nearly on it, instead of on 
opposite sides. Consider now the very asymmetrical distribu- 
tion of land and water with regard .to the equator and it hardly 
seems possible that this should be the main cause of asymmetry 
of the earth’s magnetic field. In this case one might expect 
the attracting centers to be either farther north or farther 
south, as the case may be. 
Another remarkable result is the close proximity of these 
secondary dip poles. They are only 80° apart in longitude. 
Here we may have an effect due to the heterogeneity in the 
earth’s composition.* 
It would appear then that: | 
The chief cause of distortion of the primary symmetrical 
field can be represented as due to a secondary polarization 
approximately equatorial im durection. 
The question might fairly be asked what has been gained by 
assuming that the earth has a primary polar field, uniform 
and symmetrical, and referring all distortions to a secondary 
complex field, only approximately equatorial in direction ? 
The answer is, we have placed the disturbing centers where we 
can operate upon them. We have observations of declination 
and inclination running back in this region for three centuries. 
It will be investigated in the next number whether we can 
* Prof. W. von Bezold in the paper already cited drew the ‘lines of equal depart- 
ing potential,” or as he terms them ‘‘isanomalous lines of geomagnetic potential.” 
He fully develops the theory of these lines and constructs them for 1880, making 
use of Quintus Icilius’s re-computation of the elements of terrestrial magnetism 
as based upon Gauss’s potential theory. He regarded the mean values of the ele- 
ments along parallels of latitude as the normal ones. He finds that the lines close 
around two foci, one, the north end attracting, in 26’ S. and long. 44° W.., the 
other, the south end attracting, in 49° S. and 140° E. The positions of our sec- 
ondary north poles agree very well but those of the secondary south poles are 
very different, though they are roughly in the same region. Just why this is, is 
difficult to tell at present. The theory of the potential does not prescribe that the 
vertical dip poles and the foci of maximum or minimum potential should coincide; 
this depends upon the geometric nature of the equipotential surfaces. Further- 
more, my secondary field is derived directly from observation and includes such 
effects as would not be embraced in the potentialtheory. Still the prime cause of 
want of coincidence of our south poles must be referred, doubtless, to the form 
of the secondary equipotential surfaces. As we proceed in the differentiation of 
this field, this matter may become clearer. The main fact, however, that the 
secondary polarization is apparently directed roughly east-west is sufficiently 
borne out by both investigations. Von Tillo moreover in his paper already cited 
has found that with respect to all the elements, the isanomolous lines can be 
referred to an east and to a west hemisphere. It is very much hoped that von 
Tillo’s charts will soon be published. 
