Let us now see whether we cannot assign an adequate cause for the
secular and periodic variations in the inclination and declination of
the needle. These have been generally referred to changes of
temperature, as in fact, also, the magnetism of the earth is sometimes
ascribed to galvanic or electric currents, called forth by a daily
change of temperature. Our theory gives a totally different explanation
of 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 will 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 this last case the magnetic equator is not a great
circle, neither can we suppose its effects to be an accumulation of a
fluid which is imponderable at points 90d from the plane passing through
the centre of the earth and sun, and coincident with the plane of the
central meridian, and a depressing effect on that meridian. Its precise
influence must be, from the nature of the cause, to deflect the circular
current towards the poles, in places less than 90d from the meridian,
and a contrary effect must be produced in places greater than 90d from
the meridian. Let us assume, for argument's sake, that the magnetic
poles of the earth correspond to the poles of rotation, the parallels of
latitude will, therefore, represent the ethereal currents circulating
around the globe. Now, at sunrise, the radial stream of the solar vortex
is tangential to the surface, and, therefore, can produce no change in
these currents. As the sun ascends say about 8 or 9 A.M., the radial
stream striking only the surface of the earth perpendicularly in that
place where the sun is vertical (which we will suppose at the equator),
streams off on every side, as the meridians do from the pole, and the
circles of latitude (that is the ethereal currents) being parallel to
the equator, they are met by the radial stream obliquely, and deflected
towards either pole. By this deflection they are no longer at right
angles to the meridians. But, from the principle of reaction above
noticed, the magnetic meridians will place themselves at right angles to
the current, or, in other words, the magnetic pole will change its
position on the surface of the earth with respect to that particular
place. But, in other parts of the world, the meridians are in opposite
phases at the same instant of absolute time; therefore, the magnetic
poles are not points, but wide areas enclosing the magnetic poles of all
the countries under the sun. As this conforms to observation, it is
worthy our especial attention, and may be understood by the subjoined
figure, in which the oblique curves represent the course of the
tangential current in the different positions of the sun, the parallel
lines representing the solar radial stream.
[Illustration: Fig. 24]
As the sun gains altitude the action of the radial stream is at a
greater and greater angle to the circular currents, and attains its
maximum at noon, still acting, however, after noon; but seeing that the
circular current possesses a force 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 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 anticipate from the fact that the
declinations are in _contrary directions_ in the northern and southern
hemisphere. This is owing to the ever-varying declination of the sun.
There would be such a line, no doubt, if the axis of the earth were
perpendicular to the plane of the orbit, and the magnetic pole coincided
with the pole of rotation: for then the equator would be such a line.