Bravais, and Siljerstrom, who spent a winter at Bosekop, on the coast of
Lapland (70 degrees N. lat.), and in 210 nights saw the northern lights 160
times, see the 'Comptes Rendus de l'Acad. des Sciences', t. x., p. 289, and
Martins's 'Meteorologie', 1843, p. 453. See also, Argelander in the
'Vortragen geh. in der Konigsberg Gessellschaft', bd. i., s. 259.
[footnote] **[Professor Challis of Cambridge, states that in the Aurora of
October 24th, 1847, the streamers all converged toward a single point of the
heavens, situated in or very near a vertical circle passing through the
magnetic pole. Around this point a corona was formed, the rays of which
diverged in all directions from the center, leaving a space free from light:
its azimuth was 18 degrees 41' from south to east, and its altitude 69
degrees 54'. See Professor Challis, in the 'Athenaeum', Oct. 31, 1847.] --
Tr.
In the northern latitudes,
p 196
in the immediate vicinity of the magnetic pole, the smoke-like conical
segment appears less dark, and sometimes is not even seen. Where the
horizontal force is the weakest, the middle of the luminous arch deviates
the most from the magnetic meridian.
The luminous arch remains sometimes for hours together flashing and kindling
in ever-varying undulations, before rays and streamers emanate from it, and
shoot up to the zenith. The more intense the discharges of the northern
light, the more bright is the play of colors, through all the varying
gradations from violet and bluish white to green and crimson. Even in
ordinary electricity excited by friction, the sparks are only colored in
cases where the explosion is very violent after great tension. The magnetic
columns of flame rise eithr singly from the luminous arch, blended with
black rays similar to thick smoke, or simultaneously in many opposite points
of the horizon, uniting together to torm a flickering sea of flame, whose
brilliant beauty admits of no adequate description, as the luminous waves
are every moment assuming new and varying forms. The intensity of this
light is at times so great, that Lowenorn (on the 29th of June, 1786)
recognized the coruscation of the polar light n bright sunshine. Motion
renders the phenomenon more visible. Round the point in the vault of heaven
which corresponds to the direction of the inclination of the needle, the
beams unite together to form the so-called corona, the crown of the northern
light, which encircles the summit of the heavenly canopy with a milder
radiance and unflickering emanations of light. It is only in rare instances
that a perfect crown or circle is formed, but on its completion the
phenomenon has invariably reached its maximum, and the radiations become
less frequent, shorter, and more colorless. The crown and the luminous
arches break up, and the whole vault of heaven becomes covered with
irregularly-scattered, broad, faint, almost ashy-gray luminous immovable
patches, which in their turn disappear, leaving nothing but a trace of the
dark, smoke-like segment on the horizon. There often remains nothing of the
whole spectacle but a white, delicate cloud with feathery edges, or divided
at equal distances into small roundish groups like cirio-cumuli.
This connection of the polar light with the most delicate cirrous clouds
deserves special attention, because it shows that the electro-magnetic
evolution of light is a part of a meteorological process. Terrestrial
magnetism here manifests its influence
p 197
on the atmosphere and on the condensation of aqueous vapor. The fleecy
clouds seen in Iceland by Thienemann, and which he considered to be the
northern light, have been seen in recent times by Franklin and Richardson
near the American north pole, and by Admiral Wrangel on the Siberian coast
of the Polar Sea. All remarked "that the Aurora flashed forth in the most
vivid beams when masses of cirrous strata were hovering in the upper regions
of the air, and when these were so thin that their presence could only be
recognized by the formation of a halo round the moon." These clouds
sometimes range themselves, even by day in a similar manner to the beams of
the Aurora, and then disturb the course of the magnetic needle in the same
manner as the latter. On the morning after every distinct nocturnal Aurora,
the same superimposed strata of clouds have still been observed that had
previously been luminous.*
[footnote] *John Franklin, 'Narrative of a Journey to the Shores of the
Polar Sea, in the Years 1819-1822', p. 552 and 597; Thienemann in the
'Edinburgh Philosophical Journal', vol. xx., p. 336; Farquharson, in vol.
vi., p. 392, of the same journal; Wrangel, 'Phys. Beob.', s. 59. Parry even
saw the great arch of the northern light continue throughout the day.
