L. A. Bauer— Variation of Terrestrial Magnetism. 197 
Date, 1885. 
Latitude. Mean H. M. 
40° N. 0°245 0°320 a® 
20 O7327 "348 
0 0°341 341 
20 0:292 co LL 
40 S. 0°232 °303 
Mean 0°325 a? 
The Neumayer-Petersen re-computation of the Gaussian co- 
efficients gave a value for M of 0°322 a. Our value then isin 
fair accord. H was obtained from 18 equidistant scalings of 
Neumayer’s isodynamic chart for 1885. 
Is cor M subject to a secular change? Below are the values* 
of M obtained thus far with the aid of the Gaussian theory : 
Year. Computer. M. 
1829 ° Erman-Petersen 0°32 <a 
1830 Gauss 0°331 
1880 Icilius 0°340 
1885 Neumayer-Petersen 0°322 
Mean 330 0° 
These figures do not tell anything conclusive as yet. We 
shall consequently for the present assume that the magnetic 
moment of the earth does not suffer a secular change and 
adopt as our coéfiicient.in above theoretical formulse the value 
0°330. 
We are now ready to take up the chief object of this paper 
—the localization of the causes of the present distribution of 
geomagnetism. I will asswme that if these causes were not 
_ operating the value obtained from the foregoing formule. 
would be the actually observed values. I shall consider in this 
paper, the distribution in the dep for the year 1885, scaling the 
inclination from Neumayer’s isoclinic chart at every 20° of 
longitude along parallels +60°, +40°, +20°, +10° and 0°. 
For this purpose I determine the departures of the inclinations 
thus found from those derived by formula (6) and imagine the 
ideal surface referred to above constructed with the aid of 
these departures. To give a representation of this surface, the 
topographer’s method of mapping a hilly country by means of 
contour lines could be adopted. That is, the curves of inter- 
section of equidistant geo-concentric spherical surfaces with 
the ideal surface could be drawn. The crowding together of 
these lines would indicate the position of the highest and 
lowest points of the surface or the seat of the disturbing forces. 
*G. Neumayer, ‘‘ Atlas des Erdmagnetismus,” p. 19, Gotha, 1891. 
