ll be
the poles of maximum cold, and the centres respected by the isothermal
and isogeothermal lines.

The general direction of the magnetism of the earth may be considered as
the controlling influence, therefore, in determining the position of the
magnetic needle; but there are other causes which, to some extent, will
modify the result. That half of the globe turned away from the sun will
partake of the density of the ether at that distance, which is greater
than on the side next the sun; the magnetic intensity ought, therefore,
to be greater in the night than in the day. The poles of the great
terrestrial magnets, or even the position of a magnetic needle on the
surface, are continually placed by the earth's rotation in a different
relation to the axes of the terral vortex, and the tangential current,
which is continually circulating around the globe, has its inclination
to a given meridian in a perpetual state of change. If we conceive that
there is a tendency to force the needle at right angles to this current,
we shall have an influence which varies during the day, during the year,
and during the time occupied by a complete revolution of the node. The
principal effect, however, of the horary variation of the needle is due
to the radial stream of the sun, which not only penetrates the
atmosphere, but also the solid crust of the earth. Its principal
influence is, however, an indirect influence, as we shall endeavor to
explain.

No fact in the science of electro-magnetism is, perhaps, better
established than the disposition of an ethereal current to place itself
at right angles to the magnetic meridian, and conversely, when the
current is not free to move, to place the needle at right angles to the
current. Now, the terrestrial magnet or magnets, may be considered to be
surrounded by a body of ether in rotation, which, in the earth, on its
surface, and for some distance from the surface, is made to conform to
the general rule, that is, to circulate at right angles to the magnetic
meridian. Outside this again, the ether more and more conforms to the
position of the axis of the vortex, and this position varying, it must
exert _some_ influence on the surface currents, and, therefore, change
in some degree the position of the magnetic meridian. The radial stream
comes from the sun in parallel lines, and strikes the globe and its
superficial ethereal envelope just as we have shown its action on the
atmosphere; but in thi