('Journal of the Royal Institution of Great Britain', 1828, Jan., p. 429.)
The apparently converging polar zones (streaks of clouds in the direction of
the magnetic meridian), which constantly occupied my attention during my
journeys on the elevated plateaux of Mexico and in Northern Asia, belong
probably to the same group of ciurnal phenomena.*
[footnote] *On my return from my American travels, I described the delicate
cirro-cumulus cloud, which appears uniformly divided, as if by the action of
repulsive forces, under the name of polar bands ('bandes polaires'), because
their perspective point of convergence is mostly at first in the magnetic
pole, so that the parallel rows of fleecy clouds follow the magnetic
meridian. One peculiarity of this mysterious phenomenon is the oscillation,
or occasionally the gradually progressive motion, of the point of
convergence. It is usually observed that the bands are only fully developed
in one region of the heavens, and they are seen to move first from south to
north, and then gradually from east to west. I could not trace any
connection between the advancing motion of the bands and alterations of the
currents of air in the higher regions of the atmosphere. They occur when
the air is extremely calm and the heavens are quite serene, and are much
more common under the tropics than in the temperate and frigid zones. I
have seen this phenomenon on the Andes, almost under the equator, at an
elevation of 15,920 feet, and in Northern Asia, in the plains of
Krasnojarski, south of Buchtarminsk, so similarly developed, that we must
regard the influences producing it as very widely distributed, and as
depending on general natural forces. See the important observations of
Kamtz ('Vorlesungen uber Meteorologie', 1840, s. 146), and the more recent
ones of Martins and Bravais ('Meteorologie', 1843, p. 117). In south polar
bands, composed of very delicate clouds, observed by Arqago at Paris on the
23d of June, 1844, dark rays shot upward from an arch running east and west.
We have already made mention of black rays, resembling dark smoke, as
occurring in brilliant nocturnal northern lights.
p 198
Southern lights have often been seen in England by the intelligent and
indefatigable observer Dalton and northern lights have been observed in the
southern hemisphere as far as 45 degrees latitude (as on the 14th of
January, 1831). On occasions that are by no means of rare occurrence, the
equilibrium at both poles has been simultaneously disturbed. I have
discovered with certainty that northern polar lights have been seen within
the tropics in Mexico and Peru. We must distinguish between the sphere of
simultaneous visibility of the phenomenon and the zones of the Earth where
it is seen almost nightly. Every observer no doubt sees a separate Aurora
of his own, as he sees a separate rainbow. A great portion of the Earth
simultaneously engenders these phenomena of emanations of light. Many
nights may be instanced in which the phenomenon has been simultaneously
observed in England and in Pennsylvania, in Rome and in Pekin. When it is
stated that Auroras diminish with the decrease of latitude, the latitude
must be understood to be magnetic, and as measured by its distance from the
magnetic pole. In Iceland, in Greenland, Newfoundland, on the shores of the
Slave Lake, and at Fort Enterprise in Northern Canada, these lights appear
almost every night at certain seasons of the year, celebrating with their
flashing beams, according to the mode of expression common to the
inhabitants of the Shetland Isles, "a merry dance in heaven."*
[footnote] *The northrn lights are called by the Shetland Islanders "the
merry dancers." (Kendal, in the 'Quarterly Journal of Science', new series,
vol. iv., p. 395.)
While the Aurora is a phenomenon of rare occurrence in Italy, it is
frequently seen in the latitude of Philadelphia (39 degrees 57'), owing to
the southern position of the American nagnetic pole. In the districts which
are remarkable, in the New Continent and the Siberian coasts, for the
frequent occurrence of this phenomenon, there are special regions or zones
of longitude in which the polar light is particularly bright and brilliant.*
[footnote] *See Muncke's excellent work in the new edition of Gehler's
'Physik Worterbuch', bd. vii., i., s 113-268, and especially s. 158.
