f these variations.

In the northern hemisphere, the north point of the needle moves from
east to west in the morning from about 8 1/2 A.M. to 1 1/2 P.M., and
returns to its mean position about 10 P.M. It then passes over to the
east, and again returns to its mean position about 8 or 9 A.M. The
analogy of this motion, with the horary changes in the barometer,
indicate a common origin. Humboldt, in the instructions he drew up for
the Antarctic Expedition under Sir James Ross, says: "The phenomena
of periodical variations depend manifestly on the action of _solar
heat_, operating probably through the medium of thermo electric currents
induced on the earth's surface. Beyond this rude guess, however,
_nothing is yet known of their physical cause_. It is even still a
matter of speculation whether the solar influence be a principal or only
a subordinate cause." That the sun may exert a modifying influence on
the phenomenon is not unlikely, but that he cannot be the principal
cause, is evident from the following considerations. These horary
variations of the magnetic needle are as great at the bottom of deep
mines far removed from solar influence, as on the surface. They are as
great, _ceteris paribus_ on a small island in the midst of the ocean, as
in the interior of continents, where the heating power of the surface is
vastly greater. They are extremely regular, so that between the tropics,
according to the sagacious Humboldt, "the time of the day may be known
by the direction of the needle, as well as by the height of the
barometer."

But what is the cause of these variations? This question is the most
difficult of all physical problems, and we shall only aim at indicating
the causes which are yet perhaps too intricately involved to afford a
positive numerical determination. Admitting the existence of two
principal solid masses whose general direction is from south to north,
and that these masses are more susceptible of permeation by the ethereal
fluid than the waters in which they are suspended, we have a general
solution of the position of the magnetic poles, and of the isogonic,
isoclinic, and isodynamic lines. Considering, too, that the southern
poles of these masses are the points of ingress, and the northern poles
the points of egress, it is easily understood that the ethereal medium
having the temperature of space, will cause the southern hemisphere to
be colder than the northern, and also that the magnetic poles wi