196 ZL. A. Bauer— Variation of Terrestrial Magnetism. 
D i v/a H V F 
Lat. |Obs’d Comp’d|Obs’d Comp’d) Obs’d Comp’d Obs’d Comp’d/Obs’d Comp’d|Obs’d Comp’d 
Gass: C. G. 8. CaaS) Gens. 
60 N| 42:6 0-0 |4+75:2 +73°9 40-285 +0-285/+0-14 +0:165/+0°55 +0-572|/40:56 +0-594 
40. | +09 0-0 [+596 +59°2' 40°21 +0:212/ 40:24 +0-253/4+0-43 +0-424| 0:50 0-495 
20 | +03 0:0 [+344 +36:1)+0°11 +0-113/+0°32 +0-310/+0°23 +0-226| 0°41  0:383 
0 | +07 0:0 |— 23 0:0 0:00  0:000/+0-34 +0-330/—0°02 0-000) 0:35  0:330 
20 | 41:6 0-0 |—36:0 —36-1/—0°11 —0-113/+0°30 +0:310/—0-23 —0-226] 0:39 0383 
40 | +32 0-0 |—57-6 —59°2/—0-20 —0-212)+0-24 +0-253/—0-40 —0-424| 0:47 0-495 
608 | 42:8 0-0 |—70-6 —73:9 —0-275 —0:285/+0°18 +0-165/—0°53 —0-572| 0-54 0-594 
Mean! +1°7 0-0 [+ 04  0:0/40-004  0-000/+0-251-+0-2551+0-004 0-000/+0:464-++0°468 
It will be seen that the accord is very good throughout. 
This is all the more remarkable when we consider the great 
changes encountered in the values of the magnetic elements in 
going along a parallel of latitude; as, for example, along 60° 
North, the inclination suffers a total change of 82°°5 and the 
declination along the equator a total change of 50°-7! Not- 
withstanding these great changes, the mean or normal elements 
correspond very closely to those obtained on the assumption 
that the earth is uniformly magnetized about the rotation axis. 
Why is this? Why should the “anomalies” in the distribu- 
tion so nearly cancel each other in going along a geographical 
latitude? This is certainly not a result we should expect 
a proord if the asymmetrical distribution of land and water be 
the cause of the present distribution of telluric magnetism. 
We now see the theoretical significance of von Bezold’s 
empirical formula governing the mean value of the potential 
from latitude to latitude, likewise, that of the empirical deduc- 
tion, tan I = 2 tan g given in conclusion II of preceding paper. 
What does the empirical factor, 0°330 imply? According to 
equation (2) we have: 
c= 0330 =47p 
If M is the magnetic moment of the earth, then is 
M470) (= ee: 
i. e. volume of sphere times the intensity of magnetization per 
unit of volume. With value of ¢ = 0°330 we then get a value 
of M = 0°330 a or 8°52 10" (C. G.S. units) against 8°55 x 10” 
as determined by Gauss.* Hence we have the theoretical 
interpretation of this factor. 
Furthermore, if we know the mean value of H along a 
parallel we can rapidly determine with the aid of equation (3) 
a fair value of the earth’s magnetic moment, without making 
use of the laborious Gaussian computation. To illustrate : 
*Gordon’s ‘‘ Physical Treatise on Electricity and Magnetism,” 2d ed., vol. I, 
p. 155. 
