incident with the magnetic equator. Point _a_ will be a line of no
dip, while at point _b_ there will be dip. This dip will be increased by
the action of the sun's rays, because the atmosphere under the influence
of the sun's rays has expanded the air, and has thus acquired a power to
affect the lines of magnetic force."

"All the lines passing through the heated and expanded air will, because
of its being a worse magnetic conductor, tend to open out, and the mass
of heated air will as a whole assume the condition of diamagnetic
polarity (2923). The case may be more simply stated for the facility of
recollection by saying, that the effect of the sun is to raise the
magnetic circles over the equatorial and neighbouring parts from their
normal position, in doing which the North and South dip are
simultaneously affected and increased."

Thus it can readily be seen that every day as the earth turns round on
its axis, and presents each side of the globe successively to the rays
of the sun, there should be a gradual change in the intensity of the
terrestrial magnetism. In Art. 2925, Faraday points out that the maximum
of dip would be when the sun was at its zenith or directly overhead.
With reference to the Annual Variation, Faraday points out (2882) that
if the axis of the earth were perpendicular to the plane of its orbit,
the intensity and direction of the magnetic forces might be considered
constant, but (2883) as the axis of the earth's rotation is inclined
23 deg. to the plane of the ecliptic, the two hemispheres will become
alternately warmer and colder than each other, and then a variation in
the magnetic condition may arise. The consideration of this annual
variation is further considered by Faraday in the subsequent paragraphs
to those already quoted, and I must refer the reader to them for fuller
details; I wish simply to indicate the possible explanation of the
terrestrial magnetism, in so far as that explanation is in conformity
with the aspect of the Aether submitted in Chapter IV.

With regard to secular variations Faraday points out (2880) that the
temperature of the air at the equatorial parts of the earth is greater
than in latitudes north and south, and as an elevation of temperature
diminishes the conducting power of magnetism, so the proportion of force
passing through those parts ought to be less, and that passing through
the cooler parts, greater, than if the temperature were at the same
degree over t