190 L. A. Bauer— Variation of Terrestrial Magnetism. 
a. “That the laws of terrestrial magnetism are inconsistent 
with those which belong to a permanent magnetic body.” 
b. “That they are perfectly coincident with those which 
appertain to a body in the transient state of magnetic induc- 
tion.” . 
He based these conclusions chiefly upon these facts: Biot 
(1804) and Kraft (1809) found that by taking a variable dis- 
tance between the earth’s magnetic poles or centers of attrac- 
tion, supposing them situated on a line through the earth’s 
center at right angles to the magnetic equator, considered as 
a circle, the accord between observed and computed dip became 
the more perfect the closer the two poles were made to fall 
together with the center of the earth. By supposing the cen- 
ters of magnetic attraction to be coincident with the earth’s 
center, this law was found: 
(1) tan I=2 tan @ 
I being the inclination and @ the magnetic latitude. 
Barlow obtained this identical law both empirically and 
theoretically when considering the deflections in an inclination 
needle caused by a soft iron sphere inductively magnetized.* 
He hence concluded that the earth acts like an inductively 
magnetized body. 
It will be recalled that formula (1) is identical with the law 
governing the mean inclinations along parallels of latitude as 
given in conclusion II of the preceding paper.t In the succeed- 
ing it will be shown that the same law is obtained on the assump- 
tion that the earth is a permanent uniformly magnetized 
sphere. Barlow’s deduction does not then necessarily follow. 
But let us assume that the earth’s field zs for the greater 
part an induced one. The next question that might fairly be 
asked is: What induces the earth’s magnetism ? 
Helmholtz characterized the earth’s magnetism as one of the 
most puzzling of natural forces. A volume of very respecta- 
ble dimensions could be written upon the many theories that 
have been advanced to account for the origin of tellurie mag- 
netism. It seems questionable to me, however, whether this 
problem as to the orzgin can ever be solved, owing to our in- 
ability to form an adequate conception of the earth’s condi- 
tion anterior to all observation. That theory, however, that 
will explain to us those phenomena that impress themselves 
forcibly upon us at present, viz., the variations, periodical and 
non-periodical, will probably be the basis of the one ultimately 
accepted. Ina word, we must investigate the variations of geo- 
magnetism and endeavor first to account for them before 
* See his ‘‘Essay on Magnetic Attractions,” 1st ed., 1820, 2nd ed., 1823. 
+ See this Journal, August, 1895, Art, XII. 
