Variation Phenomenon of Terrestrial Magnetism. 499 



earth would then consist of a solid external crust covering a 

 liquid shell, leaving the interior of the globe in its primitive 

 gaseous condition. 



It is not necessary for the argument to assign any particular 

 thickness for the solid and liquid spherical shells, although there 

 are reasons for thinking that the thickness is not very consider- 

 able, as recent investigations have shown that the elementary 

 substances, at great depths, are of much higher specific gravity 

 than those on which the mean density of the earth has been 

 determined. 



The remarkable relations which subsist between terrestrial 

 magnetic disturbances and the period of sun-spot frequency, 

 abundantly show that the solar vapors are highly electro- 

 dynamic. It may consequently be inferred that a spherical mass 

 of incandescent vapors of similar constitution to those in the 

 sun, and enclosed within the solid terrestrial envelope, would 

 also be similarly electro-dynamic. 



It is also postulated that the glowing terrestrial spheroid at 

 one period of its history revolved with its axis of rotation at 

 right angles to the plane of its orbit, and the spheroid being 

 symmetrically electro-dynamic, the compass needle would point 

 due north and south on every meridian of its surface. Now, 

 if we further assume that the polar axis of the solid and liquid 

 globe was tilted over about its centre at a definite angle, with- 

 out affecting the plane of rotation of the internal electro- 

 dynamic sphere of vapor, and if the latter rotated with a some- 

 what slower motion than the solid external shell, then would 

 all the principal phenomena of terrestrial magnetism be man- 

 ifested. 



It is not necessary to consider the causes which produced the 

 present inclination of the earth's equator to the plane of its 

 orbit, beyond suggesting that it may have been brought about 

 by a disturbance of equilibrium through the rupture of the 

 lunar ring on the transition of that body from the annular to 

 the spherical form." 



This is the foundation of his theory. Assuming now that 

 the earth has passed through the following electro-dynamic and 

 electro magnetic stages, he attempts to reproduce them. 



(1.) " The electro-dynamic condition of the terrestrial globe, 

 at a period of its history when the crust, from its high tem- 

 perature, was non-magnetic, its surface of uniform curvature, 

 and the electro-dynamic polar foci of the internal sphere of 

 vapors inclined at a definite angle from the poles of the 

 earth's axis." 



(2.) " The electro-dynamic and electro-magnetic condition of 

 the earth, when its outer crust was uniform in curvature and 

 had cooled down sufficiently to become permanently magnetic, 



