maximum of intensity for the whole surface of the Earth is 2.052, and the
minimum 0.706. Both phenomena occur in the southern hemisphere; the former
in 73 degrees 47' S. lat., and 169 degrees 30'E. long. from Paris, near
Mount Crozier, west-northwest of the south magnetic pole, at a place where
Captain James Ross found the inclination of the needle to be 87 degrees 11'
(Sabine, 'Contributions to Terrestrial Magnetism', 1843, No. 5, p. 231); the
latter, observed by Erman at 19 degrees 59' S. lat., and 37 degrees 24' W.
long. from Paris, 320 miles eastward from the Brazilian coast of Espiritu
Santo (Erman, 'Phys. Beob.', 1841, s. 570), at a point where the inclination
is only 7 degrees 55'. The actual ratio of the two intensities is therefore
as 1 to 2.906. It was long believed that the greatest intensity of the
magnetic force was only two and a half times as great as the weakest
exhibited on the Earth's surface. (Sabine, 'Report on Magnetic Intensity',
p. 82.)
If the intensity near the magnetic south pole
p 188
be expressed by 2.052 (the unit still employed being the intensity which I
discovered on the magnetic equator in Northern Peru), Sabine found it was
only 1.624 at the magnetic north pole near Melville Island (70 degrees 27'
north lat.), while it is 1.803 at New York, in the United States, which has
almost the same latitude as Naples.
The brilliant discoveries of Oersted, Arago, and Faraday have established a
more intimate connection between the electric tension of the atmosphere and
the magnetic tension of our terrestrial globe. While Oestred has discovered
that electricity excites magnetism in the neighborhood of the conducting
body, Faraday's experiments have elicited electric currents from the
liberated magnetism. Magnetism is one of the manifold forms under which
electricity reveals itself. The ancient vague presentiment of the identity
of electric and magnetic attraction has been verified in our own times.
"When electrum (amber)," says Pliny, in the spirit of the Ionic natural
philosophy of Thales,* is 'animated' by friction and heat, it will attract
bark and dry leaves precisely as the loadstone attracts iron."
[footnote] *Of amber (succinum, glessum) Pliny observes (xxxvii., 3),
"Genera ejus plura. Attritu digitorum accepta caloris anima trahunt in se
paleas ac folia arida quae levia sunt, ac ut magnes lapis ferri ramenta
quoque." (Plato, 'in Timaeo', p. 80. Martin, 'Etude sur le Timee', t. ii.,
p. 343-346. Strabo, xv., p. 703, Casaub,; Clemens Alex., 'Strom.', ii., p.
370, where, singularly enough, a difference is made between [Greek words])
When Thales, in Aristot., 'de Anima', 1, 2, and Hippias, in Diog. Laert.,
i., 24, describe the magnet and amber as possessing a soul, they refer only
to a moving principle.
The same words may be found in the literature of an Asiatic nation, and
occur in a eulogium on the loadstone by the Chinese physicist Kuopho.*
[footnote] *"The magnet attracts iron as amber does the smallest grain of
mustard seed. It is like a breath of wind which mysteriously penetrates
through both, and communicates itself with the rapidity of an arrow." These
are the words of Kuopho, a Chinese panegyrist on the magnet, who wrote in
the beginning of the fourth century. (Klaproth, 'Lettre a M. A. de Humboldt,
sur l'Invention de la Boussole', 1834, p. 125.)
I observed with astonishment,
p 189
on the woody banks of the Orinoco, in the sports of the natives, that the
excitement of electricity by friction was known to these savage races, who
occupy the very lowest place in the scale of humanity. Children may be seen
to rub the dry, flat, and shining seeds or husks of a trailing plant
(probably a 'Negretia') until they are able to attract threads of cotton and
pieces of bamboo cane. That which thus delights the naked copper-colored
Indian is calculated to awaken in our minds a deep and earnest impression.
What a chasm divides the electric pastime of these savages from the
discovery of a metallic conductor discharging its electric shocks, or a pile
composed of many chemically-decomposing substances, or a light-engendering
magnetic apparatus! In such a chasm lie buried thousands of years that
compost the history of the intellectual development of mankind!