The existence
p 199
of local influences can not, therefore, be denied in these cases. Wrangel
saw the brilliancy diminish as he left the shores of the Polar Sea, about
Mischne-Kolymsk. The observations made in the North Polar expedition appear
to prove that in the immediate vicinity of the magnetic pole the development
of light is not in the least degree more intense or frequent than at some
distance from it.
The knowledge which we at present possess of the altitude of the polar light
is based on measurements which from their nature, the constant oscillation
of the phenomenon of light, and the consequent uncertainty of the angle of
parallax, are not deserving of much confidence. The results obtained,
setting aside the older data, fluctuate between several miles and an
elevation of 3000 or 4000 feet; and, in all probability, the northern lights
at different times occur at very different elevations.*
[footnote] *Farquharson in the 'Edinburgh Philos. Journal', vol. xvi., p.
304; 'Philos. Transact.' for 1829, p. 113.
[The height of the bow of light of the Aurora seen at the Cambridge
Observatory, March 19, 1847, was determined by Professors Challis, of
Cambridge, and Chevallier, of Durham, to be 177 miles above the surface of
the Earth. See the notice of this meteor in 'An Account of the Aurora
Borealis of Oct. 24, 1847', by John H. Morgan, Esq., 1848.] -- Tr.]
The most recent observers are disposed to place the phenomenon in the region
of clouds, and not on the confines of the atmosphere; and they even believe
that the rays of the Aurora may be affected by winds and currents of air, if
the phenomenon of light, by which alone the existence of an electro-magnetic
current is appreciable, be actually connected with matrial groups of
vesicles of vapor in motion, or, more correctly speaking, if light penetrate
them, passing from one vesicle to another. Franklin saw near Great Bear
Lake a beaming northern light, the lower side of which he thought
illuminated a stratum of clouds, while, at a distance of only eighteen
geographical miles, Kendal, who was on watch throughout the whole night, and
never lost sight of the sky, perceived no phenomenon of light. The
assertion, so frequently maintained of late, that the rays of the Aurora
have been seen to shoot down to the ground between the spectator and some
neighboring hill, is open to the charge of optical delusion, as in the cases
of strokes of lightning or of the fall of fire-balls.
Whether the magnetic storms, whose local character we have illustrated by
such remarkable examples, share noise as well as light in common with
electric storms, is a question
p 200
that has become difficult to answer, since implicit confidence is no longr
yielded to the relations of Greenland whale-fishers and Siberian
fox-hunters. Northern lights appear to have become less noisy since their
occurrences have been more accurately recorded. Parry, Franklin, and
Richardson, near the north pole; Thienemann in Iceland; Gieseke in
Greenland; Lotur, and Bravais, near the North Cape; Wrangel and Anjou, on
the coast of the Polar Sea, have together seen the Aurora thousands of
times, but never heard any sound attending the phenomenon. If this negative
testimony should not be deemed equivalent to the positive counter-evidence
of Hearne on the mouth of the Copper River and of Henderson in Iceland, it
must be remembered that, although Hood heard a noise as of quickly-moved
musket-balls and a slight cracking sound during an Aurora, he also noticed
the same noise on the following day, when there was no northern light to be
seen; and it must not be forgotten that Wrangel and Gieseke were fully
convinced that the sound they had heard was to be ascribed to the
contraction of the ice and the crust of the snow on the sudden cooling of
the atmosphere. The belief in a crackling sound has arisen, not among the
people generally, but rather among learned travelers, because in earlier
times the northern light was declared to be an effect of atmospheric
electricity, on account of the luminous manifestation of the electricity in
rarefied space, and the observers found it easy to hear what they wished to
hear. Recent experiments with very sensitive electrometers have hitherto,
contrary to the expectation generally entertained, yielded only negative
results. The condition of the electricity in the atmosphere*
p 291
is not found to be changed during the most intense Aurora; but, on the other
hand, the three expressions of the power of terrestrial magnetism,
declination, inclination and intensity, are all affected by polar light, so
that in the same night, and at different periods of the magnetic
development, the same end of the needle is both attracted and repelled.
