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                                 AURORÆ:
                      THEIR CHARACTERS AND SPECTRA.

                                   BY
                        J. RAND CAPRON, F.R.A.S.

    “And now the Northern Lights begin to burn, faintly at first,
    like sunbeams playing in the waters of the blue sea. Then a
    soft crimson glow tinges the heavens. There is a blush on
    the cheek of night. The colours come and go; and change from
    crimson to gold, from gold to crimson. The snow is stained with
    rosy light. Twofold from the zenith, east and west, flames
    a fiery sword; and a broad band passes athwart the heavens,
    like a summer sunset. Soft purple clouds come sailing over the
    sky, and through their vapoury folds the winking stars shine
    white as silver. With such pomp as this is Merry Christmas
    ushered in, though only a single star heralded the first
    Christmas.”—LONGFELLOW.

                                 LONDON:
                   E. & F. N. SPON, 46 CHARING CROSS.
                                NEW YORK:
                            446 BROOM STREET.
                                  1879.

                     PRINTED BY TAYLOR AND FRANCIS,
                      RED LION COURT, FLEET STREET.

                                   TO
                  PROF. CHARLES PIAZZI SMYTH, F.R.S.E.,
                     ASTRONOMER ROYAL FOR SCOTLAND,
               ONE OF THE EARLIEST SPECTROSCOPIC OBSERVERS
                                   OF
                     THE AURORA AND ZODIACAL LIGHT,
                               THIS VOLUME
                                   IS
                         RESPECTFULLY DEDICATED
                                   BY
                               THE AUTHOR.

PREFACE.

Probably few of the phenomena of Nature so entirely charm and interest
scientific and non-scientific observers alike as the Aurora Borealis, or
“Northern Lights” as it is popularly called. Whether contemplated as the
long low quiescent arc of silver light illuminating the landscape with
a tender radiance, as broken clouds and columns of glowing ruddy light,
or as sheaves of golden rays, aptly compared by old writers to aerial
spears, such a spectacle cannot fail at all times to be a subject of
admiration, in some cases even of awe.

Hence it is no wonder that the Aurora has always received a considerable
amount of attention at the hands of scientific men. Early explorers of
the Arctic Regions made constant and important observations of it and its
character; and the list of references to works given in the Appendix will
show how often it formed the subject of monographs and communications to
learned Societies. The early contributions seem relatively more numerous
than those of a later date; and the substance of them will be found well
summed up in Dr. Brewster’s ‘Edinburgh Encyclopædia’ (1830), article
“Aurora.” A most complete and able epitome of our more recent experience
and knowledge of the Aurora and its spectrum has been contributed by
my friend Mr. Henry R. Procter to the present (9th) edition of the
‘Encyclopædia Britannica,’ article “Aurora Polaris.” It is, however,
a drawback to Encyclopædic articles that their matter is of necessity
condensed, and that they rarely have the very desirable aid of drawings
and engravings to illustrate their subjects. In spite, therefore, of the
exhaustive way, both as to fact and theory, in which the contributor to
the ‘Encyclopædia Britannica’ has realized his task, it seemed to me
there was still room left for a popular treatise, having for its object
the description of Auroræ, their characters and spectra. The question
of the Aurora spectrum seems the more worthy of extended discussion in
that it still remains an unsolved problem. In spite of the observations
and researches of Ångström, Lemström, and Vogel abroad, and of Piazzi
Smyth, Herschel, Procter, Backhouse, and others at home, the goal is
not yet reached; for while the faint and more refrangible lines are but
doubtfully referred to air, the bright and sharp red and green lines,
which mainly characterize the spectrum, are as yet unassociated with any
known analogue.

With these views, and to incite to further and closer observations, I
have been induced to publish the present volume as a sort of Auroral
Guide. For much of the history of the Aurora I am indebted to, and
quote from former articles and records, including the two excellent
Encyclopædic ones before referred to. Mr. Procter, Mr. Backhouse, and my
friend Mr. W. H. Olley have each kindly furnished me with much in the way
of information and suggestion. Dr. Schuster has lent me tubes showing the
true oxygen spectrum; while Herr Carl Bock, the Norwegian naturalist, has
enabled me to reproduce a veritable curiosity, viz. a picture in oil
painted by the light of a Lapland Aurora. The experiments detailed in
Part III. were suggested by the earlier ones of De la Rive, Varley, and
others, and demonstrate the effect of the magnet on electric discharges.
For assistance in these I am indebted to my friend Mr. E. Dowlen.

The illustrations are mainly from original drawings of my own. Those from
other sources are acknowledged. Messrs. Mintern have well reproduced
in chromo-lithography the coloured drawings illustrating the Auroræ,
moon-patches, &c.

TABLE OF CONTENTS.

                                 PART I.

                     THE AURORA AND ITS CHARACTERS.

                               CHAPTER I.

  The Aurora as known to the Ancients: pp. 1-5.

                               CHAPTER II.

  Some general Descriptions of Auroræ: pp. 6-15.

      By Sir John Franklin: pp. 6, 7.

      By Rev. James Farquharson: pp. 7, 8.

      By Mons. Lottin (Lardner): pp. 8-11.

      By Lieut. Weyprecht (Payer): pp. 11-14.

      Article in ‘Edinburgh Encyclopædia:’ pp. 14, 15.

                              CHAPTER III.

  Some specific Descriptions of Auroræ: pp. 16-30.

      Capt. Sabine’s Auroræ: p. 16.

      Aurora seen at Sunderland, February 8th, 1817: p. 16.

      Dr. Hayes’s Aurora, 6th January, 1861: pp. 16, 17.

      Prof. Lemström’s Aurora, 1st September, 1868: pp. 17, 18.

      Mr. J. R. Capron’s Aurora, October 24th, 1870: pp. 18, 19.

      Mr. Barker’s red and white Auroræ, 9th November (1870?): p. 19.

      Mr. J. R. Capron’s Aurora, February 4th, 1872: pp. 19, 20.

      Cardiff, Aurora seen at: p. 21.

      Mr. J. R. Capron’s Aurora, February 4th, 1874: pp. 21, 22.

      Mr. Herbert Ingall’s Aurora, July 18th, 1874: pp. 22, 23.

      Mr. J. R. Capron’s white Aurora, September 11th, 1874: pp. 23, 24.

      Dr. Allnatt’s Aurora, June 9th, 1876: p. 24.

      Herr Carl Bock’s Lapland Aurora, October 3rd, 1877: p. 25.

      Rev. T. W. Webb’s Aurora: pp. 25, 26.

      English Arctic Expedition, 1875-76: p. 26.

      Aurora Australis: pp. 26-29.

      Prof. Piazzi Smyth’s typical Auroræ: pp. 29, 30.

                               CHAPTER IV.

  Phenomena simulating Auroræ: pp. 31-32.

      Auroric Lights (Kinahan): pp. 31, 32.

      Luminous Arch: p. 32.

                               CHAPTER V.

  Some qualities of the Aurora: pp. 33-51.

      Noises attending Auroræ: pp. 33, 34.

      Colours of Aurora: pp. 35-37.

      Height of Aurora: pp. 37-40.

      Phosphorescence attending Aurora: pp. 41-44.

      Aurora and Ozone: pp. 44, 45.

      Polarization of Aurora light: pp. 45, 46.

      Number of Auroræ: p. 46.

      Duration of Aurora: p. 47.

      Travelling of Auroræ (Donati): pp. 47, 48.

      Geographical distribution of Auroræ (Fritz and Loomis): pp. 48, 49.

      Extent and principal zone of the Aurora: pp. 50, 51.

                               CHAPTER VI.

  Aurora in connexion with other Phenomena: pp. 52-69.

      Auroræ and Clouds: pp. 52-54.

      Aurora and Thunder-storms: pp. 54, 55.

      Aurora and Magnetic Needle: pp. 55-58.

      Auroræ, Magnetic Disturbances, and Sun-spots: pp. 58-62.

      Aurora and Electricity: pp. 62-64.

      Aurora and Meteoric Dust: pp. 64, 65.

      Aurora and Planets Venus and Jupiter: pp. 66, 67.

      Aurora and Zodiacal Light: pp. 67-69.

                              CHAPTER VII.

  Aurora-like patches on the partially-eclipsed Moon: pp. 70-77.

                              CHAPTER VIII.

  Aurora and Solar Corona: pp. 78-81.

                               CHAPTER IX.

  Supposed Causes of the Aurora: pp. 82-87.

      Prof. Lemström’s Theory: pp. 87, 88.

      Theories of MM. Becquerel and De la Rive: p. 88.

      M. Planté’s electric experiments: pp. 89, 90.

                                PART II.

                       THE SPECTRUM OF THE AURORA.

                               CHAPTER X.

  Spectroscope adapted for the Aurora: pp. 91-93.

      Spectrum of Aurora described: pp. 94-100.

      Flickering of the green line: p. 100.

      Mr. Backhouse’s graphical spectra of four Auroræ: p. 101.

      Lord Lindsay’s Aurora-spectrum, October 21, 1870: p. 102.

      Spectrum of the Aurora Australis: pp. 103, 104.

      Professor Piazzi Smyth’s Aurora-spectra: p. 104.

      Author’s Catalogue of the Auroral lines: pp. 104-106.

      Theories in relation to the Aurora and its spectrum: pp. 106, 107.

                               CHAPTER XI.

  The comparison of some Tube and other Spectra with the Spectrum of
        the Aurora: pp. 108-120.

      Hydrogen-tube: pp. 108-110.

      Carbon- and oxygen-tubes: pp. 110-114.

      Geissler mercury-tube and barometer mercurial vacuum: pp. 114, 115.

      Air-tubes: pp. 115, 116.

      Phosphorescent tube: pp. 116, 117.

      Spark in air: p. 117.

      Spark over water: p. 117.

      Phosphoretted-hydrogen flame: pp. 117, 118.

      Iron-spectrum: p. 118.

      Spectrum of mercury: p. 119.

      Table of coincidences: p. 119.

                              CHAPTER XII.

  Notes on Professor Ångström’s Theory of the Aurora-spectrum: pp. 121-127.

                              CHAPTER XIII.

  The Oxygen-spectrum in relation to the Aurora (Procter and Schuster):
       pp. 128-131.

                                PART III.

       MAGNETO-ELECTRIC EXPERIMENTS IN CONNEXION WITH THE AURORA.

                       INTRODUCTION: pp. 133-135.

                              CHAPTER XIV.

  Examination of Geissler-tubes under action of the Magnet: pp. 136-146.

      Nitrogen-tubes: pp. 136, 137.

      Oxygen-tubes: p. 138.

      Hydrogen-tubes: pp. 138, 139.

      Water-gas tube: p. 139.

      Ammonia-tube: pp. 139, 140.

      Carbonic-acid tube: p. 140.

      Chlorine-tubes: pp. 140, 141.

      Iodine-tubes: pp. 141-143.

      Bromine-tubes: pp. 143, 144.

      Silicic fluoride-tubes: p. 144.

      Sulphuric-acid tubes: pp. 144, 145.

      Sulphur-tube: pp. 145, 146.

                               CHAPTER XV.

  Effect of Magnet on a capillary Glass Tube: pp. 147, 148.

      Action of magnet on a bar of heavy glass: pp. 147, 148.

                              CHAPTER XVI.

  Effect of Magnet on wide Air (Aurora) tube: pp. 149-153.

      Stratification (note on): pp. 149, 150.

      Effect of magnet on Plücker (air-) tube: pp. 150-152.

      Effect of magnet on Plücker tube (tin chloride): pp. 152, 153.

      Effect of magnet on tin-chloride Geissler tube: p. 153.

                              CHAPTER XVII.

  Effect of Magnet on bulbed Phosphorescent Tube: pp. 154-156.

      Effect of magnet on small phosphorescent (powder) tubes: pp. 156,
         157.

      Lighting-up tubes with one wire only (Marquis of Salisbury’s
         observations): pp. 157, 158.

                             CHAPTER XVIII.

  Action of the Magnet on the Electric Spark: pp. 159, 160.

                              CHAPTER XIX.

  The Discharge _in vacuo_ in Larger Vessels, and Magnetic Effects
       thereon: pp. 161-165.

      Some of Baron Reichenbach’s magnetic researches tested: pp. 165, 166.

  Summary of the foregoing Experiments and their Results: p. 167.

                               CHAPTER XX.

  Some concluding Remarks: pp. 168-171.

                               APPENDICES.

  A. References to some Works and Essays on the Aurora: pp. 173, 174.

  B. Extracts from the Manual and Instructions for the (English) Arctic
       Expedition, 1875: pp. 175-181.

  C. Extracts from Parliamentary Blue-book, containing the “Results
       derived from the Arctic Expedition, 1875-76:” pp. 182-188.

  D. Aurora and Ozone: pp. 189-193.

  E. Dr. Vogel’s Inquiries into the Spectrum of the Aurora: pp. 194-207.

LIST OF PLATES.

  Plate.