The incessant change or oscillatory motion which we discover in all magnetic
phenomena, whether in those of the inclincation, declination, and intensity
of these forces, according to the hours of the day and the night, and the
seasons and the course of the whole year, leads us to conjecture the
existence of very various and partial systems of electric currents on the
surface of the Earth. Are these currents, as in Seebeck's experiments,
thermo-magnetic, and excited directly from unequal distribution of heat? or
should we not rather regard them as induced by the position of the Sun and
by solar heat?*
[footnote] *"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 as 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 in the phenomena of terrestrial magnetism."
('Observations to be made in the Antarctic Expedition', 1840, p. 35.)
Have the rotation of the planets, and the different degrees of velocity
which the individual zones acquire, according to their respective distances
from the equator, any influence on the distribution of magnetism? Must we
seek the seat of these currents, that is to say, of the disturbed
electricity, in the atmosphere, in the regions of planetary space, or in the
polarity of the Sun and Moon? Galileo, in his celebrated 'Dialogo', was
inclined to ascribe the parallel direction of the axis of the Earth to a
magnetic point of attraction seated in universal space.
If we represent to ourselves the interior of the Earth as fused and
undergoing an enormous pressure, and at a degree of temperature the amount
of which we are unable to assign,
p 190
we must renounce all idea of a magnetic nucleus of the Earth. All magnetism
is certainly not lost until we arrive at a white heat,* and it is manifested
when iron is at a dark red heat, however different, therefore, the
modifications may be which are excited in substances in their molecular
state, and in the coercive force depending upon that condition in
experiments of this nature, there will still remain a considerable thickness
of the terrestrial stratum, which might be assumed to be the seat of
magnetic currents.
[footnote] *Barlow, in the 'Philos. Trans.' for 1822, Pt. i., p. 117; Sir
David Brewster, 'Treatise on Magnetism', p. 129. Long before the times of
Gilbert and Hooke, it was taught in the Chinese work 'Ow-thea-tsou' that
heat diminished the directive force of the magnetic needle. (Klaproth,
'Lettre a M. A. de Humboldt, sur l'Invention de la Boussole', p. 96.)
The old explanation of the horary variations of declination by the
progressive warming of the Earth in the apparent revolution of the Sun from
east to west must be limited to the uppermost surface, since thermometers
sunk into the Earth, which are now being accurately observed at so many
different places, show how slowly the solar heat penetrates even to the
inconsiderable depth of a few feet. Moreover, the thermic condition of the
surface of water, by which two thirds of our planet is covered, is not
favorable to such modes of explanation, when we have reference to an
immediate action and not to an effect of induction in the aërial and
aqueous investment of our terrestrial globe.
In the present condition of our knowledge, it is impossible to afford a
satisfactory reply to all questions regarding the ultimate physical causes
of these phenomena. It is only with reference to that which presents itself
in the triple manifestations of the terrestrial force, as a measurable
relation of space and time, and as a stable element in the midst of change,
that science has recently made such brilliant advances by the aid of the
determination of mean numerical values. From Toronto in Upper Canada to the
Cape of Good Hope and Van Diemen's Land, from Paris to Pekin, the Earth has
been covered, since 1828, with magnetic observatories,* in which every
regular
p 191
or irregular manifestation of the terrestrial force is detected by
uninterrupted and simultaneous observations. A variation
p 192
of 1/40000th of the magnetic intensity is measured, and at certain epochs,
observations are made at intervals of 2 1/2 minutes, and continued for
twenty-four hours consecutively.
[footnote] *As the first demand for the establishment of these
observatories (a net-work of stations, provided with similar instruments)
proceeded from me, I did not dare to cherish the hope that I should live
long enough to see the time when both hemispheres should be uniformly
covered with magnetic houses under the associated activity of able
physicists and astronomers. This has, however, been accomplished, and
chiefly through the liberal and continued support of the Russian and British
governments.
[footnote continues] In the years 1806 and 1807, I and my friend and
fellow-laborer, Herr Oltmanns, while at Berlin, observed the movements of
the needle, especially at the times of the solstices and equinoxes, from
hour to hour, and often from half hour to half hour, for five or six days
and nights uninterruptedly. I had persuaded myself that continuous and
uninterrupted observations of several days and nights (observatio perpetua)
were preferable to the single observations of many months. The apparatus, a
Prony's magnetic telescope, suspended in a glass case by a thread devoid of
torsion, allowed angles of seven or eight seconds to be read off on a
finely-divided scale, placed at a proper distance, and lighted at night by
lamps. Magnetic perturbations (storms), which occasionally recurred at the
same hour on several successive nights, led me even then to desire extremely
that similar apparatus should be used to the east and west of Berlin, in
order to distinguish general terrestrial phenomena from those which are mere
local disturbances, depending on the inequality of heat in different parts
of the Earth, or on the cloudiness of the atmosphere. My departure to
Paris, and the long period of political disturbance that involved the whole
of the west of Europe, prevented my wish from being then accomplished.