[footnote] *[Mr. James Glaisher, of the Royal Observatory, Greenwich, in
his interesting 'Remarks on the Weather during the Quarter ending December
31st, 1847', says, "It is a fact well worthy of notice, that from the
beginning of this quarter till the 29th of December, the electricity of the
atmosphere was almost always in a neutral state, so that no signs of
electricity were shown for several days together by any of the electrical
instruments." During this period there were 'eight' exhibitions of the
Aurora Borealis, of which one was the peculiarly bright display of the
Aurora Borealis, of which one was the peculiarly bright display of the
meteor on the 24th of October. These frequent exhibitions of brilliant
Aurorae seem to depend upon many remarkable meteorological relations, for we
find, according to Mr. Glaisher's statement in the paper to which we have
already alluded, that the previous fifty years afford no parallel season to
the closing one of 1847. The mean temperature of evaporation and of the dew
point, the mean elastic force of vapor, the mean reading of the barometer,
and the mean daily range of the readings of the thermometers in air, were
all greater at Greenwich during that season of 1847 than the average range
of many preceding years.] -- Tr.
The assertion made by Parry, on the strength of the data yielded by his
observations in the neighborhood of the magnetic pole at Melville Island,
that the Aurora did not disturb, but rather exercised a calming influence on
the magnetic needle, has been satisfactorily refuted by Parry's own more
exact researches,* detailed in his journal, and by the admirable
observations of Richardson, Hood, and Franklin in Northern Canada, and
lastly by Bravais and Lottin in Lapland.
[footnote] *Kamtz, 'Lehrbuch der Meteorologie', bd. iii., s. 498 and 501.
The process of the Aurora is, as has already been observed, the restoration
of a disturbed condition of equilibrium. The effect on the needle is
different according to the degree of intensity of the explosion. It was
only unappreciable at the gloomy winter station of Bosekop when the
phenomenon of light was very faint and aptly compared to the flame which
rises in the closed circuit of a voltaic pile between two points of carbon
at a considerable distance apart, or, according to Fizeau, to the flame
rising between a silver and a carbon point, and attracted or repelled by the
magnet. This analogy certainly sets aside the necessity of assuming the
existence of metallic vapors in the atmosphere, which some celebrated
physicists have regarded as the substratum of the northern light.
When we apply the indefinite term 'polar light' to the luminous phenomenon
which we ascribe to a galvanic current, that is to say, to the motion of
electricity in a closed circuit, we merely indicate the local direction in
which the evolution of light is most frequently, although by no means
invariably, seen. This phenomenon derives the greater part of its
importance from the fact that the Earth becomes 'self-luminous', and that as
a planet, besides the light which it receives from the central body, the
Sun, it shows itself capable in itself of developing light. The intensity
of the terrestrial light, or, rather the luminosity which is diffused,
exceeds, in cases of the brightest colored radiation toward the zenith, the
light of the Moon in its first quarter. Occasionally, as on the 7th of
January, 1831, printed characters could be read without difficulty. This
almost uninterrupted development of light
p 202
in the Earth leads us by analogy to the remarkable process exhibited in
Venus. The portion of this planet which is not illumined by the Sun often
shines with a phosphorescent light of its own. It is not improbable that
the Moon, Jupiter, and the comets shine with an independent light, besides
the reflected solar light visible through the polariscope. Without speaking
of the problematical but yet ordinary mode in which the sky is illuminated,
when a low cloud may be seen to shine with an uninterrupted flickering light
for many minutes together, we still meet with other instances of terrestrial
development of light in our atmosphere. In this category we may reckon the
celebrated luminous mists seen in 1783 and 1831; the steady luminous
appearance exhibited without any flickeriing in great clouds observed by
Rozier and Beccaria; and lastly, as Arago* well remarks, the faint diffused
light which guides the steps of the traveler in cloudy, starless, and
moonless nights in autumn and winter, even when there is no snow on the
ground.
[footnote] *Arago, on the dry fogs of 1783 and 1831, which illuminated the
night, in the 'Annuaire du Bureau des Longitudes', 1832, p. 246 and 250;
and, regarding extraordinary luminous appearances in clouds without storms,
see 'Notices sur la Tonnerre', in the 'Annuaire pour l'an. 1838', p. 279-285.