      I. The Aurora during the Ice-pressure              _To face page_ 14

     II. Aurora seen by Dr. Hayes, 6th January, 1861          ”    ”    16

    III. Aurora, Guildford, Oct. 24, 1870                     ”    ”    18

     IV. Aurora, Guildford, Feb. 4, 1872; Eclipsed Moon,
           Aug. 23, 24, 1877                                  ”    ”    20

      V. Corona, Graphical Auroræ, Zodiacal Light, &c.        ”    ”    21

     VI. Aurora, Guildford, Feb. 4, 1874; Spectrum des
           Nordlichts (Vogel)                                 ”    ”    22

    VII. Aurora, Kyle Akin, Isle of Skye, Sept. 11, 1874      ”    ”    24

   VIII. Herr Carl Bock’s Lapland Aurora, Oct. 3, 1877        ”    ”    25

     IX. Compared Aurora and other Spectra. Loomis’s
           curves of Auroras, Magnetic Declination, and
           Solar Spots                                        ”    ”    59

      X. Spectroscope, Micrometer, Tubes                      ”    ”    91

     XI. Aurora-spectra, Candle-spectrum                      ”    ”   102

    XII. Aurora-spectrum, Solar spectrum, and
           Candle-spectrum                                    ”    ”   104

   XIII. Vogel’s Aurora-lines, Aurora-lines near G, and in
           the red and green                                  ”    ”   108

    XIV. Aurora, Hydrocarbons, Oxygen                         ”    ”   110

     XV. Aurora and Air-tubes, &c.                            ”    ”   115

    XVI. Aurora, Phosphoretted Hydrogen, Iron, &c.            ”    ”   117

   XVII. Effect of Magnet on Tubes and Spark                  ”    ”   134

  XVIII. Same, and Oxygen-spectrum                            ”    ”   154

PART I.

THE AURORA AND ITS CHARACTERS.

CHAPTER I.

THE AURORA AS KNOWN TO THE ANCIENTS.

[Sidenote: Seneca’s ‘Quæstiones Naturales,’ Lib. I. c. xiv. Description
of Auroræ.]

In Seneca’s ‘Quæstiones Naturales,’ Lib. I. c. xiv., we find the
following:—“Tempus est, alios quoque ignes percurrere, quorum diversæ
figuræ sunt. Aliquando emicat stella, aliquando ardores sunt, aliquando
fixi et hærentes, nonnunquam volubiles. Horum plura genera conspiciantur.
Sunt _Bothynoë_[1], quum velut corona cingente introrsus igneus cœli
recessus est similis effossæ in orbem speluncæ. Sunt _Pithitæ_[2], quum
magnitudo vasti rotundique ignis dolio similis, vel fertur vel in uno
loco flagrat. Sunt _Chasmata_[3], quum aliquod cœli spatium desedit,
et flammam dehiscens, velut in abdito, ostentat. Colores quoque omnium
horum plurimi sunt. Quidam ruboris acerrimi, quidam evanidæ ac levis
flammæ, quidam candidæ lucis, quidam micantes, quidam æqualiter et sine
eruptionibus aut radiis fulvi.

…

[Sidenote: Seneca, c. xv.]

C. xv. “Inter hæc ponas licet et quod frequenter in historiis legimus,
cœlum ardere visum: cujus nonnunquam tam sublimis ardor est ut inter ipsa
sidera videatur, nonnunquam tam humilis ut speciem longinqui incendii
præbeat.

“Sub Tiberio Cæsare cohortes in auxilium Ostiensis coloniæ cucurrerunt,
tanquam conflagrantis, quum cœli ardor fuisset per magnam partem noctis,
parum lucidus crassi fumidique ignis.”

[Sidenote: Translation.]

We may translate this:—“It is time other fires also to describe, of which
there are diverse forms.

“Sometimes a star shines forth; at times there are fire-glows, sometimes
fixed and persistent, sometimes flitting. Of these many sorts may be
distinguished. There are Bothynoë, when, as within a surrounding corona,
the fiery recess of the sky is like to a cave dug out of space. There are
Pithitæ, when the expanse of a vast and rounded fire similar to a tub
(dolium) is either carried about or glows in one spot.

“There are Chasmata, when a certain portion of the sky opens, and gaping
displays the flame as in a porch. The colours also of all these are
many. Certain are of the brightest red, some of a flitting and light
flame-colour, some of a white light, others shining, some steadily and
yellow without eruptions or rays.

…

“Amongst these we may notice, what we frequently read of in history, the
sky is seen to burn, the glow of which is occasionally so high that it
may be seen amongst the stars themselves, sometimes so near the Earth
(humilis) that it assumes the form of a distant fire. Under Tiberius
Cæsar the cohorts ran together in aid of the colony of Ostia as if it
were in flames, when the glowing of the sky lasted through a great part
of the night, shining dimly like a vast and smoking fire.”

[Sidenote: Auroræ frequently read of in history.]

From the above passages many striking particulars of the Aurora may be
gathered; and by the division of the forms of Aurora into classes it is
evident they were, at that period, the subject of frequent observation.
The expression “et quod frequenter in historiis legimus” shows, too, that
the phenomena of Auroral displays were a matter of record and discussion
with the writers of the day.

Various forms of Aurora may be recognized in the passages from Chap.
xiv.; while in those from Chap. xv. a careful distinction is drawn
between the Auroræ seen in the zenith or the upper regions of the sky,
and those seen on the horizon or apparently (and no doubt in some cases
actually) near the Earth’s surface.

[Sidenote: A spurious Aurora.]

The description of the cohorts running to the fire only to find it an
Aurora, calls to mind the many similar events happening in our own days.
Not, however, but that a mistake may sometimes occur in an opposite
direction. In the memoirs of Baron Stockmar an amusing anecdote is
related of one Herr von Radowitz, who was given to making the most of
easily picked up information. A friend of the Baron’s went to an evening
party near Frankfort, where he expected to meet Herr von Radowitz.
On his way he saw a barn burning, stopped his carriage, assisted the
people, and waited till the flames were nearly extinguished. When he
arrived at his friend’s house he found Herr von Radowitz, who had
previously taken the party to the top of the building to see an Aurora,
dilating on terrestrial magnetism, electricity, and so forth. Radowitz
asked Stockmar’s friend, “Have you seen the beautiful Aurora Borealis?”
He replied, “Certainly; I was there myself; it will soon be over.” An
explanation followed as to the barn on fire: Radowitz was silent some ten
minutes, then took up his hat and quietly disappeared.

[Sidenote: Auroræ as portents.]

It is probable that many of the phantom combats which are recorded to
have appeared in forms of fire in the air on the evenings preceding great
battles might be traced to Auroræ, invested with distinct characteristics
by the imagination of the beholders. Auroræ are said to have appeared
in the shape of armies of horse and foot engaged in battle in the sky
before the death of Julius Cæsar, which they were supposed to foretell.
For more than a year before the siege and destruction of Jerusalem by
Titus Vespasian, the Aurora was said to have been frequently visible in
Palestine.

Josephus, in his ‘Wars of the Jews’ (Whiston’s Translation, Book VI.
chap. v. sect. 3), in referring to the signs and wonders preceding the
destruction of Jerusalem, speaks of a star or comet, and that a great
light shone round about the altar and the holy house, which light lasted
for half an hour, and that a few days after the feast of unleavened bread
a certain prodigious and incredible phenomenon appeared—“for before
sunsetting chariots and troops of soldiers in their armour were seen
running about among the clouds, and surrounding of cities.” (This, if an
Aurora, must have been an instance of a daylight one.)

We find in Book II. of Maccabees, chap. v. verses 1, 2, 3, 4 (B.C. about
176 years):—

“1. About this same time Antiochus prepared his second voyage into Egypt:

“2. And then it happened that through all the city, for the space almost
of forty days, there were seen horsemen running in the air, in cloth of
gold, and armed with lances like a band of soldiers.

“3. And troops of horsemen in array, encountering and running one against
another, with shaking of shields and multitude of pikes, and drawing
of swords and casting of darts, and glittering of golden ornaments and
harness of all sorts.

“4. Wherefore every man prayed that that apparition might turn to good.”

[Sidenote: Early descriptions of Auroræ.]

In Aristotle’s ‘De Meteoris,’ Lib. I. c. iv. and v., the Aurora is
described as an appearance resembling flame mingled with smoke, and of a
purple red or blue colour. Pliny (Lib. II. c. xxvii.) speaks of a bloody
appearance of the heavens which seemed like a fire descending on the
earth, seen in the third year of the 107th Olympiad, and of a light seen
in the nighttime equal to the brightness of the day, in the Consulship
of Cæcilius and Papirius (Lib. II. c. xxxiii.), both of which may be
referred to Auroræ.

In the ‘Annals of Philosophy,’ vol. ix. p. 250, it is stated that
the Aurora among English writers is first described by Matthew of
Westminster, who relates that in A.D. 555 lances were seen in the air
(“quasi species lancearum in aëre visæ sunt a septentrionali usque ad
occidentem”).

In the article in the ‘Edinb. Encyc.’ vol. iii. (1830), the Aurora (known
to the vulgar as “streamers” or “merry dancers”) is distinguished in
two kinds—the “tranquil” and the “varying.” Musschenbroek enumerates as
forms:—_trabs_, “the beam,” an oblong tract parallel to the horizon;
_sagitta_, “the arrow;” _faces_, “the torch;” _capra saltans_, “the
dancing goat;” _bothynoë_, “the cave,” a luminous cloud having the
appearance of a recess or hollow in the heavens, surrounded by a corona;
_pithiæ_, “the tun,” an Aurora resembling a large luminous _cask_. The
two sorts of Auroræ distinguished as the “bothynoë” and “pithiæ” are
evidently taken from the passage in Seneca’s ‘Quæstiones’ before quoted.
In ‘Liberti Fromondi Meteorologicorum’ (London, 1656), Lib. II. cap. v.
“De Meteoris supremæ regionis aëris,” art. 1. De Capra, Trabe, Pyramide,
&c., these and other fantastic forms attributable to Auroræ are more
fully described.

In the article “Aurora Polaris,” Encyc. Brit. edit. ix., we find noted
that from a curious passage in Sirr’s ‘Ceylon and the Cingalese,’
vol. ii. p. 117, it would seem that the Aurora, or something like it,
is visible occasionally in Ceylon, where the natives call it “Buddha
Lights,” and that in many parts of Ireland a scarlet Aurora is supposed
to be a shower of blood. The earliest mentioned Aurora (in Ireland)
was in 688, in the ‘Annals of Cloon-mac-noise,’ after a battle between
Leinster and Munster, in which Foylcher O’Moyloyer was slain.

In the article in the Edinb. Encyc. before referred to it is stated that
it was not much more than a century ago that the phenomenon had been
noticed to occur with frequency in our latitudes.

Dr. Halley had begun to despair of seeing one till the fine display of
1716.

[Sidenote: Early notices of Auroræ not frequent in our latitudes.]

The first account on record in an English work is said to be in a book
entitled ‘A Description of Meteors by W. F. D. D.’ (reprinted, London,
1654), which speaks of “burning spears” being seen January 30, 1560. The
next is recorded by Stow as occurring on October 7, 1564; and, according
to Stow and Camden, an Aurora was seen on two nights, 14th and 15th
November, 1574.

Twice, again, an Aurora was seen in Brabant, 13th February and 28th
September, 1575. Cornelius Gemma compared these to spears, fortified
cities, and armies fighting in the air. Auroræ were seen in 1580 and 1581
in Wirtemberg, Germany.

Then we have no record till 1621, when an Aurora, described by Gassendi
in his ‘Physics,’ was seen all over France, September 2nd of that year.

In November 1623 another, described by Kepler, was seen all over Germany.

From 1666 to 1716 no appearance is recorded in the ‘Transactions of the
French Academy of Sciences;’ but in 1707 one was seen in Ireland and at
Copenhagen; while in 1707 and 1708 the Aurora was seen five times.

The Aurora of 1716, occurring after an interval of eighty years, which
Dr. Halley describes, was very brilliant and extended over much country,
being seen from the west of Ireland to the confines of Russia and the
east of Poland, extending nearly 30° of longitude, and from about the
50th degree of latitude, over almost all the north of Europe, and in all
places exhibiting at the same time appearances similar to those observed
in London. An Aurora observed in Bologna in 1723 was stated to be the
first that had ever been seen there; and one recorded in the ‘Berlin
Miscellany’ for 1797 is called a very unusual phenomenon. Nor did Auroræ
appear more frequent in the Polar Regions at that time, for Cælius
states that the oldest inhabitants of Upsala considered the phenomenon
as quite rare before 1716. Anderson, of Hamburg, writing about the same
time, says that in Iceland the inhabitants themselves were astonished
at the frequent Auroræ then beginning to take place; while Torfæus, the
Icelander, who wrote in 1706, was old enough to remember the time when
the Aurora was an object of terror in his native country.

According to M. Mairan, 1441 Auroræ were observed between A.D. 583 and
1751, of which 972 were observed in the winter half-years and 469 during
the summer half-years. In our next Chapter we propose to give some
general descriptions of Auroræ from comparatively early sources.

CHAPTER II.

SOME GENERAL DESCRIPTIONS OF AURORÆ.

[Sidenote: Sir John Franklin’s description.]

Sir John Franklin (‘Narrative of a Journey to the Shores of the Polar Sea
in the years 1819, 1820, 1821, 1822’) describes an Aurora in these terms:—

[Sidenote: Parts of the Aurora: beams, flashes, and arches.]

“For the sake of perspicuity I shall describe the several parts of the
Aurora, which I term beams, flashes, and arches.

“The beams are little conical pencils of light, ranged in parallel lines,
with their pointed extremities towards the earth, generally in the
direction of the dipping-needle.

[Sidenote: Formation of the Aurora.]

“The flashes seem to be scattered beams approaching nearer to the
earth, because they are similarly shaped and infinitely larger. I have
called them flashes, because their appearance is sudden and seldom
continues long. When the Aurora first becomes visible it is formed like
a rainbow, the light of which is faint, and the motion of the beams
undistinguishable. It is then in the horizon. As it approaches the zenith
it resolves itself into beams which, by a quick undulating motion,
project themselves into wreaths, afterwards fading away, and again and
again brightening without any visible expansion or contraction of matter.
Numerous flashes attend in different parts of the sky.”

