318 L. A. Bauer— Variation of Terrestrial Magnetism. 
The plus isapoclinics are again simpler in form than the 
minus ones, and from this fact one could argue that if we 
were to construct the “ equiabspotentials,” the foci of the plus 
equal-departing potentials would fall more closely together 
with that of the plus isapoclinics, than the one of the minus 
equiabspotentials with that of the minus isapoclinics.* In other 
words, in the Indian and Pacific Oceans another disturbing 
factor seems to come into play. I reached the same conclusion 
when investigating the secular motion of a free magnetic 
needle.t I found, namely, that the secular magnetic wave 
travels in the main westward, but in eastern Asia, in the Pacific 
Ocean and on the western coast of America we have evidences 
of a wave, seemingly of a smaller amplitude, that travels 
eastward. We are thus succeeding in gradually drawing the 
lines closer and closer around the disturbing factors of terres- 
trial magnetism. And this is very important since theory 
cannot determine the deflecting centers for us; observation 
must point them out to us. 
The next thing to which I wish to draw attention is the 
question at the head of this paper—what ts the secular motion 
of the secondary magnetic poles? By comparing both dia- 
grams it will be seen that beyond a doubt the whole secondary 
magnetic system has been moving along the equator westwardly 
trom 1780 to 1885. 
To make this clearer the following figures{ are given: 
Position in 
1780. 1829. 1885. 
Lat. | Long. | Lat. | Long. | Lat. | Long. 
Py pore fr.AGa: ar. “Gr. 
Secondary magnetic north pole._|10° 8. | 30° W.§} 10° S.} 40° W./20° S.| 45° W. 
i is south 2-22) 10 No "65: 0 50 E.| 5 §.| 40° EH. 
y 2 equator 0 21. ok. | 20 1 0 a WwW. 
cuts the geographical i 0 1252 2 K. 0 187 #.1 0 191 E. 
The assignment of the positions of the poles is liable to an 
uncertainty of about 5° in latitude and in longitude. It is 
especially interesting to include in the comparison the results 
for 1829 as they have been derived from Erman and Petersen’s 
figures based upon the Gaussian potential theory of terrestrial 
magnetism. | 
* See footnote to No. II, p. 200. 
+ See “ Beitraege zur Kenniniss des Wesens der Saecular-Variation des Erd- 
magnetismus,” p. 35, first footnote and p. 44. 
+ The results here given are slight improvements upon the quantities already 
published, 
§ In the “abstract” published in Science, June 21, 1895, p. 675, this was 
erroneously given as 50° W. instead of 30° W. 
| Erman and Petersen: Grundlagen der Gaussischen Theorie und die Erschein- 
ungen des Erdmagnetismus im Jahre, 1829. Berlin, 1874. Table of computed 
inclinations. 