As in polar light or the electro-magnetic storm, a current of brilliant and
often colored light streams through the atmosphere in high latitudes, so
also in the torrid zones between the tropics, the ocean simultaneously
develops light over a space of many thousand square miles. Here the magical
effect of light is owing to the forces of organic nature. Foaming with
light, the eddying waves flash in phosphorescent sparks over the wide
expanse of waters, where every scintillation is the vital manifestation of
an invisible animal world. So varied are the sources of terrestrial light!
Must we still suppose this light to be latent, and combined in vapors, in
order to explain 'Moser's images produced at a distance' -- a discovery in
which reality has hitherto manifested itself like a mere phantom of the
imagination.
As the internal heat of our planet is connected on the one hand with the
generation of electro-magnetic currents and the process of terrestrial light
(a consequence of the magnetic storm), it, on the other hand, discloses to
us the chief source of geognostic phenomena. We shall consider these in
their connection with and their transition from merely dynamic disturbances,
from the elevation of whole continents and mountain chains to the
development and effusion of gaseous and
p 203
liquid fluids, of hot mud, and of those heated and molten earths which
become solidified into crystalline mineral masses. Modern geognosy, the
mineral portion of terrestrial physics, has made no slight advance in having
investigated this connection of phenomena. This investigation has led us
away from the delusive hypothesis, by which it was customary formerly to
endeavor to explain, individually every expression of force in the
terrestrial globe: it shows us the connection of the occurrence of
heterogeneous substances with that which only appertains to changes in space
(disturbances or elevations), and groups together phenomena which at first
sight appeared most heterogeneous, as thermal springs, effusion of carbonic
acid and sulphurous vapor, innocuous salses (mud eruptions), and the
dreadful devastation of volcanic mountains.*
[footnote] *[See Mantell's 'Wonders of Geology', 1848, vol. i., p. 34, 36,
105; also Lyell's 'Principles of Geology', vol. ii., and Daubeney 'On
Volcanoes', 2d ed., 1848, Part ii., ch. xxxii., xxxiii.] -- Tr.
In a general view of nature, all these phenomena are fused together in one
sole idea of the reaction of the interior of a planet on its external
surface. We thus recognize in the depths of the earth, and in the increase
of temperature with the increase of depth from the surface, not only the
germ of disturbing movements, but also of the gradual elevation of whole
continents (as mountain chains on long fissures), of volcanic eruptions, and
of the manifold production of mountains and mineral masses. The influence
of this reaction of the interior on the exterior is not, however, limited to
inorganic nature alone. It is highly probable that, in an earlier world,
more powerful emanations of carbonic acid gas, blended with the atmosphere,
must have increased the assimilation of carbon in vegetables, and that an
inexhaustible supply of combustible matter (lignites and carboniferous
formations) must have been thus buried in the upper strata of the earth by
the revolutions attending the destruction of vast tracts of forest. We
likewise perceive that the destiny of mankind is in part dependent on the
formation of the external surface of the earth, the direction of mountain
tracts and high lands, and on the distribution of elevated continents. It
is thus granted to the inquiring mind to pass from link to link along the
chain of phenomena until it reaches the period when, in the solidifying
process of our planet, and in its first transition from the gaseous form to
the agglomeration of matter, that portion of the inner heat of the Earth was
developed, which does not belong to the action of the Sun.
This material taken from pages 204-248
COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1
by Alexander von Humboldt
Translated by E C Otte
from the 1858 Harper & Brothers edition of Cosmos, volume 1
--------------------------------------------------
p 204
In order to give a general delineation of the causal connection of
geognostical phenomena, we will begin with those whose chief characteristic
is dynamic, consisting in motion and in change in space. Earthquakes
manifest themselves by quick and successive vertical, or horizontal, or
rotatory vibrations.*
[footnote] *[See Daubeney 'On Volcanoes', 2d ed., 1848, p. 509.] -- Tr.