that around any body in space there are existing aetherial
concentric spheres or shells which are equipotential surfaces, or
surfaces of equal pressure, and that these surfaces coincide with the
electric equipotential surfaces, as shown in Art. 80.

Not only so, but they coincide with the magnetic shells which the lines
of force actually form around a circular and globular magnet, as the
earth. For it must not be forgotten that these lines of force exist
equally on all sides of the earth, and therefore really form a spherical
shell, or to speak more correctly an aetherial electro-magnetic shell,
which is an oblate spheroid in shape, partaking of the shape of the
earth or other planet which the lines of force surround.

If these shells were divided into two equal halves, the line so dividing
them would be called the magnetic equator, and on that line any magnet
would set itself in a horizontal position, so that all round the earth
on the magnetic equator would correspond to a line of no dip. At the
magnetic poles, a magnet would set itself vertically, or at an angle of
90 deg., and between these two parts, the place of no dip, and that of
90 deg., the dip gradually changes as illustrated in the figure. Again,
in relation to the magnetism of the earth we find that there are certain
variations in the magnetic force, which not only influence the dip at
any place, but also the intensity at that place. The variations in
Magnetic Force are chiefly three--

1st. Diurnal Variations.
2nd. Annual Variations.
3rd. Secular Variations.

Let us look at these three variations from the standpoint of the
magnetic lines of force which exist around the earth, and around every
planet. In relation to the variations of the magnetic forces upon the
surface of the earth, Faraday points out that these variations are
caused by the action of the sun's rays upon the terrestrial lines of
force. He uses the following figure to illustrate his meaning. Let _H_
be the sun, _E_ the earth--

[Illustration: Fig: 21.]

He writes as follows: "If the magnetic and astronomical poles of our
earth be supposed to be coincident, then North and South poles will also
represent the North and South magnetic poles, and the different curves
cutting the earth will sufficiently represent a course of magnetic lines
as they occur at, or about, the surface of the earth. _H_ represents the
sun, and _a_ the place immediately underneath it, which is also
co