(OErsted's great discovery (1820) of the intimate connection between
electricity and magnetism again excited a general interest (which had long
flagged) in the periodical variations of the electro-magnetic tension of the
Earth. Arago, who many years previously had commenced in the Observatory at
Paris, with a new and excellent declination instrument by Gambey, the
longest uninterrupted series of horary observations which we possess in
Europe, showed by a comparison with simultaneous observations of
perturbation made at Kasan, what advantages might be obtained from
corresponding measurements of declination. When I returned to Berlin, after
an eighteen years' residence in France, I had a small magnetic house erected
in the autumn of 1828, not only with the view of carrying on the work
commenced in 1806, but more with the object that simultaneous observations
at hours previously determined might be made at Berlin, Paris, and Freiburg,
at a depth of 35 fathoms below the surface. The simultaneous occurrence of
the perturbations, and the parallelism of the movements for October and
December, 1829, were then graphically represented. (Pogg., 'Annalen', bd.
xix., s. 357, taf. i.-iii.) An expedition into Northern Asia, undertaken in
1829, by command of the Emperor of Russia, soon gave me an opportunity of
working out my plan on a larger scale. The plan was laid before a select
committee of one of the Imperial Academies of Science, and, under the
protection of the Director of the Mining Department, Count von Cancrin, and
the excellent superintendence of Professor Kupffer, magnetic stations were
appointed over the whole of Northern Asia, from Nicolajeff, in the line
through Catharinenburg, Barnaul, and Nertschinsk, to Pekin.
[footnote continues] The year 1832 ('Gottinger gelehrte Anzeigen', st. 206)
is distinguished as the great epoch in which the profound author of a
general theory of terrestrial magnetism, Friedrich Gauss, erected apparatus,
constructed on a new principle, in the Gottingen Observatory. The magnetic
observatory was finished in 1834, and in the same year Gauss distributed new
instruments, with instructions for their use, in which the celebrated
physicist, Wilhelm Weber, took extreme interest, over a large portion of
Germany and Sweden, and the whole of Italy. ('Resultate der Beob. des
Magnetischen Verceins in Jahr' 1338, s. 135, and Poggend., 'Annalen.' bd.
xxxiii., s. 426.) In the magnetic association that was now formed with
Gottingen for its center, simultaneous observations have been undertaken
four times a year since 1836, and continued uninterruptedly for twenty-four
hours. The periods, however, do not coincide with those of the equinoxes
and solstices, which I had proposed and followed out in 1830. Up to this
period, Great Britain, in possession of the most extensive commerce and the
largest navy in the world, had taken no part in the movement which since
1828 had begun to yield important results for the more fixed ground-work of
terrestrial magnetism. I had the good fortune, by a public appeal from
Berlin which I sent in April 1836, to the Duke of Sussex, at that time
President of the Royal Society (Lettre de M. de Humboldt a S. A. R. le Duc
de Sussex, sur les moyens propres a perfectionner la connaissance du
magnetisme terrestre par l'establissement des stations magnetiques et
d'observations correspondantes), to excite a friendly interest in the
undertaking which it had so long been the chief object of my wish to carry
out. In my letter to the Duke of Sussex I urged the establishment of
permanent stations in Canada, St. Helena, the Cape of Good Hope, the Isle of
France, Ceylon, and New Holland, which five years previously I had advanced
as good positions. The Royal Society appointed a joint physical and
meteorological committee, which not only proposed to the government the
establishment of fixed magnetic observatories in both hemispheres, but also
the equipment of a naval expedition for magnetic observations in the
Antarctic Seas. It is needless to proclaim the obligations of science to
the great activity of Sir John Herschel, Sabine, Airy, and Lloyd, as well as
the powerful support that was afforded by the British Association for the
Advancement of Science at their meeting held at Newcastle in 1838. In June,
1839, the Antarctic magnetic expedition, under the command of Captain James
Clark Ross, was fully arranged; and now, since its successful return, we
reap the double fruits of the highly important geographical discoveries
around the south pole, and a series of simultaneous observations at eight or
ten magnetic stations.
