rce of re-action, that is, that the
magnetism of the earth is ever striving to bring these currents to their
natural direction, an hour or two after noon, the currents tend again to
the equator, and the maximum deflection is passed, and finally ceases a
few hours after sunset. Now let us attend to what is going on on the
opposite side of the world. The radial stream passing over the polar
regions, now produces a contrary effect; the ethereal atmosphere of the
great magnet is accumulated on the farthest side from the sun, by the
action of the radial stream passing over the polar region, the parallel
currents are now bent towards the equator, being at a maximum in places
where it is an hour or two past midnight. Before they were concave to
the equator, and now they are convex; the magnetic meridian is therefore
deflected the contrary way to what it was in the day time, by the same
principle of reaction. After the maximum, say at 4 A.M., the deflection
gradually ceases, and the magnetic meridian returns to its mean position
at 8 or 9 A.M. These times, however, of maximum and minimum, must vary
with the time of the year, or with the declination of the sun, with the
position of the moon in her orbit, with the perigee of the orbit, and
with the place of the ascending node; there are also minor influences
which have an effect, which present instrumental means cannot render
appreciable.

What says observation? The needle declines from its mean position in the
whole northern hemisphere to the westward, from about 8.30 A.M., until
1.30 P.M.; it then gradually returns to its mean position by 10 A.M.
After 10 P.M., it passes over to the eastward, and attains its maximum
deflection about three or four hours after midnight, and is found again
at its mean position about 9 A.M. Now, this is precisely the direction
of the deviation of the magnetic meridian, the needle therefore only
follows the meridian, or still continues to point to the temporary
magnetic pole. And although we have assumed, for the sake of simplicity,
that the mean magnetic pole corresponds to the pole of rotation; in
truth there are two magnetic poles, neither of which correspond; yet
still the general effect will be the same, although the numerical
verification will be rendered more difficult.

In the southern hemisphere the effect is the reverse, (this southern
hemisphere, however, must be considered separated from the northern by
the magnetic equator, and not by