[Sidenote: Arches of the Aurora.]

Sir John Franklin then points out that this mass would appear like an
arch to a person situated at the horizon by the rules of perspective,
assuming its parts to be equidistant from the earth; and mentions a
case when an Aurora, which filled the sky at Cumberland House from
the northern horizon to the zenith with wreaths and flashes, assumed
the shape of arches at some distance to the southward. He then
continues:—“But the Aurora does not always make its first appearance as
an arch. It sometimes rises from a confused mass of light in the east
or west, and crosses the sky towards the opposite point, exhibiting
wreaths of beams or coronæ boreales on its way. An arch also, which is
pale and uniform at the horizon, passes the zenith without displaying any
irregularity or additional brilliancy.” Sir John Franklin then mentions
seeing three arches together, very near the northern horizon, one of
which exhibited beams and even colours, but the other two were faint and
uniform. (See example of a doubled arc Aurora observed at Kyle Akin,
Skye, Plate VII.)

He also mentions an arch visible to the southward exactly similar to one
in the north. It appeared in fifteen minutes, and he suggests it probably
had passed the zenith before sunset. The motion of the whole body of
the Aurora from the northward to the southward was at angles not more
than 20° from the magnetic meridian. The centres of the arches were as
often in the magnetic as in the true meridian. A delicate electrometer,
suspended 50 feet from the ground, was never perceptibly affected by the
Aurora.

[Sidenote: Aurora does not often appear until some hours after sunset.]

Sir John Franklin further remarks that the Aurora did not often appear
immediately after sunset, and that the absence of that luminary for some
hours was in general required for the production of a state of atmosphere
favourable to the generation of the Aurora.

[Sidenote: Aurora seen in daylight.]

On one occasion, however (March 8th, 1821), he observed it distinctly
previous to the disappearance of daylight; and he subsequently states
that on four occasions the coruscations of the Aurora were seen very
distinctly before daylight had disappeared.

[In the article “Aurora Polaris,” Encyc. Brit. edit. ix., the
Transactions of the Royal Irish Academy, 1788, are referred to, where Dr.
Usher notices that the Aurora makes the stars flutter in the telescope;
and that, having remarked this effect strongly one day at 11 A.M., he
examined the sky, and saw an Auroral corona with rays to the horizon.

Instances are by no means rare of the principal Aurora-line having
been seen in waning sunlight, and in anticipation of an Aurora which
afterwards appeared.]

[Sidenote: The Rev. James Farquharson’s observations. Auroral arch.
Passage across the zenith.]

The Rev. James Farquharson, from the observation of a number of
Auroræ in Aberdeenshire in 1823 (‘Philosophical Transactions,’ 1829),
concluded:—that the Aurora follows an invariable order in its appearance
and progress; that the streamers appear first in the north, forming an
arch from east to west, having its vertex at the line of the magnetic
meridian (when this arch is of low elevation it is of considerable
breadth from north to south, having the streamers placed crosswise in
relation to its own line, and all directed towards a point a little
south of the zenith); that the arch moves forward towards the south,
contracting laterally as it approaches the zenith, and increasing its
intensity of light by the shortening of the streamers and the gradual
shifting of the angles which the streamers near the east and west
extremities of the arch make with its own line, till at length these
streamers become parallel to that line, and then the arch is seen in
a narrow belt 3° or 4° only in breadth, stretching across the zenith
at right angles to the magnetic meridian; that it still makes progress
southwards, and after it has reached several degrees south of the zenith
again enlarges its breadth by exhibiting an order of appearances the
reverse of that which attended its progress towards the zenith from the
north; that the only conditions that can explain and reconcile these
appearances are that the streamers of the Aurora are vertical, or nearly
so, and form a deep fringe which stretches a great way from east to
west at right angles to the magnetic meridian, but which is of no great
thickness from north to south, and that the fringe moves southward,
preserving its direction at right angles to the magnetic meridian.

[Sidenote: M. Lottin’s observations.]

Dr. Lardner, in his ‘Museum of Science and Art,’ vol. x. p. 189 _et
seq._, alludes to a description of “this meteor” (_sic_) supplied by M.
Lottin, an officer of the French Navy, and a Member of the Scientific
Commission to the North Seas. Between September 1838 and April 1839,
being the interval when the sun was constantly below the horizon, this
savant observed nearly 150 Auroræ. During this period sixty-four were
visible, besides many concealed by a clouded sky, but the presence of
which was indicated by the disturbances they produced upon the magnetic
needle.

The succession of appearances and changes presented by these “meteors” is
thus graphically described by M. Lottin:—

[Sidenote: Formation of the auroral bow.]

“Between four and eight o’clock P.M. a light fog, rising to the altitude
of six degrees, became coloured on its upper edge, being fringed with the
light of the meteor rising behind it. This border, becoming gradually
more regular, took the form of an arch, of a pale yellow colour, the
edges of which were diffuse, the extremities resting on the horizon.
This bow swelled slowly upwards, its vertex being constantly on the
magnetic meridian. Blackish streaks divided regularly the luminous arc,
and resolved it into a system of rays. These rays were alternately
extended and contracted, sometimes slowly, sometimes instantaneously,
sometimes they would dart out, increasing and diminishing suddenly in
splendour. The inferior parts, or the feet of the rays, presented always
the most vivid light, and formed an arc more or less regular. The length
of these rays was very various, but they all converged to that point
of the heavens indicated by the direction of the southern pole of the
dipping-needle. Sometimes they were prolonged to the point where their
directions intersected, and formed the summit of an enormous dome of
light.

[Sidenote: It ascends to the zenith. Reaches the zenith.]

“The bow then would continue to ascend toward the zenith. It would
suffer an undulatory motion in its light—that is to say, that from
one extremity to the other the brightness of the rays would increase
successively in intensity. This luminous current would appear several
times in quick succession, and it would pass much more frequently from
west to east than in the opposite direction. Sometimes, but rarely, a
retrograde motion would take place immediately afterward; and as soon
as this wave of light had run successively over all the rays of the
Aurora from west to east, it would return in the contrary direction
to the point of its departure, producing such an effect that it was
impossible to say whether the rays themselves were actually affected by
a motion of translation in a direction nearly horizontal, or if this
more vivid light was transferred from ray to ray, the system of rays
themselves suffering no change of position. The bow, thus presenting
the appearance of an alternate motion in a direction nearly horizontal,
had usually the appearance of the undulations or folds of a ribbon or
flag agitated by the wind. Sometimes one, and sometimes both of its
extremities would desert the horizon, and then its folds would become
more numerous and marked, the bow would change its character and assume
the form of a long sheet of rays returning into itself, and consisting
of several parts forming graceful curves. The brightness of the rays
would vary suddenly, sometimes surpassing in splendour stars of the
first magnitude; these rays would rapidly dart out, and curves would be
formed and developed like the folds of a serpent; then the rays would
affect various colours, the base would be red, the middle green, and the
remainder would preserve its clear yellow hue. Such was the arrangement
which the colours always preserved. They were of admirable transparency,
the base exhibiting blood-red, and the green of the middle being
that of the pale emerald; the brightness would diminish, the colours
disappear and all be extinguished, sometimes suddenly and sometimes by
slow degrees. After this disappearance fragments of the bow would be
reproduced, would continue their upward movement and approach the zenith;
the rays, by the effect of perspective, would be gradually shortened;
the thickness of the arc, which presented then the appearance of a large
zone of parallel rays, would be extended; then the vertex of the bow
would reach the magnetic zenith, or the point to which the south pole of
the dipping-needle is directed. At that moment the rays would be seen in
the direction of their feet. If they were coloured they would appear as
a large red band, through which the green tints of their superior parts
could be distinguished, and if the wave of light above mentioned passed
along them their feet would form a long sinuous undulating zone; while
throughout all these changes the rays would never suffer any oscillation
in the direction of their axis, and would constantly preserve their
mutual parallelisms.

[Sidenote: Multiple bows. Corona formed.]

“While these appearances are manifested new bows are formed, either
commencing in the same diffuse manner or with vivid and ready formed
rays; they succeed each other, passing through nearly the same phases,
and arrange themselves at certain distances from each other. As many as
nine have been counted having their ends supported on the earth, and in
their arrangement resembling the short curtains suspended one behind the
other over the scene of a theatre, and intended to represent the sky.
Sometimes the intervals between these bows diminish, and two or more of
them close upon each other, forming one large zone traversing the heavens
and disappearing towards the south, becoming rapidly feeble after passing
the zenith. But sometimes also, when this zone extends over the summit of
the firmament from east to west, the mass of rays appear suddenly to come
from the south, and to form, with those from the north, the real boreal
corona, all the rays of which converge to the zenith. This appearance of
a crown, therefore, is doubtless the mere effect of perspective; and an
observer placed at the same instant at a certain distance to the north or
to the south would perceive only an arc.

“The total zone, measuring less in the direction north and south than in
the direction east and west, since it often leans upon the corona, would
be expected to have an elliptical form; but that does not always happen:
it has been seen circular, the unequal rays not extending to a greater
distance than from eight to twelve degrees from the zenith, while at
other times they reach the horizon.

“Let it then be imagined that all these vivid rays of light issue forth
with splendour, subject to continual and sudden variations in their
length and brightness; that these beautiful red and green tints colour
them at intervals; that waves of light undulate over them; that currents
of light succeed each other; and in fine, that the vast firmament
presents one immense and magnificent dome of light, reposing on the
snow-covered base supplied by the ground, which itself serves as a
dazzling frame for a sea calm and black as a pitchy lake. And some idea,
though an imperfect one, may be obtained of the splendid spectacle which
presents itself to him who witnesses the Aurora from the Bay of Alten.

[Sidenote: Duration of corona.]

“The corona when it is formed only lasts for some minutes; it sometimes
forms suddenly, without any previous bow. There are rarely more than two
on the same night, and many of the Auroras are attended with no crown at
all.

[Sidenote: Disappearance of Aurora.]

“The corona becomes gradually faint, the whole phenomenon being to
the south of the zenith, forming bows gradually paler and generally
disappearing before they reach the southern horizon. All this most
commonly takes place in the first half of the night, after which the
Aurora appears to have lost its intensity; the pencils of rays, the
bands, and the fragments of bows appear and disappear at intervals. Then
the rays become more and more diffused, and ultimately merge into the
vague and feeble light which is spread over the heavens, grouped like
little clouds, and designated by the name of auroral plates (plaques
aurorales). Their milky light frequently undergoes striking changes in
the brightness, like motions of dilatation and contraction, which are
propagated reciprocally between the centre and the circumference, like
those which are observed in marine animals called Medusæ. The phenomena
become gradually more faint, and generally disappear altogether on the
appearance of twilight. Sometimes, however, the Aurora continues after
the commencement of daybreak, when the light is so strong that a printed
book may be read. It then disappears, sometimes suddenly; but it often
happens that, as the daylight augments, the Aurora becomes gradually
vague and undefined, takes a whitish colour, and is ultimately so mingled
with the cirro-stratus clouds that it is impossible to distinguish it
from them.”

Lieutenant Weyprecht has grandly described forms of Aurora in Payer’s
‘New Lands within the Arctic Circle’ (vol. i. p. 328 _et seq._) as
follows:—

[Sidenote: Lieut. Weyprecht’s description. Formation of arches.]

“There in the south, low on the horizon, stands a faint arch of light.
It looks as it were the upper limit of a dark segment of a circle; but
the stars, which shine through it in undiminished brilliancy, convince
us that the darkness of the segment is a delusion produced by contrast.
Gradually the arch of light grows in intensity and rises to the zenith.
It is perfectly regular; its two ends almost touch the horizon, and
advance to the east and west in proportion as the arch rises. No beams
are to be discovered in it, but the whole consists of an almost uniform
light of a delicious tender colour. It is transparent white with a
shade of light green, not unlike the pale green of a young plant which
germinates in the dark. The light of the moon appears yellow contrasted
with this tender colour, so pleasing to the eye and so indescribable in
words, a colour which nature appears to have given only to the Polar
Regions by way of compensation. The arch is broad, thrice the breadth,
perhaps, of the rainbow, and its distinctly marked edges are strongly
defined on the profound darkness of the Arctic heavens. The stars shine
through it with undiminished brilliancy. The arch mounts higher and
higher. An air of repose seems spread over the whole phenomenon; here and
there only a wave of light rolls slowly from one side to the other. It
begins to grow clear over the ice; some of its groups are discernible.
The arch is still distant from the zenith, a second detaches itself from
the dark segment, and this is gradually succeeded by others. All now
rise towards the zenith; the first passes beyond it, then sinks slowly
towards the northern horizon, and as it sinks loses its intensity. Arches
of light are now stretched over the whole heavens; seven are apparent at
the same time on the sky, though of inferior intensity. The lower they
sink towards the north the paler they grow, till at last they utterly
fade away. Often they all return over the zenith, and become extinct just
as they came.

[Sidenote: Band of light appears. Second band and rays.]