A great English astronomer and physicist has calculated* that the mass of
observations which are in progress will accumulate in the course of three
years to 1,958,000.
[footnote] *See the article on 'Terrestrial Magnetism', in the 'Quarterly
Review' 1840, vol. lxvi., p. 271-312.
Never before has so noble and cheerful a spirit presided over the inquiry
into the 'quantitative' relations of the laws of the phenomena of nature.
We are, therefore, justified in hoping that these laws, when compared with
those which govern the atmosphere and the remoter regions of space, may, by
degrees, lead us to a more intimate acquaintance with the genetic conditions
of magnetic phenomena. As yet we can only boast of having opened a greater
number of paths which may possibly lead to an explanation of this subject.
In the physical science of terrestrial
p 193
magnetism, which must not be confounded with the purely mathematical branch
of the study, those persons only will obtain perfect satisfaction who, as in
the science of the meteorological processes of the atmosphere conveniently
turn aside the practical bearing of all phenomena that can not be explained
according to their own views.
Terrestrial magnetism, and the electro-dynamic forces computed by the
intellectual Ampere,* stand in simultaneous and intimate connection with the
terrestrial or polar light, as well as with the internal and external heat
of our planet, whose magnetic poles may be considered as the poles of cold.**
[footnote] *Instead of ascribing the internal heat of the Earth to the
transition of matter from a vapor-like fluid to a solid condition, which
accompanies the formation of the planets, Ampere has propounded the idea,
which I regard as highly improbable, that the Earth's temperature may be the
consequence of the continuous chemical action of a nucleus of the metals of
the earths and alkalies on the oxydizing external crust. "It can not be
doubted," he observes in his masterly 'Theorie des Phenomenes
Electro-dynamiques', 1826, p. 199, "that electro-magnetic currents exist in
the interior of the globe, and that these currents are the cause of its
temperature. They arise from the action of a central metallic nucleus,
composed of the metals discovered by Sir Humphrey Davy, acting on the
surrounding oxydized layer."
[footnote] **The remarkable connection between the curvature of the
magnetic lines and that of my isothermal lines was first detected by Sir
David Brewster. See the 'Transactions of the Royal Society of Edinburgh',
vol. ix., 1821, p. 318, and 'Treatise on Magnetism', 1837, p. 42, 44, 47,
and 268. This distinguished physicist admist two cold poles (poles of
maximum cold) in the northern hemisphere, an American one near Cape Walker
(73 degrees lat., 100 degrees W. long.), and an Asiatic one (73 degrees
lat., 80 degrees E. long.); whence arise, according to him, two hot and two
cold meridians, i.e., meridians of greatest heat and cold. Even in the
sixteenth century, Acosts ('Historia Natural de las Indias', 1589, lib. i.,
cap. 17), grounding his opinion on the observations of a very experienced
Portuguese pilot, taught that there were four lines without declination. It
would seem from the controversy of Henry Bond (the author of 'The Longitude
Found', 1676) with Beckborrow, that this view in some measure influenced
Halley in his theory of four magnetic poles. See my 'Examen Critique de
l'Hist. de la Geographie', t. iii., p. 60.
The bold conjecture hazarded one hundred and twenty-eight years since by
Halley,* that the Aurora Borealis was a magnetic phenomenon, has acquired
empirical certainty from Faraday's brilliant discovery of the evolution of
light by magnetic forces.
[footnote] *Halley, in the 'Philosophical Transactions', vol. xxix. (for
1714-1716), No. 341.
