Sy ae ae 
L. A. Bauer— Variation of Terrestrial Magnetism. 195 
ship, by Peter Barlow.* By turning back it will be seen that 
(6) is identical with Biot’s and Kraft’s formula which these 
investigators found empirically when considering the magnetic 
action of the earth. In obtaining (6) the only assumption 
made was that the earth was uniformly magnetized about the 
geographical axis. Barlow’s argument, based on equation (6), 
that the earth is enductwely magnetized is, consequently, not 
necessarily true.t . 
From (1) it will be seen that if we substitute for 4zya’, M, 
the magnetic moment of the earth, the external action of the 
uniform magnetized sphere is identical with that produced by 
an infinitely small magnet having the same magnetic moment, 
and situated at the center of the earth. This explains Biot’s 
and Kraft’s result that the computed dip became the more per- 
fect the nearer the magnetic poles were moved to the center 
of the earth. 
Inside the sphere the potential will reduce to 
=> See (8) 
from which it appears that the force — “= = 47 is every- 
where constant in direction and intensity, i. e. inside the sphere 
the field is uniform. 
Now let us see how closely the above theoretical results will 
give us the mean values of the various elements along geo- 
graphical parallels of latitude. To obtain the computed values 
below I have adopted provisionally for the value of ¢ von 
Bezold’s empirical coéfficient, viz: 0°330. The observed values 
are the means of equidistant scalings of magnetic charts along 
parallels of latitude. The values for Dand I have been taken 
from Table I of my former paper (No. [). Those for ¥, H, 
V and F have been borrowed from General A. von Tillo.t 
* See his ‘‘ Essay on Magnetic Attractions.” 
+ This formula (6) is very significant. It tells us that the inclination is inde- 
pendent of ¢, or the intensity of magnetization. Hence changes in c will not affect 
I as long as the axis of magnetization coincides with the geographical axis. Any 
change in I will have to be referred to a change in direction of magnetic axis. 
Compare Table I of No. I of these contributions (August number) and see how 
nearly the mean inclination for a geographical parallel has remained constant for 
a century. It will be doubly interesting now to determine definitely whether the 
slight fluctuation exhibited is real and not to be described to inaccuracy of chart 
data. 
t+ Comptes Rendus No. 15 (Oct. 8, 1894): Magnétisme moyen du globe et 
isanomales du magnétisme terrestre. Note by M. Alexis de Tillo. 