“It is seldom, however, that an Aurora runs a course so calm and so
regular. The typical dark segment, which we see in treatises on the
subject, in most cases does not exist. A thin bank of clouds lies on
the horizon. The upper edge is illuminated; out of it is developed a
band of light, which expands, increases in intensity of colour, and
rises to the zenith. The colour is the same as in the arch, but the
intensity of the colour is stronger. The colours of the band change in
a never-ceasing play, but place and form remain unaltered. The band is
broad, and its intense pale green stands out with wonderful beauty on
the dark background. Now the band is twisted into many convolutions, but
the innermost folds are still to be seen distinctly through the others.
Waves of light continually undulate rapidly through its whole extent,
sometimes from right to left, sometimes from left to right. Then, again,
it rolls itself up in graceful folds. It seems almost as if breezes high
in the air played and sported with the broad flaming streamers, the ends
of which are lost far off on the horizon. The light grows in intensity,
the waves of light follow each other more rapidly, prismatic colours
appear on the upper and lower edge of the band, the brilliant white of
the centre is enclosed between narrow stripes of red and green. Out of
one band have now grown two. The upper continually approaches the zenith,
rays begin to shoot forth from it towards a point near the zenith to
which the south pole of the magnetic needle, freely suspended, points.

[Sidenote: Corona formed.]

“The band has nearly reached it, and now begins a brilliant play of rays
lasting for a short time, the central point of which is the magnetic
pole—a sign of the intimate connexion of the whole phenomenon with the
magnetic forces of the earth. Round the magnetic pole short rays flash
and flare on all sides, prismatic colours are discernible on all their
edges, longer and shorter rays alternate with each other, waves of light
roll round it as a centre. What we see is the auroral corona, and it
is almost always seen when a band passes over the magnetic pole. This
peculiar phenomenon lasts but a short time. The band now lies on the
northern side of the firmament, gradually it sinks, and pales as it
sinks; it returns again to the south to change and play as before. So
it goes on for hours, the Aurora incessantly changes place, form, and
intensity. It often entirely disappears for a short time, only to appear
again suddenly, without the observers clearly perceiving how it came and
where it went; simply, it is there.

[Sidenote: Single-rayed band.]

“But the band is often seen in a perfectly different form. Frequently
it consists of single rays, which, standing close together, point in an
almost parallel direction towards the magnetic pole. These become more
intensely bright with each successive wave of light; hence each ray
appears to flash and dart continually, and their green and red edges
dance up and down as the waves of light run through them. Often, again,
the rays extend through the whole length of the band, and reach almost
up to the magnetic pole. These are sharply marked, but lighter in colour
than the band itself, and in this particular form they are at some
distance from each other. Their colour is yellow, and it seems as if
thousands of slender threads of gold were stretched across the firmament.
A glorious veil of transparent light is spread over the starry heavens;
the threads of light with which this veil is woven are distinctly marked
on the dark background; its lower border is a broad intensely white band,
edged with green and red, which twists and turns in constant motion. A
violet-coloured auroral vapour is often seen simultaneously on different
parts of the sky.

[Sidenote: Aurora in stormy weather. Fragments.]

“Or, again, there has been tempestuous weather, and it is now, let us
suppose, passing away. Below, on the ice, the wind has fallen; but the
clouds are still driving rapidly across the sky, so that in the upper
regions its force is not yet laid. Over the ice it becomes somewhat
clear; behind the clouds appears an Aurora amid the darkness of the
night. Stars twinkle here and there; through the opening of the clouds we
see the dark firmament, and the rays of the Aurora chasing one another
towards the zenith. The heavy clouds disperse, mist-like masses drive on
before the wind. Fragments of the northern lights are strewn on every
side: it seems as if the storm had torn the Aurora bands to tatters,
and was driving them hither and thither across the sky. These threads
change form and place with incredible rapidity. Here is one! lo, it is
gone! Scarcely has it vanished before it appears again in another place.
Through these fragments drive the waves of light: one moment they are
scarcely visible, in the next they shine with intense brilliancy. But
their light is no longer that glorious pale green; it is a dull yellow.
It is often difficult to distinguish what is Aurora and what is vapour;
the illuminated mists as they fly past are scarcely distinguishable from
the auroral vapour which comes and goes on every side.

[Sidenote: Bands. Rays reach the pole. No noise.]

“But, again, another form. Bands of every possible form and intensity
have been driving over the heavens. It is now eight o’clock at night,
the hour of the greatest intensity of the northern lights. For a moment
some bundles of rays only are to be seen in the sky. In the south a
faint, scarcely visible band lies close to the horizon. All at once it
rises rapidly, and spreads east and west. The waves of light begin to
dart and shoot, some rays mount towards the zenith. For a short time it
remains stationary, then suddenly springs to life. The waves of light
drive violently from east to west, the edges assume a deep red and green
colour, and dance up and down. The rays shoot up more rapidly, they
become shorter; all rise together and approach nearer and nearer to the
magnetic pole. It looks as if there were a race among the rays, and that
each aspired to reach the pole first. And now the point is reached, and
they shoot out on every side, to the north and the south, to the east and
the west. Do the rays shoot from above downwards, or from below upwards?
Who can distinguish? From the centre issues a sea of flames: is that sea
red, white, or green? Who can say? It is all three colours at the same
moment! The rays reach almost to the horizon: the whole sky is in flames.
Nature displays before us such an exhibition of fireworks as transcends
the powers of imagination to conceive. Involuntarily we listen; such
a spectacle must, we think, be accompanied with sound. But unbroken
stillness prevails; not the least sound strikes on the ear. Once more
it becomes clear over the ice, and the whole phenomenon has disappeared
with the same inconceivable rapidity with which it came, and gloomy night
has again stretched her dark veil over everything. This was the Aurora
of the coming storm—the Aurora in its fullest splendour. No pencil can
draw it, no colours can paint it, and no words can describe it in all its
magnificence.”

A reproduction of the woodcut in Payer’s ‘Austrian Arctic Voyages,’
illustrating some of the features of the above description, will be found
on Plate I.

In the ‘Edinburgh Encyclopædia,’ article “Aurora,” we find:—

[Sidenote: Descriptions of Auroræ in high Northern latitudes.]

“In high Northern latitudes the Auroræ Boreales are singularly
resplendent, and even terrific.

“They frequently occupy the whole heavens, and, according to the
testimony of some, eclipse the splendour of stars, planets, and moon, and
even of the sun itself.

[Sidenote: In Siberia.]

“In the south-eastern districts of Siberia, according to the description
of Gmelin, cited and translated by Dr. Blagden (Phil. Trans. vol. lxxiv.
p. 228), the Aurora is described to begin with single bright pillars,
rising in the north, and almost at the same time in the north-east,
which, gradually increasing, comprehend a large space of the heavens,
rush about from place to place with incredible velocity, and finally
almost cover the whole sky up to the zenith, and produce an appearance
as if a vast tent were expanded in the heavens, glittering with gold,
rubies, and sapphires. A more beautiful spectacle cannot be painted; but
whoever should see such a northern light for the first time could not
behold it without terror.”

[Sidenote: Maupertius’s description at Oswer-Zornea.]

Maupertius describes a remarkable Aurora he saw at Oswer-Zornea on the
18th December, 1876. An extensive region of the heavens towards the south
appeared tinged of so lively a red that the whole constellation of Orion
seemed as if dyed in blood. The light was for some time fixed, but soon
became movable, and, after having successively assumed all the tints of
violet and blue, it formed a dome of which the summit nearly approached
the zenith in the south-west.

[Sidenote: Red Auroræ rare in Lapland.]

Maupertius adds that he observed only two of the red northern lights in
Lapland, and that they are of very rare occurrence in that country.

The observations of Carl Bock, the Norwegian naturalist, kindly
communicated by him to me, and detailed in Chapter III., quite confirm
this observation of Maupertius as to the rare occurrence of red Auroræ in
Lapland, he having only seen one.

[Illustration: Plate I.]

CHAPTER III.

SOME SPECIFIC DESCRIPTIONS OF AURORÆ, INCLUDING RESULTS OF THE ENGLISH
ARCTIC EXPEDITION, 1875-76.

_Captain Sabine’s Auroræ._

[Sidenote: Captain Sabine’s Auroræ.]

Captain Sabine describes Auroræ seen at Melville Island (Parry’s first
voyage, January 15). Towards the southern horizon an ordinary Aurora
appeared. The luminous arch broke into masses streaming in different
directions, always to the east of the zenith.

[Sidenote: Curvature of arches towards each other.]

The various masses seemed to arrange themselves in two arches, one
passing near the zenith and a second midway between the zenith and the
horizon, both north and south, but curving towards each other. At one
time a part of the arch near the zenith was bent into convolutions like a
snake in motion and undulating rapidly.

_Aurora seen at Sunderland, February 8th, 1817._

(‘Annals of Philosophy,’ vol. ix. p. 250.)

[Sidenote: Aurora seen at Sunderland, Feb. 8, 1817. Formation of corona.]

It began about 7 P.M. during a strong gale from the N.W., with single
bright streamers in the N. and N.W., which covered a large space and
rushed about from place to place with amazing velocity, and had a fine
tremulous motion, illuminating the hemisphere as much as the moon does
eight or nine days from change. About 11 o’clock part of the streamers
appeared as if projected south of the zenith and looked like the pillars
of an immense amphitheatre, presenting a most brilliant spectacle and
seeming to be in a lower region of the atmosphere, and to descend and
ascend in the air for several minutes. (This appears to have been the
formation of a corona.) One streamer passed over Orion, but neither
increased nor diminished its splendour.

_Description of Aurora by Dr. Hayes, 6th January, 1861._

[Sidenote: Dr. Hayes’s Aurora, 6th January, 1861.]

‘Recent Polar Voyages’ contains a narrative of the voyage of Dr. Hayes,
who sailed from Boston on the 6th of July, 1860, and wintered at Port
Foulbe. He witnessed a remarkable display of the Aurora Borealis on the
morning of the 6th January, 1861.

[Sidenote: Development of Aurora.]

The darkness was so profound as to be oppressive. Suddenly, from the rear
of the black cloud which obscured the horizon, flashed a bright ray.
Presently an arch of many colours fixed itself across the sky, and the
Aurora gradually developed.

[Sidenote: Rays changed to glow.]

The space within the arch was filled by the black cloud; but its borders
brightened steadily, though the rays discharged from it were exceeding
capricious, now glaring like a vast conflagration, now beaming like the
glow of a summer morn. More and more intense grew the light, until, from
irregular bursts, it matured into an almost uniform sheet of radiance.
Towards the end of the display its character changed. Lurid fires flung
their awful portents across the sky, before which the stars seemed to
recede and grow pale.

[Sidenote: Mixed colours. Colours change. Tongues of white flame formed.]

The colour of the light was chiefly red; but every tint had its turn,
and sometimes two or three were mingled; blue and yellow streamers shot
across the terrible glare, or, starting side by side from the wide
expanse of the radiant arch, melted into each other, and flung a strange
shade of emerald over the illuminated landscape. Again this green subdues
and overcomes the red; then azure and orange blend in rapid flight,
subtle rays of violet pierce through a broad flash of yellow, and the
combined streams issue in innumerable tongues of white flame, which mount
towards the zenith.

The illustration which accompanies this description in the work is
reproduced on Plate II., and forcibly reminds one of the “curtains” of
the Aurora described in the preceding Chapter by Mons. Lottin.

[Illustration: Plate II.]

_Prof. Lemström’s Aurora of 1st September, 1868._

[Sidenote: Prof. Lemström’s Aurora, 1st September, 1868.]

In the first Swedish Expedition, 1868, some remarkable observations were
made on the appearance of luminous beams around the tops of mountains,
which M. Lemström showed by the spectroscope to be of the same nature as
Auroræ.

[Sidenote: Aurora from earth’s surface. Yellow-green line seen.]

On the 1st September, 1868, on the Isle of Amsterdam in the Bay of
Sweerenberg, there was a light fall of snow, and the snowflakes were
observed falling obliquely. All at once there appeared a luminous
phenomenon which, starting from the earth’s surface, shot up vertically,
cutting the direction of the falling snowflakes, and this appearance
lasted for some seconds. On examination with a spectroscope the
yellow-green line was found by Lemström (but of feeble intensity) when
the slit of the instrument was directed towards a roof or other object
covered with snow, and even in the snow all round the observer.

[Sidenote: Lemström’s conclusions.]

M. Lemström concluded that an electric discharge of an auroral nature,
which could only be detected by means of the spectroscope, was taking
place on the surface of the ground all around him, and that, from a
distance, it would appear as a faint display of Aurora.

[It should, however, here be noted that the reflection of an Aurora from
a white or bright surface would give, in a fainter degree, the spectrum
of the Aurora itself; and, apart from the phenomena seen by the eye, the
case fails to be conclusive that an Aurora on the surface of the ground
was examined.]

_Mr. J. R. Capron’s Aurora of October 24th, 1870._

[Sidenote: Mr. J. R. Capron’s Aurora, Oct. 24, 1870. Silver glow in
north. Phosphorescent cloud-streamers. Crimson masses on horizon.
Coloured streamers. Corona formed. Aurora fades away.]

The description, from my notes made at the time of this fine display, is
as follows:—“Last evening (October 24) the Aurora Borealis was again most
beautifully seen here (Guildford). At 6 P.M. indications of the coming
display were visible in the shape of a bright silver glow in the north,
which contrasted strongly with the opposite dark horizon. For two hours
this continued, with the addition from time to time of a crimson glow in
the north-east, and of streamers of opaque-white phosphorescent cloud,
shaped like horse-tails (very different from the more common transparent
auroral diverging streams of light), which floated upwards and across
the sky from east and west to the zenith. At about 8 o’clock the display
culminated; and few observers, I should think, ever saw a more lovely
sky-picture. Two patches of intense crimson light about this time massed
themselves on the north-east and north-west horizon, the sky between
having a bright silver glow. The crimson masses became more attenuated
as they mounted upwards; and from them there suddenly ran up bars or
streamers of crimson and gold light, which, as they rose, curved towards
each other in the north, and, ultimately meeting, formed a glorious arch
of coloured light, having at its apex an oval white luminous corona
or cloud of similar character to the phosphorescent clouds previously
described, but brighter. At this time the spectator appeared to be
looking at the one side of a cage composed of glowing red and gold bars,
which extended from the distant parts of the horizon to a point over
his head. Shortly after this the Auroral display gradually faded away,
and at 9 o’clock the sky was of its usual appearance, except that the
ordinary tint seemed to have more of indigo, probably by contrast with
the marvellous colours which had so lately shone upon it.”