The northern light is preceded by premonitory signs. Thus, in the morning
before the occurrence of the phenomenon, the irregular horary course of the
magnetic needle generally indicates a disturbance of the equilibrium in the
distribution of
p 194
terrestrial magnetism.*
[footnote] *[The Aurora Borealis of October 24th, 1847, which was one of
the most brilliant ever known in this country, was preceded by great
magnetic disturbance. On the 22d of October the maximum of the west
declination was 23 degrees 10'; on the 23d the position of the magnet was
continually changing, and the extreme west declinations were between 22
degrees 44' and 23 degrees 37';on the night between the 23d and 24th of
October, the changes of position were very large and very frequent, the
magnet at times moving across the field so rapidly that a difficulty was
experienced in following it. During the day of the 24th of October there
was a constant change of position, but after midnight, when the Aurora began
perceptibly to decline in brightness, the disturbance entirely ceased. The
changes of position of the horizontal-force magnet were as large and as
frequent as those of the declination magnet, but the vertical-force magnet
was at no time so much affected as the other two instruments. See 'On the
Aurora Borealis, as it was seen on Sunday evening, October 24th, 1847, at
Blackheath,' by James Glaisher, Esq., of the Royal Observatory, Greenwich,
in the 'London, Edinburgh, and Dublin Philos. Mag and Journal of Science for
Nov.', 1847, by John H. Morgan, Esq. We must not omit to mention that
magnetic disturbance is now registered by a 'photographic' process: the
self-registering photographic apparatus used for this purpose in the
Observatory at Greenwich was designed by Mr. Brooke, and another ingenious
instrument of this kind has been invented by Mr. F. Ronalds, of the Richmond
Observatory.] -- Tr.
When this disturbance attains a great degree of intensity, the equilibrium
of the distribution is restored by a discharge attended by a development of
light "The Aurora* itself is, therefore, not to be regarded as an externally
manifested cause of this disturbance, but rather as a result of telluric
activity, manifested on the one side by the appearance of the light, and on
the other by the vibrations of the magnetic needle."
[footnote] *Dove, in Poggend., 'Annalen', bd. xx., s. 341; bd. xix., s.
388. "The declination needle acts in very nearly the same way as an
atmospheric electrometer, whose divergence in like manner shows the
increased tension of the electricity before this has become so great as to
yield a spark." See also, the excellent observations of Professor Käwmtz,
in his 'Lehrbuch der Meteorologie', bd. iii., s. 511-519, and Sir David
Brewster, in his 'Treatise on Magnetism', p. 280. Regarding the magnetic
properties of the galvanic flame, or luminous arch from a Bunsen's carbon
and zinc battery, see Casselmann's 'Beobachtungen' (Marburg, 1844), s. 56-62.
The splendid appearance of colored polar light is the act of discharge, the
termination of a magnetic storm, as in an electrical storm a development of
light -- the flash of lightning -- indicates the restoration of the
disturbed equilibrium in the distribution of the electricity. An electric
storm is generally confined to a small space beyond the limits of which the
condition of the atmospheric electricity remains unchanged. A magnetic
storm, on the other hand,
p 193
shows its influence on the course of the needle over large portions of
continents, and, as Arago first discovered far from the spot where the
evolution of light was visible. It is not improbable that, as
heavily-charged threatening clouds, owing to frequent transitions of the
atmospheric electricity to an opposite condition, are not always discharged,
accompanied by lightning, so likewise magnetic storms may occasion
far-extending disturbances in the horary course of the needle, without there
being any positive necessity that the equilibrium of the distribution should
be restored by explosion, or by the passage of luminous effusions from one
of the poles to the equator, or from pole to pole.
In collecting all the individual features of the phenomenon in one general
picture, we must not omit to describe the origin and course of a perfectly
developed Aurora Borealis. Low down in the distant horizon, about the part
of the heavens which is intersected by the magnetic meridian, the sky which
was previously clear is at once overcast. A dense wall of bank of cloud
seems to rise gradually higher and higher, until it attains an elevation of
8 or 10 degrees. The color of the dark segment passes into brown or
violet; and stars are visible through the cloudy stratum, as when a dense
smoke darkens the sky. A broad, brightly-luminous arch, first white, then
yellow, encircles the dark segment; but as the brilliant arch appears
subsequently to the smoky gray segment, we can not agree with Argelander in
ascribing the latter to the effect of mere contrast with the bright luminous
margin.*
[footnote] *Argelander, in the important observations on the northern light
embodied in the 'Vorträgen gehalten in der physikalish-okonomischen
Gessellschaft zu Konigsberg', bd. i., 1834, s. 257-264.
The highest point of the arch of light is, according to accurate
observations made on the subject,* not generally in the magnetic meridian
itself, but from 5 degrees to 18 degrees toward the direction of the
magnetic declination of the place.**