the geographical one,) the needle
declines to the eastward in the morning, and goes through the same
changes, substituting east for west, and west for east. Does observation
decide this to be to be a fact also? Most decidedly it does; and this
alone may be considered a positive demonstration, that the theory which
explains it is true. The contrary deflection of the needle in the
northern and southern hemisphere may be formally proclaimed as utterly
beyond the reach of the common theory of magnetism to explain. This
difficulty arises from considering the needle as the disturbed body
instead of the earth; and also from the fact that the effect of solar
heat must be common to needles in both hemispheres, and act upon similar
poles, and consequently the deflection must be in the same direction.

But a still more capital feature is presented by the discovery of
Colonel Sabine, that the deflection is in contrary directions at the
Cape of Good Hope, at the epoch of the two equinoxes. This arises from
the great angle made by the magnetic meridian at this place, with the
terrestrial meridian--the variation being by Barlow's tables, 30d to the
westward. The sun varies in declination 47d throughout the year. At the
southern solstice, therefore the radial stream strikes the circular
current on the southern side, and deflects it towards the equator,
rendering the declination to the westward in the morning; but at the
northern solstice the radial stream strikes the current on its northern
side, and the deflection is eastward in the morning. And the vicinity of
the Cape of Good Hope is, perhaps, the only part of the world where this
anomaly will obtain; as it is necessary not only that the declination
shall be considerable, but also that the latitude shall not be very
great.

Observation also determines that the amount of the horary variation
increases with the latitude. Near the equator, according to Humboldt, it
scarcely amounts to three or four minutes, whilst it is from thirteen to
fourteen minutes in the middle of Europe. The theory explains this also;
for as the circles recede from the equator, the angles made by their
planes with the direction of the radial stream increases, and hence the
force of deflection is greater, and the effect is proportioned to the
cause. We have also a satisfactory explanation of the fact that there
has not yet been discovered a line of _no variation of horary
declination_ as we might reasonably anti