[Sidenote: T. F.’s description of same at Torquay. Mr. Gibbs’s report in
London.]

T. F., describing the same Aurora from Torquay, says it showed itself at
sundown, attained its maximum at 8, and lasted until 11. At 8 o’clock
more than half the visible heavens was one sea of colour; the general
ground greenish yellow and pale rose, with extensive shoals of deep rose
in the east and west; while from the north, streaming upwards to and
beyond the zenith, were tongues and brushes of rosy red, so deep that the
sky between looked black. Mr. Gibbs reported that in London, at about 8
o’clock, brilliant crimson rays shot up to the zenith, and the sky seemed
one mass of fire.

A facsimile of my water-colour sketch of this fine discharge is given on
Plate III.

[Illustration: Plate III.]

_Mr. Barker’s (superposed) red and white Auroræ, 9th November (1870?)._

[Sidenote: Mr. Barker’s Auroræ, 9th November (1870?). Red Aurora. White
Aurora.]

On the 9th November (1870?) Mr. Barker saw at New Haven (U. S.) a most
magnificent crimson Aurora. At about a quarter to 6 P.M. it consisted of
a brilliant streamer shooting up from the north-western horizon. This
was continued in a brilliant red, but rather nebulous, mass of light
passing upwards and to the north. Its highest points were from 30° to 40°
in altitude. A white Aurora, consisting of bright streamers, appeared
simultaneously and extended round to the north-east. Prof. Newton
informed Mr. Barker that he had observed an equally brilliant red patch
of auroral light in the north-east five or ten minutes earlier.

[Sidenote: Red seen through white.]

Since the lower end of the red streamers was much lower than that of the
white, it would seem as if the red were seen through the white, the red
being most remote.

[Sidenote: Crimson line not seen in white Aurora.]

Spectroscopic observations of this Aurora were made. The crimson Aurora
lasted less than half an hour, and then disappeared. In the white Aurora,
which remained, the crimson line could not be seen.

[Sidenote: Carl Bock’s vibrating rays.]

It may be here noted that during the Aurora seen by Carl Bock in
Lapland, and painted by him by its own light (described, p. 25), he had
the impression of sets of vibrating rays behind each other, and in the
drawing it looks as if streamers were seen behind an arc.

[Illustration: Plate IV.]

_Mr. J. R. Capron’s Aurora of February 4th, 1872._

[Sidenote: Mr. J. R. Capron’s Aurora, Feb. 4, 1872. Masses of
phosphorescent vapour. Rose tints appeared. Aurora from behind clouds.
Formation of corona. Duration of corona. Streamers from corona. Rain
during Aurora. Wind during night. Phosphorescent clouds preceded the
Aurora in daylight.]

My description of this Aurora as seen at Guildford, and as given at the
time, is as follows:—“Last evening, returning from church a little before
8 P.M., the sky presented a weird and unusual aspect, which at once
struck the eye. A lurid tinge upon the clouds which hung around suggested
the reflection of a distant fire; while scattered among these, torn and
broken masses of vapour, having a white and phosphorescent appearance,
and quickly changing their forms, reminded me of a similar appearance
preceding the great Aurora of 24th October, 1870. Shortly some of these
shining white clouds or vapours partly arranged themselves in columns
from east to west, and at the same time appeared the characteristic
patches of rose-coloured light which are often seen in an auroral
display. About 8 o’clock the clouds had to a certain extent broken away,
and the Aurora shone out from behind heavy banks of vapour, which still
rested on the eastern horizon, the north-west horizon being free from
cloud and glowing brightly with red light. And now, at about 8.15, was
presented a most beautiful phenomenon. While looking upwards, I saw a
corona or stellar-shaped mass of white light form in the clear blue sky
immediately above my head[4], not by small clouds or rays collecting,
but more in the way that a cloud suddenly forms by condensation in the
clear sky on a mountain top, or a crystal shoots in a transparent liquid,
having too, as I thought, an almost traceable nucleus or centre, from
which spear-like rays projected. From this corona in a few seconds shot
forth diverging streamers of golden light, which descended to and mingled
with the rosy patches of the Aurora hanging about the horizon. The spaces
of sky between the streamers were of a deep purple (probably an effect of
contrast). The display of the corona, though lasting a few minutes only,
was equal to, if not excelling in beauty, the grand display of October
1870, before described, in which case, however, a ring or disk of white
light of considerable size took the place of the stellar-shaped corona.
What struck me particularly was the corona developing itself as from a
centre in the clear sky, and the diverging streamers apparently shooting
downwards, whereas in general the streamers are seen to shoot up from the
horizon and converge overhead. The effect may have been an illusion; but,
if so, it was a remarkable one. The general Aurora lasted for some time,
till it was lost in a clouded sky; and, in fact, rain was descending at
one time while the Aurora was quite bright. Strong wind prevailed during
the night[5]. The Aurora was probably very extensive, as the evening,
notwithstanding the clouds, was nearly as bright as moonlight. The
peculiar clouds referred to must have preceded the Aurora in daylight,
as I recollect seeing them at 6.30 as we went to church.”

[Sidenote: Aurora predicted.]

They had even then a peculiarly wild, ragged, and phosphorescent
appearance, and so much resembled some I had seen to accompany the Aurora
of October 1870, that I predicted (as came to pass) a display later in
the evening. A _facsimile_ of my water-colour sketch of this Aurora is
given on Plate IV. fig. 1, while the corona and rays are represented
(with rather too hard an outline) on Plate V. fig. 2.

[Illustration: Plate V.]

_Description of an Aurora seen at Cardiff._

[Sidenote: Aurora seen at Cardiff. Formation of corona.]

An Aurora was seen at Cardiff. A dusky red aspect of the sky towards
the north, and extending itself across the zenith westward, made its
appearance about half-past 5 P.M. The lights reached their greatest
intensity at 6 o’clock, when the sky was suffused with a rich crimson
glow, a broad band of colour reaching from N.E. to W. A corona of deep
hue, having rugged sharply defined edges, stood out prominently in the
zenith, apparently on a parallel plane to the earth, and having its
centre almost immediately over the head of the spectator.

[Sidenote: Radii thrown out from corona.]

From this corona, elliptic in form, and in its broadest diameter about
four times the size of the moon, there were thrown out brilliant silvery
blue radii, extending to the N.E. and W. horizon, and presenting the
appearance of a vast cupola of fire.

[Sidenote: Rain fell when Aurora died out.]

At half-past 6 the lights died completely out, leaving masses of cloud
drifting up from the south, and a shower of rain fell. The corona was
remarked upon as unusual. At Edinburgh the sky was brilliant for several
hours. (The date of this Aurora is uncertain, as the account is from an
undated newspaper cutting. It is supposed to be in February 1872, but
could hardly have been on the 4th, as the Aurora of that date did not
reach its maximum development at Edinburgh till 8 P.M.)

_Mr. J. R. Capron’s Aurora, seen at Guildown, Guildford, February 4th,
1874._

[Sidenote: Silvery brightness in N.E. Light-cloud, which moved from E. to
W. Formation of arc in N. Streamers. Horizontal clouds of misty light.]

About 7 P.M. my attention was drawn to a silvery brightness in the
north-east. Above, and still more to the east, was a bright cloud of
light, which looked dense and misty, and gave one the impression of an
illuminated fog-cloud. The edges were so bright that the adjacent sky,
but for the stars shining in it, might, by contrast, have been taken for
a dark storm-cloud. The light-cloud expanded upwards until its apex
became conical, and then moved rapidly from east, along the northern
horizon, until it reached the due west, where it rested, and formed for
some time a luminous spot in the sky. About the same time a long low arch
of light formed along the northern horizon, having a brighter patch at
each extremity; and these being higher in the sky, the arch and turned-up
ends were in shape like a Tartar bow. This bow was permanent; and later
on a cloud of rose-coloured light formed in the east, looking like the
reflection of a distant fire. From the bow also shot up curved streamers
of silver light towards the zenith, which at one time threatened to form
a corona. This, however, did not happen, and the Aurora gradually faded
away, until, when the moon rose about 8, a silver tinge in the east alone
remained. I should also mention that fleecy horizontal clouds of misty
light floated in the north above the bow across the streamers.

Mr. H. Taylor informed me he saw a similar Aurora some three weeks
before, in which the bright horizontal light and short white streamers
were the main characters. I am not sure that the horizontal light-clouds
were not actual mist-clouds illuminated by reflection of the Aurora;
not so, however, I think, the first-mentioned cloud, which had more the
appearance of the _aura_ in the large end of an illuminated Geissler tube.

[Sidenote: Spectrum of the Aurora described.]

I examined the Aurora with a Browning direct-vision spectroscope, and
found Ångström’s line quite bright, and by the side of it three faint
and misty bands towards the blue end of the spectrum upon a faintly
illuminated ground. I could also see at times a bright line beyond
the bands towards the violet. There was not light enough to take any
measurements of position of the lines.

I made a pencil sketch of this Aurora, at the time when the light-cloud
had moved W. and the arc formed, and of the spectrum. These drawings are
reproduced on Plate VI. figs. 1 and 1_a_.

[Illustration: Plate VI.]

_Mr. Herbert Ingall’s Aurora, July 18th, 1874._

[Sidenote: Mr. Herbert Ingall’s Aurora, July 18, 1874. Haze canopy
formed. Bright bluish flames appeared. Beams and streamers appeared.
Oscillatory motion of rays.]

An Aurora of July 18th, 1874, seen by Mr. Herbert Ingall at Champion
Hill, S.E., was described by him as an extraordinary one. About 11
the sky was clear; at midnight the sky was covered by a sort of haze
canopy, sometimes quite obscuring the stars, and then suddenly fading
away. Mr. Ingall was shortly after remarking the sky in the S.E. and S.
horizon as being more luminous than usual, when his attention was drawn
to a growing brightness in the S.W., and a moment afterwards bright
bluish flames “swept over the S.W. and W. horizon, as if before a high
wind. They were not streamers, but bright blue flames.” They lasted
about a minute and faded; but about two minutes afterwards a glowing
luminosity appeared in the W.S.W., and broke into brilliant beams and
streamers. The extreme rays made an angle of 90° with each other, the
central ray reaching an altitude of 50°. The extreme divergence of the
streamers (indicating their height above the earth’s surface), and
their direction (from W.S.W. to E.N.E.) at right angles to the magnetic
meridian, suggested to Mr. Ingall a disturbance of an abnormal character.
The rays had an oscillatory motion for about fifteen seconds, and then
disappeared, “as if a shutter had suddenly obscured the source of light.”

[Sidenote: Mr. Ingall’s remarks corroborated.]

Mr. Ingall’s remarks were corroborated by an observer in lat. 54° 46´
6″·2 N., long. 6h 12m 19s·75 W. The display, however, was more brilliant,
and the intensity of light at midnight illuminated the whole district
as with an electric light. The rays, too, bore tints differing from one
another; the largest seemed to partake of the nature of the blue sky,
while the smaller ones, running parallel with the horizon, were ever
changing from blue to orange-red.

[Sidenote: Rev. C. Gape saw flashes or streaks of a pale blue colour.]

On June 25 (same year?), between 9 and 10 o’clock, the Rev. Chas. Gape
saw at Rushall Vicarage, Scole, Norfolk, in the E.S.E., very frequent
flashes or streaks of a pale blue colour darting from the earth towards
the heavens like an Aurora. The day had been dull and close, with distant
thunder. In the E.S.E. it was dark, but overhead and everywhere else it
was clear and starry.

_Mr. J. R. Capron’s White Aurora of September 11th, 1874._

[Sidenote: Mr. J. R. Capron’s white Aurora of Sept. 11, 1874.]

On September 11, 1874, we were at Kyle Akin, in the Isle of Skye. The
day had been wet and stormy, but towards evening the wind fell and the
sky became clear. About 10 P.M. my attention was called to a beautiful
Auroral display.

[Sidenote: Double arc of pure white light in the N.]

No crimson or rose tint was to be seen, but a long low-lying arc of the
purest white light was formed in the north, and continued to shine with
more or less brilliancy for some time. The arc appeared to be a double
one, by the presence of a dark band running longitudinally through it.

[Sidenote: White streamers. Auroral bow believed to be near the earth.]

Occasional streamers of equally pure white light ran upwards from
either end of the bow. The moon was only a day old, but the landscape
was lighted up as if by the full moon; and the effect of Kyle Akin
lighthouse, the numerous surrounding islands, and the still sea between
was a true thing of beauty. The display itself formed a great contrast
to the more brilliant but restless forms of Auroræ generally seen. I
particularly noticed a somewhat misty and foggy look about the brilliant
arc, giving it almost a solid appearance. The space of sky between the
horizon and the lower edge of the arc was of a deep indigo colour,
probably the effect of contrast. I had a strong impression that the bow
was near to the earth, and was almost convinced that the eastern end and
some fleecy clouds in which it was involved were between myself and the
peaks of some distant mountains.

I have not seen any other account of this Aurora, of which I was able at
the time to obtain a sketch. This is reproduced on Plate VII. It was a
lovely sight, and wonderfully unlike the cloud-accompanied and crimson
Auroræ which I had seen in the South.

It is noticed in Parry’s ‘Third Voyage’ that the lower edge of the
auroral arch is generally well defined and unbroken, and the sky beneath
it so exactly like a dark cloud (to him often of a brownish colour), that
nothing could convince to the contrary, if the stars, shining through
with undiminished lustre, did not discover the deception.

[Sidenote: No trace of brown colour in segment of sky below the arc.]

I saw no trace of brown colour. The segment below the arch resting on the
horizon was of a deep indigo colour.

[Illustration: Plate VII.]

_Dr. Allnatt’s Aurora, June 9th, 1876._

[Sidenote: Dr. Allnatt’s Aurora, June 9, 1876. Band of auroral light
appeared. Streaks of cirro-stratus divided the Aurora. Want of electric
manifestations attributed to absence of sun-spots.]

Dr. Allnatt, writing to the ‘Times’ from Abergele, North Wales, near
the coast of the Irish Channel, reported an Aurora on the night of the
9th June, 1876. After a cool and gusty day, with a strong N.E. wind
and a disturbed sea, there appeared at 11 P.M. in the N. horizon a
broad band of vivid auroral light, homogeneous, motionless, and without
streamers. About midnight a long attenuated streak of black cirro-stratus
stretched parallel with the horizon, and divided the Aurora into nearly
symmetrical sections. On the preceding day the sky was covered with dark
masses of electric cloud of weird and fantastic forms. The season had
been singularly unproductive of high electric manifestations, which Dr.
Allnatt thought might be attributable to the comparative absence of spots
on the solar disk. [It may here be noted how conspicuous the years 1877
and 1878 have been for absence of Sun-spots and of Auroræ.]

[Illustration: Plate VIII.]

_Herr Carl Bock’s Lapland Aurora, 3rd October, 1877._

[Sidenote: Herr Carl Bock’s Lapland Aurora, 3rd Oct. 1877. Lapland Auroræ
generally of the yellow type.]

In January 1878 I had the pleasure to meet, at the Westminster Aquarium,
Herr Carl Bock, the Norwegian naturalist, who accompanied four
Laplanders, two men and two women, with sledges, tents, &c., on their
visit to this country. The Laplanders (as mentioned elsewhere) did not
confirm the accounts of noises said to have been observed by Greenlanders
and others during the Aurora. Carl Bock mentioned to me that the displays
he saw in Lapland were most brilliant, but generally of the yellow type
(the Laplanders called the Aurora “yellow lights”). He saw only one red
Aurora. He kindly lent me a picture (probably in its way unique), an
oil-painting of an Aurora Borealis, entirely sketched by the light of the
Aurora itself.

[Sidenote: A picture painted by light of the Aurora. Movement of the
rays. Inner edge of arc fringed with rays.]

The painting is remarkable for the tender green of the sky, an effect
probably due to a mixture of the ordinary sky colour with the yellow
light of the Aurora. This picture was taken at Porsanger Fjord, in lat.
71° 50´, on 3rd October, 1877. It lasted from 9 P.M. till about 11
P.M. The rays kept continually moving, and certain of them seemed in
perspective and behind the others. It will be noticed that the _inner_
edge of the arc is fringed with rays, contrary to the sharp and definite
margin which is usually presented. Probably two Auroræ or auroral forms
were seen—a quiescent arc in front, and a set of moving streamers beyond.
Two larger and brighter patches of light are seen at each extremity of
the arc, as in the case of the Aurora seen by me at Guildown, February
4th, 1874, which, indeed, the display much resembles. A reduced facsimile
of Herr Bock’s excellent picture is given on Plate VIII.

[Sidenote: Aurora of longitudinal rays.]

Herr Bock also acquainted me that on the following day he saw an
Aurora in which the lines of light, instead of being vertical, were
longitudinal, and were continually swept along in several currents. They
were not so strong as in the former case.

_Rev. T. W. Webb’s Aurora._

[Sidenote: Rev. T. W. Webb’s Aurora. Arc resolved into sets of streamers
moving in opposite directions.]

The Rev. T. W. Webb has described to me in a letter an Aurora very like
that seen by Carl Bock in Lapland, and apparently the prevailing type
in those regions. An arc similar to that figured by Carl Bock appeared
in the N.W., and seemed to resolve itself into two sets of streamers
moving in opposite directions (or the one set might be fixed and the
other moving), like the edges of two great revolving toothed wheels. This
lasted but for a few seconds; but during that interval the tints were
varied and brilliant, including blue and green.

_The English Arctic Expedition 1875-76, under Capt. Sir George Nares._

[Sidenote: English Arctic Expedition, 1875-76. Instructions for use of
officers. Appendix B. Capt. Sir G. Nares’s report. True Auroræ seldom
observed, and displays faint. Citron-line observed on only two displays.
Appendix C.]

In anticipation of the starting of this Expedition, some instructions
for the use of the officers in connexion with the hoped for display of
brilliant Auroræ were prepared:—as to general features of the Auroræ, by
Professor Stokes; as to Polarization, by Dr. William Spottiswoode; and as
to Spectrum work, by Mr. Norman Lockyer and myself. As these instructions
were somewhat elaborate, and will apply to all Auroral displays, I have
supplied a copy of them in Appendix B. They were unfortunately not
brought into requisition, for want of the Auroræ themselves. Capt. Sir
George Nares has reported to the Admiralty, under date 5th December,
1877, as follows:—“Although the auroral glow was observed on several
occasions between 25 October, 1875, and 26 February, 1876, true Auroræ
were seldom observed; and the displays were so faint, and lasted so short
a time, and the spectrum observations led to such poor results, that no
special report has been considered necessary. Although the citron-line
was observed occasionally, on only two displays of the Aurora was it well
defined, and then for so short a time that no measure could be obtained.”
(For Sir George Nares’s further Report see Appendix C, containing
extracts from blue-book, ‘Results derived from the Arctic Expedition,
1875-6.’)

_Aurora Australis._

[Sidenote: Aurora Australis. Mr. Forster’s description. Long columns of
white light spreading over the whole sky.]

In an article on Auroræ in high Southern latitudes (Phil. Trans. No.
461, and vol. liv. No. 53), we find that Mr. Forster, who as naturalist
accompanied Capt. Cook on his second voyage round the world, says:—“On
February 17th, 1773, in south latitude 58°, a beautiful phenomenon was
observed during the preceding night, which appeared again this, and
several following nights. It consisted of long columns of a clear white
light shooting up from the horizon to the eastward almost to the zenith,
and gradually spreading over the whole southern part of the sky. These
columns were sometimes bent sideways at their upper extremities; and
though in most respects similar to the northern lights of our hemisphere,
yet differed from them in being always of a whitish colour, whereas ours
assume various tints, especially those of a fiery and purple hue. The sky
was generally clear when they appeared, and the air sharp and cold, the
thermometer standing at the freezing point.” This account agrees very
closely in particulars with Capt. Maclear’s notice of Aurora Australis
[after referred to], and especially in the marked absence of red Auroræ.

The height of the barometer does not appear to be mentioned, the
temperature being apparently much the same as in the more recent cases.

[Sidenote: Capt. Maclear’s Aurora Australis, 3rd March, 1874. Light of
pale yellow tint only.]

In a letter dated from H.M.S. ‘Challenger,’ North Atlantic, April 10th,
1876, Capt. Maclear was good enough to communicate to me some particulars
of an Aurora Australis seen 3rd March 1874, in lat. 54° S., long. 108° E.
The letter is mainly descriptive of the spectrum (which will be described
in connexion with the general question of the spectrum of the Aurora).
It states that the red line was looked for in vain, and that the light
appeared of a _pale yellow_, and had none of the rosy tint seen in the
northern displays.

[Sidenote: Capt. Maclear’s Auroræ described in ‘Nature.’]

Capt. Maclear has since contributed to ‘Nature,’ of 1st November 1877, a
description of four Auroræ seen from the ‘Challenger’ in high southern
latitudes (including the one communicated to me). He speaks of the
opportunity of observing as not frequent, either from the rarity of
the phenomena, or because the dense masses of cloud prevalent in those
regions prevented their being seen except when exceptionally bright.
There were four appearances described:—

[Sidenote: Feb. 9, 1874.]

(1.) At 1.30 on the morning of February 9th, 1874, preceded by a watery
sunset, lat. 57° S. and long. 75° E., bar. 29·0 in., ther. 35°; brilliant
streaks to the westward. Day broke afterwards with high cirrus clouds and
clear horizon.

[Sidenote: Feb. 21, 1874.]

(2.) At 9.30 P.M., February 21, 1874, lat. 64° S., long. 89° E., bar.
28·8 in., ther. 31°; one bright curved streamer. The Aurora preceded a
fine morning with cumulo-stratus clouds, extending from Jupiter (which
appeared to be near the focus) through Orion and almost as far beyond.
Under this a black cloud, with stars visible through it. Real cumuli
hid great part of the remainder of the sky, but there were two vertical
flashing rays which moved slowly to the right (west). Generally the
Aurora was still bright.

[Sidenote: March 3, 1874. Auroral line found in light to southward.]

(3.) At midnight, March 3rd, 1874, lat. 53° 30´ S., long. 109° E., bar.
29·1, ther. 36°, after some days’ stormy weather, a brilliant sunset,
followed by a fine morning. Soon after 8 P.M. the sky began to clear
and the moon shone out. Noticing the light to the southward to be
particularly bright, Capt. Maclear applied the spectroscope, and found
the distinguishing auroral line.

[Sidenote: Brilliant white clouds seen.]

About midnight the sky was almost clear, but south were two or three
brilliant light clouds, colour very white-yellow, shape cumulo-stratus.
From about west to near south extended a long feathery light of the same
colour, parallel with the horizon, and between south and west there
appeared occasionally brilliant small clouds. The upper edges seemed
hairy, and gave one the idea of a bright light behind a cloud. The forms
changed, but no particular order was noticed.

(Here follows a description of the spectrum, and the mode in which a
delineation by the lines was obtained.)

[Sidenote: March 6, 1874. Capt. Maclear suggests whether a low barometer
has to do with the absence of red.]

(4.) At 8 P.M., March 6th, 1874. This was a slight Aurora, seen to the
southward; after this the clouds changed to high cirrus. Capt. Maclear
suggests whether a low barometer has any thing to do with the absence
of red in the spectrum, the normal state of the barometer being an inch
lower in those regions than in more temperate latitudes.

[Sidenote: Barometer falls after the Aurora, and strong gale from the S.
or S.W. follows.]

Edin. Encyc. vol. iii. article “Aurora.” Dr. Kirwan observed that the
barometer commonly falls after the Aurora. Mr. Winn, in the seventy-third
volume of the Phil. Trans., makes the same remark, and says that in
twenty-three instances, without fail, a strong gale from the south or
south-west followed the appearance of an Aurora. If the Aurora were
bright, the gale came on within twenty-four hours, but was of no long
continuance; if the light was faint and dull, the gale was less violent,
longer in coming, and longer in duration.

[Sidenote: Pale yellow glow rare in the Aurora Borealis.]

The pale yellow-coloured glow referred to by Capt. Maclear is, in my
experience, rare in the Aurora Borealis. It is probably the “æqualiter et
sine eruptionibus aut radiis _fulvi_,” described by Seneca (_antè_, p.
1), and may probably belong to more southern climes.

[Sidenote: Spectrum of Auroræ Australes extends more into the violet.]

We shall see too, by-and-by, that these Auroræ Australes as to spectrum
extend more into the violet than the Aurora Borealis. The yellow, as
complementary to violet, is likely thus to make (in the absence of the
red) its appearance.

It is, however, somewhat singular that Carl Bock found almost exclusively
yellow Auroræ in Lapland.

In Proctor’s ‘Borderland of Science,’ article “The Antarctic Regions,”
we find quoted a passage from a letter by Capt. Howes, of the ‘Southern
Cross,’ in which a graphic description is given of a Southern Aurora:—

[Sidenote: Capt. Howes’s description of a Southern Aurora.]

“At about half-past one on the 2nd of last September the rare phenomenon
of the Aurora Australis manifested itself in a most magnificent manner.
Our ship was off Cape Horn, in a violent gale, plunging furiously into
a heavy sea, flooding her decks, and sometimes burying her whole bows
beneath the waves. The heavens were as black as death, not a star was to
be seen, when the brilliant spectacle first appeared.

[Sidenote: Balls of electric fire resting on mast-heads &c.]

“I cannot describe the awful grandeur of the scene; the heavens gradually
changed from murky blackness till they became like vivid fire, reflecting
a lurid glowing brilliancy over every thing. The ocean appeared like
a sea of vermilion lashed into fury by the storm, the waves dashing
furiously over our side, ever and anon rushed to leeward in crimson
torrents. Our whole ship—sails, spars, and all—seemed to partake of
the same ruddy hues. They were as if lighted up by some terrible
conflagration. Taking all together—the howling, shrieking storm, the
noble ship plunging fearlessly beneath the crimson-crested ways, the
furious squalls of hail, snow, and sleet, drifting over the vessel, and
falling to leeward in ruddy showers, the mysterious balls of electric
fire resting on our mast-heads, yard-arms, &c., and, above all, the awful
sublimity of the heavens, through which coruscations of auroral light
would shoot in spiral streaks, and with meteoric brilliancy,—there was
presented a scene of grandeur surpassing the wildest dreams of fancy.”

The foregoing picture presents a singular contrast to the yellow-white
Auroræ described as seen in high southern latitudes by Capt. Maclear,
and is interesting as a southern Aurora of a red or ruddy tint. Looking,
however, at the extreme rarity of red Auroræ in those latitudes, and
the description of “mysterious balls of electric fire resting on our
mast-heads, yard-arms, &c.” (a phenomenon not often noticed in connexion
with the Aurora), it suggests itself that the case in question may have
been an instance not of a true Aurora, but of an electric display, with
conditions approaching those experienced by travellers who have found
themselves in mountainous districts surrounded by storm-clouds charged
with electricity[6].

_Prof. Piazzi Smyth’s Typical Auroræ._

[Sidenote: Prof. Piazzi Smyth’s typical Auroræ.]

Prof. Piazzi Smyth was kind enough lately to send me the fourteenth
volume of the ‘Astronomical Observations made at the Royal Observatory,
Edinburgh, during the years 1870-1877.’ This volume, amongst its other
interesting matter, affords some valuable information on the subject
of the Aurora Borealis. The Aurora plates are five in number, three
comprising some well-executed chromo-lithographs of typical Auroræ, from
sketches made by Prof. Smyth, the other two plates being of the Aurora
spectrum. The Auroræ delineated are thus described:—

[Sidenote: Aug. 6, 1871, quiescent arc. August 21, 1871, active arc.]

Plate 5. (August 6, 1871.) An example of a mild quiescent kind of auroral
arc, with dark cavernous substratum. (August 21, 1871.) An example of a
bright large active arc darting out rays.

[Sidenote: Sept. 7, 1871, arc streamers and clouds. May 8, 1871, double
arc (longitudinal).]

Plate 6. (September 7, 1871.) An auroral arc, with streamers and dark
clouds, and maintaining a bright appearance though in proximity to
the moon. (May 8, 1871.) A double-arched auroral arc (the arches are
longitudinally arranged).

[Sidenote: April 28, 1871, multiple arc. Oct. 25, 1870, coloured Aurora.]

Plate 7. (April 28, 1871.) A multiple-arched arc of Aurora with
moonlight. (October 25, 1870.) A case of grandest coloured Auroræ, or
Aurora superb and almost universal.

All the foregoing drawings are very vivid and striking, and form a most
interesting set of typical forms of Auroræ.

According to my own experience, the Aurora with arches arranged
longitudinally, thus, [Illustration], is the rarest of all the forms. I
have not met with it myself, nor do I recollect an illustration of one
other than Prof. Smyth’s.

CHAPTER IV.

PHENOMENA SIMULATING AURORÆ.

_Auroric Lights (Kinahan)._

[Sidenote: Mr. Kinahan’s Auroric Lights. White and red. White light
appears in pencils radiating from a point.]

Mr. G. Henry Kinahan writes to ‘Nature,’ from Ovoca, under date January
27th, 1877, and speaks of two distinct kinds of light so classed—one
brilliant and transparent, of a white yellowish-blue or yellowish-red
colour, while the other is semi-opaque and of a bloody red colour, the
latter being considered in Ireland a forerunner of bad weather. The first
kind generally appears as intermittent pencils of light that suddenly
appear and disappear.

[Sidenote: Frequently not stationary, but jumping about.]

Usually they proceed or radiate from some point near the north of the
horizon; but Mr. Kinahan has frequently seen them break from a point in
the heavens, not stationary, but jumping about within certain limits.
Sometimes these lights occur as suddenly flashing clouds of light of a
white colour, but at other times of blue and reddish yellow.

[Sidenote: In daylight like sun-rays. Red light appears in clouds
floating upwards or diffused.]

If this class of lights is watched into daylight, they appear somewhat
like faint rays of a rising sun. One morning, while travelling in West
Galway in the twilight, they were very brilliant, and quite frightened
Mr. Kinahan’s car-driver, who thought the sun was going to rise in the
north instead of the east. The second, or bloody red light, usually
occurs in clouds floating in one direction up into the heavens, but often
diffused over a portion of the sky. Mr. Kinahan has never seen them
coming from the east, and on only a few occasions from the south, but
generally from the west, north-west, or north.

[Sidenote: Red light appears as dirty misty clouds in daylight, or as a
mist or misty rays.]

If both kinds of light appear at the same time, the second while passing
over the first dims it. If the second class is watched into daylight,
they appear as dirty misty clouds that suddenly form and disappear
without the spectator being able to say where they come from or where
they go to, or as a hazy mist over a portion of the sky, that suddenly
appears and disappears, or as misty rays proceeding from a point in the
horizon. Generally, when these clouds occur, there is a bank of black
clouds to the westward.

[Sidenote: Season since October 1876 prolific in auroric light.]

Mr. Kinahan then speaks of the season as having been prolific in auroric
light, as there had been few nights since the 1st October then last
(1876) in which they did not appear. On many occasions they were late
in the night, being very common and brilliant during the dark days of
December, a few hours before dawn (about 5 o’clock). Each time there was
a fine day they appeared also, and the weather broke again.

[Sidenote: Mr. J. Allan Broun questions nature of these lights, as Aurora
is seldom seen at 5 A.M. in this country. On 77 occasions seen only twice
so early. Season was of marked infrequency elsewhere.]

Mr. Jno. Allan Broun refers to this graphic account of Mr. Kinahan’s,
and concludes there must have been some mistake as to the nature of
these “auroric lights,” as the Aurora Borealis is very rarely seen at
5 A.M. in this country. In the years 1844 and 1845, during which the
Aurora was sought for at Makerstown every hour of the night, it was
observed in 77 nights on an average of nearly three hours each night;
but it was seen only twice so early, and that with a bright or brilliant
Aurora, which remained during five hours on the first occasion, and
from 6 P.M. to 6 A.M. on the second. Parts of the phenomenon seen by
Mr. Kinahan, Mr. Broun also could not say he had ever seen; and if Mr.
Kinahan’s observations could have been confirmed it would have been most
important, especially as made so frequently at the epoch of minimum.
The description is in many respects a sufficiently recognizable one of
auroral discharges; but the frequent appearance in early morning is
certainly unusual, and few if any Auroræ seem to have been recorded as
appearing elsewhere in Great Britain during the time which Mr. Kinahan
refers to as so prolific (see, however, Dr. Allnatt’s, _antè_, p. 24). In
fact, the season in question was one of marked infrequency (see English
Arctic Expedition Report, _antè_, p. 26). Mr. Buchan furnished Mr. Broun
with a note of Auroræ seen in the stations of the Scottish Meteorological
Society during the year 1876, and they were 42 in number, 26 in the first
half, and 16 in the second half of the year. The greater part were seen
in the most northerly stations, including the Orkney, Shetland, and Faroe
Islands, and only 9 south of the Forth.

_Luminous Arch._

[Sidenote: Luminous arch, Sept. 11, 1814. Height above horizon 6 to 9
miles.]

In the ‘Annals of Philosophy,’ vol. iv. p. 362, there is a minute
description of a luminous arch which appeared in the sky on the night
of Sunday, September 11th, 1814, and was seen in the west of England
opposite the Irish Sea, the west part of the south of Scotland, and part
of the west of Ireland. It was described as a part of either a body of
dense greyish-white light, or a mass of luminous matter in the shape of
an arch. Its height above the horizontal line was estimated at not more
than 9 nor less than 6 miles.

[Sidenote: It moved southward, and was assumed to differ from the Aurora.]

Its direction when first seen was N. 80° E., and S. 80° W. It moved to
the southward. It was assumed to differ from the Aurora Borealis in
wanting coruscations, and in its having a much paler light.

CHAPTER V.

SOME QUALITIES OF THE AURORA.

_Noises attending Auroræ._

[Sidenote: Noises attending Auroræ. Gmelin affirms them. Other testimony
to them. Musschenbroek. Cavallo. Nairne. Belknap.]

In the Edinb. Encyc., Gmelin is stated, in continuation of his
description of an Arctic Aurora, to add:—“For however fine the
illumination may be, it is attended, as I have heard from the relation of
many persons, with such a hissing, cracking, and rushing noise through
the air, as if the largest fireworks were playing off.” To describe what
they then heard, the natives are said to use the expression, “Spolochi
chodjat”—that is, The raging host is passing. The hunter’s dogs, too,
are also described as so much frightened when the Auroræ overtake the
hunters, that they will not move, but lie obstinately on the ground till
the noise has passed. This account of noises seems to be confirmed by
other testimony. They are stated to have been heard at Hudson’s Bay and
in Sweden; and Musschenbroek mentions that the Greenland whale-fishers
assured him they had frequently heard the noise of the Aurora Borealis,
but adds that “no person in Holland had ever experienced this
phenomenon.” Mr. Cavallo declares he “has repeatedly heard a crackling
sound proceeding from the Aurora Borealis” (Elements of Nat. or Exper.
Phil. vol. iii. p. 449). Mr. Nairne mentions that in Northampton, when
the northern lights were very bright, he is confident he perceived a
hissing or whizzing sound. Mr. Belknap of Dover, New Hampshire, North
America, testifies to a similar fact (American Trans. vol. ii. p. 196).

[Sidenote: Sir John Franklin negatives them.]

Sir John Franklin mentions, in his ‘Journey to the Shores of the Polar
Sea’:—“Nor could we distinguish its (the Aurora’s) rustling noise, of
which, however, such strong testimony has been given to us that no doubt
can remain of the fact.”

[Sidenote: March 11th. Hissing noise heard during Aurora’s passage.
Explained to arise from the snow.]

In detail, he mentions he never heard any sound that could be
unequivocally considered as originating in the Aurora, although he had
had an opportunity of observing that phenomenon for upwards of 200 nights
(the Aurora was registered at Bear Lake 343 times without any sound being
heard to attend its motions); but the uniform testimony of the natives
and all the older residents in the country induced him to believe that
its motions were sometimes audible. On the 11th March, at 10 P.M., a
body of Aurora rose N.N.W.; and after a mass had passed E. by S., the
remainder broke away in portions, which crossed about 40° of the sky
with great rapidity. A hissing noise, like that of a bullet passing
through the air, was heard, which seemed to proceed from the Aurora;
but Mr. Wentzel assured the party the noise was occasioned by severe
cold succeeding mild weather, and acting upon the surface of the snow
previously melted in the sun’s rays. A similar noise was heard the next
morning.

[Sidenote: Capt. Sabine also negatives noise.]

In Parry’s first voyage, Captain Sabine describes an Aurora seen at
Melville Island, and adds that the Aurora had the appearance of being
_very near_ the party, but _no sound could be heard_.

[Sidenote: Article “Aurora Polaris,” Encyc. Brit., suggests noises as not
improbable.]

In the article “Aurora Polaris,” Encyc. Brit, edition ix., the writer
admits the evidence of scientific Arctic voyagers having listened in vain
for such noises; but, referring to the statements of Greenlanders and
others on the subject, concludes there is no _à priori_ improbability
of such sounds being occasionally heard, since a somewhat similar sound
accompanies the brush-discharge of the electric machine.

[Sidenote: Payer negatives and discredits noises.]

Payer, of the Austrian Polar Expedition (1872-1874), states that the
Aurora was never accompanied by noise, and discredits the alleged
accounts of noises in the Shetlands and Siberia.

[Sidenote: As also Weyprecht.]

Lieut. Weyprecht, of the same expedition, says (_antè_, p.
14):—“Involuntarily we listen; such a spectacle must, we think, be
accompanied with sound, but unbroken silence prevails, not the least
sound strikes on the ear.”

[Sidenote: Herr Carl Bock negatives noises in the case of Lapland Auroræ.]

Herr Carl Bock, who accompanied the Laplanders visiting this country (at
the Westminster Aquarium) in 1877-78, and who witnessed many brilliant
auroral displays in Lapland, assured me he could trace no noise, except
on one occasion, when he heard a sort of rustling, which he attributed to
the wind. The Laplanders themselves did not associate any special noise
with the Aurora.

[Sidenote: Auroral noises in telephone. Ringing sound in vacuum-tube
under influence of magnet.]

It has been recently stated, in an article on the Telephone in ‘Nature,’
that Professor Peirce “has observed the most curious sounds produced
from a telephone in connexion with a telegraph wire during the Aurora
Borealis;” but no further details are given. In experimenting with a
silicic fluoride vacuum-tube between the poles of an electro-magnet, I
found, on the magnet being excited, that the capillary stream of blue
light was decreased in volume and brightness, and at the same time from
within the tube a peculiar whistling or slightly metallic ringing sound
was heard.

[Sidenote: Adverse conclusion as to noises accompanying Aurora.]

I certainly have never met with an instance of noise accompanying an
Aurora and traced to it. On the whole the balance of evidence seems quite
adverse to any proof of noises proper ordinarily accompanying an Aurora.

_Colours of the Aurora._

[Sidenote: Colours of the Aurora. Sir John Franklin’s views. Other
observers have described all colours of spectrum. Violet rare. Crimson
indicates coming Aurora.]

Sir John Franklin considered the colours in the Polar Aurora did not
depend on the presence of any luminary, but were generated by the motion
of the beams, and then only when that motion was rapid and the light
brilliant. The lower extremities, he says, quivered with a fiery red
colour, and the upper with orange. He also saw violet in the former.
Other observers have, in their various descriptions of Auroræ, mentioned
the colours of the rays or beams as red, crimson, green, yellow, &c.; in
fact, comprising the range of the spectrum. Violet seems less frequently
mentioned. The red or crimson colour is frequently the first indication
of the coming Aurora, and is usually seen on or near the horizon. The
colours have frequently been observed to shift or change.

[Sidenote: Prof. Piazzi Smyth describes colours of Aurora of Feb. 4,
1872, as seen at Edinburgh.]

Prof. Piazzi Smyth, in a letter to ‘Nature,’ describing the Aurora of
February 4th, 1872, as seen at Edinburgh, says that when the maximum
development was reached all the heavens were more or less covered with
pink ascending streamers, except towards the N., which was dark and
grey—first by means of a long low arch of blackness, transparent to large
stars, and then by the streamers which shot up from this arch, which were
green and grey only for several degrees of their height, and only became
pink as they neared the zenith. The red streamers varied from orange to
rose-pink, red rose, and damask rose.

The Professor pointed out that the spectroscope knew no variety of reds
giving one red line only, and attributed this to the mixing up of rays
and streamers of blackness out of the long low arch. When the Aurora
faded away a true starlight-night sky appeared; so that evidently the
dark arch and streamers were as much part of the Aurora as the green and
red lights.

[Sidenote: Dr. Allnatt at Frant describes vivid colours of same Aurora.]

Dr. Allnatt, at Frant, found in the case of the same Aurora the
south-western part of the heavens tinged by a bright crimson band.
A dark elliptical cloud extending from S. to S.E. was illuminated
at its upper edge with a pale yellow light, and sent up volumes of
carmine radii interspersed with green and the black alternating matter
characteristic of elemental electricity. Almost due E., and of about 25
degrees elevation, was a bright insulated spot of vivid emerald-green,
which appeared almost sufficiently intense to cast a faint shadow from
intercepting objects. At 7 o’clock the Aurora had passed the zenith, and
the sky presented a weird and wonderful appearance. A dark rugged cloud,
some 8 degrees E. of the zenith, was surrounded by electric light of all
hues—carmine, green, yellow, blood-red, white, and black; and the bright
spot still existed in the south.

[Sidenote: Descriptions at Blackburn and Cambridge. Lapland Auroræ
yellow.]

At Blackburn, in Lancashire, the rays were described as glowing in the
N.E. from silvery white to deepest crimson; and at Cambridge the same
Aurora was described as of a brilliant carmine tint. The Auroræ seen
in Lapland by Herr Carl Bock, were, he informed me, almost invariably
yellow; he saw only one red one.

[Sidenote: Hydrogen vacuum-tube suggestive of Aurora colours.]

The behaviour of a hydrogen Geissler vacuum-tube will be subsequently
referred to in the Chapter on the comparison of some tubes with the
Aurora spectrum, and is suggestive as to Aurora colours.

[Sidenote: Variation of tints in.]

The capillary part of this tube, when lighted by a small coil, was found
to vary in tint—silver-white, bright green, and crimson being each in
succession the dominant colour, according to the working of the break of
the coil. When a spectroscope was used, the red, blue, and violet lines
of the gas were seen to change in intensity in accordance with the light
colour seen in the tube.

[Sidenote: Variation of colour in nitrogen tube under influence of
magnet.]

A Geissler nitrogen vacuum-tube was also so arranged that the capillary
part of it should be vertically between the conical extremities of the
armatures of a large electro-magnet, the armatures just being clear of
the outside of the tube. The tube was then lighted up by a small coil,
and the magnet excited by four large double-plate bichromate cells.

[Sidenote: Change from rosy to violet hue.]

The stream of light was steady and brilliant, and, except at the violet
pole, of the rosy tint peculiar to a nitrogen vacuum-tube. On excitation
of the electro-magnet, the discharge was seen to diminish in volume, with
an apparent increase in impetuosity; and not only the capillary part, but
in a less degree the bulbs also of the tube, changed from a rosy to a
well-marked violet hue.

[Sidenote: Photographic plates taken. Difference in.]

We several times connected and disconnected the magnet with its
batteries, but always with the same result. Of the spectrum of the
capillary part of this tube we took photographic plates with quartz
prisms and lenses, taking care that all things should be as equal as
possible, the apparatus undisturbed, and the time of exposure exactly
the same. One plate was taken with the tube in its normal condition, the
other while it was under the influence of the magnet. The spectra were
identical, except that the plate of the tube influenced by the magnet
was decidedly the brightest, and was found to penetrate more into the
violet region (the Author’s ‘Photographed Spectra,’ p. 60, plate xxv.).
These plates effectually corroborated the change of colour, as the violet
ray would have more photographic effect than the rosy. The identity of
the spectra of the capillary part proved that the change in colour could
not have proceeded from an extension of the violet glow. (A similar
experiment will be found also detailed in Part III. Chapter XII.)

_Height of the Aurora._

[Sidenote: Height of Aurora. Sir John Franklin considers it within the
region of the clouds. At no great elevation.]

Sir John Franklin (Narrative of a Journey on the Shores of the Polar Sea
in the years 1819, ’20, ’21, ’22) says:—“My notes upon the appearance of
the Aurora coincide with those of Dr. Richardson in proving that that
phenomenon is frequently seated within the region of the clouds, and that
it is dependent in some degree upon the cloudy state of the atmosphere.”
And further:—“The observations of Dr. Richardson point particularly to
the Aurora being formed at no great elevation, and that it is dependent
upon certain other atmospheric phenomena, such as the formation of one or
other of the various modifications of cirro-stratus.”

Sir John Franklin also refers to notes from the Journal of Lieut. Robert
Hood, R.N., on an Aurora:—

[Sidenote: Observations of Lieut. Robert Hood and Dr. Richardson. A beam
not more than 7 miles from the earth. An arch 7 miles from the earth.]

The observations were made at Basquian House, and at the same time by Dr.
Richardson at Cumberland House, quadrants and chronometers having been
prepared for the purpose. On the 2nd April the altitude of a brilliant
beam was 10° 0´ 0″ at 10h 1m 0s at Cumberland House. Fifty-five miles
S.S.W. it was not visible. It was estimated that the beam was not more
than 7 miles from the earth, and 27 from Cumberland House. On the 6th
April the Aurora was for some hours in the zenith at that place, forming
a confused mass of flashes and beams; and in lat. 53° 22´ 48″ N., long.
103° 7´ 17″, it appeared in the form of an arch, stationary, about 9°
high, and bearing N. by E. It was therefore 7 miles from the earth.

[Sidenote: An arch between 6 and 7 miles from the earth.]

On the 7th April the Aurora was again in the zenith before 10 P.M. at
Cumberland House, and in lat. 53° 36´ 40″ N., long. 102° 31´ 41″. The
altitude of the highest of two concentric arches at 9h P.M. was 9°, at 9h
30m it was 11° 30´, and at 10h 0m 0s P.M. 15° 0´ 0″, its centre always
bearing N. by E. During this time it was between 6 and 7 miles from the
earth. [The bearings are true, not magnetic.]

[Sidenote: Sir John Franklin’s remarks.]

Sir J. Franklin says this was opposed to the general opinion of
meteorologists of that period: he also noticed he had sometimes seen an
attenuated Aurora flashing across the sky in a single second, with a
quickness of motion inconsistent with the height of 60 or 70 miles, the
least that had hitherto been ascribed to it.

[Sidenote: Dr. Richardson’s conclusions.]

The needle was most disturbed, February 13, 1821, P.M., at a time when
the Aurora was distinctly seen passing between a stratum of cloud and
the earth; and it was inferred from this and other appearances that the
distance of the Aurora from the earth varied on different nights. Dr.
Richardson concludes that his notes prove, independent of all theory,
that the Aurora is occasionally seated in a region of the air below a
species of cloud which is known to possess no altitude; and is inclined
to infer that the Aurora Borealis is constantly accompanied by, or
immediately precedes, the formation of one or other of the forms of
cirro-stratus.

[Sidenote: Captain Parry observed Auroræ near the Earth’s surface. Sir W.
R. Grove’s observation at Chester. Mr. Ladd’s observation at Margate. The
author’s observation at Kyle Akin, Skye.]

Captain Parry observed Auroræ near the earth’s surface; and records that
he and two companions saw a bright ray of the Aurora shoot down from the
general mass of light between him and the land, which was distant some
3000 yards. Sir W. R. Grove (‘Correlation of Physical Forces’) saw an
Aurora at Chester, when the flashes appeared close, so that gleams of
light continuous with the streamers were to be seen between him and the
houses—“he seemed to be in the Aurora.” Mr. Ladd, of Beak Street, Regent
Street, has related to me an appearance he was struck with, and examined
carefully. Standing in the evening in Margate Harbour, he saw a white ray
of the Aurora, which, apparently shooting downwards, was clearly placed
between his eye and the opposite head of the pier, which projected into
the sea. Mr. Ladd also informed me that Prof. Balfour assured him that
such an appearance was not unusual. In the double-arc Aurora seen by me
in the Isle of Skye, September 11, 1874 (described _antè_, p. 23), I had
a strong impression that the bow was near the earth, and thought that
the eastern end, and some fleecy clouds in which it was involved, were
between myself and the peaks of the distant mountains.

[Sidenote: Dalton’s calculation of 100 miles. Backhouse’s 50 to 100
miles. Prof. Newton, mean 130 miles. Elevation of Auroræ cannot exceed a
few miles.]

In the article “Aurora Polaris,” Encyc. Brit., edition ix., Dalton is
instanced as having calculated the height of an Aurora in the north of
England at 100 miles; and Backhouse as having made many calculations,
with the result of an average height of 50 to 100 miles. Prof. Newton,
too, is quoted for the height of 28 Auroræ (calculated by one observation
of altitude and amplitude of an arch) as ranging from 33 to 281 miles,
with a mean of 130 miles. It is, however, pointed out that a height of
62 miles above the earth’s surface would imply a vacuum attainable with
difficulty, even with the Sprengel pump. This difficulty is then met by a
reference to the observed altitude of some meteors, and to a suggestion
of Prof. Herschel’s that electric repulsion may carry air or other matter
up to a great height. Dr. Lardner (‘Museum of Science and Art,’ vol. x.
p. 192) speaks of the height of Auroræ as not certainly ascertained; but
considers them atmospheric phenomena scarcely above the region of the
clouds, and does not think it probable that their elevation in any case
can exceed a few miles.

[Sidenote: M’Clintock’s observations. Capt. Ross saw Auroræ on an
ice-cliff, which he attributed to electric action.]

M’Clintock, after noticing that the beams of the Aurora were most
frequently seen in the direction of open water, says that in some cases
patches of light could be plainly seen a few feet above a small mass of
vapour over an opening in the ice. Captain Ross, in his Antarctic voyage,
saw the bright line of the Aurora forming a range of vertical beams along
the top of an ice-cliff; and suggested this was produced by electrical
action taking place between the vaporous mist thrown upwards by the waves
against the berg, and the colder atmosphere with which the latter was
surrounded.

[Sidenote: Bergman estimates height as 468 miles.]

Bergman, from a mean of 30 computations, makes the height of the
phenomenon to be 72 Swedish (about 468 English) miles.

[Sidenote: Boscovich 825 miles.]

Father Boscovich calculated the height of an Aurora Borealis observed on
the 16th December, 1727, to have been 825 miles.

[Sidenote: Mairan 600 miles. Euler several thousand miles. Dr. Blagden
about 100 miles.]

Mairan supposed the far greater number of Auroræ to be at least 600
miles above the surface of the earth. Euler assigned them an elevation
of several thousands of miles. Dr. Blagden, however, limited their
height to about 100 miles, which he supposed to be the region of
fireballs—remarking that instances were upon record in which northern
lights had been seen to join and form luminous balls, darting about with
great velocity, and even leaving a train behind them like common meteors
(Phil. Trans. vol. lxxiv. p. 227).

[Sidenote: Dalton 150 miles.]

Mr. Dalton, from an observation of the luminous arches on a base of 22
miles, found the altitude of the Aurora to be about 150 miles (Dalton’s
‘Meteorological Observations and Essays,’ 1793, pp. 54, 153).

[Sidenote: Dr. Thompson assumes considerable height. His table. Average
of 31 observations, 500 miles.]

Dr. Thompson, ‘Annals of Philosophy,’ vol. iv. p. 429 (1814), assumes
that the height of the beams above the surface of the earth was much
greater than that of most other meteorological appearances, and gives
(p. 430) a table of Auroræ, mainly taken from Bergman, Opusc. v. p. 291,
of 31 Auroræ observed in the years 1621 to 1793, with heights in English
miles. The lowest is, 23rd February, 1784, London (Cavendish), 62 miles;
the highest, 23rd October, 1751, Fournerius, 1006 miles! The average
of the 31 estimated observations gives a height of about 500 miles. It
is not stated how these observations were obtained, though methods are
mentioned how they might be.

[Sidenote: Prof. Heis’s instrument for determining height of Auroræ.]

Prof. Heis, of Münster, exhibited at the recent Scientific Loan
Collection at South Kensington (‘Official Catalogue,’ 3rd edit. p. 296,
No. 1231) an instrument for the determination of the position of the
point of convergence of the rays of the Aurora, and for determining the
height of the Aurora. A ball resting in a pan was to be brought into
position, so that several diverging pencils of Aurora, when properly
viewed, were covered by the rod which passed through the centre of the
ball. The point of the rod (which could be moved up and down in the
ball), when the instrument was set to the astronomical meridian, showed
the azimuth and altitude of the converging point of the pencils of light.
This point of convergence does not coincide with the point to which the
inclination-needle directs. From the deviation of the two points, the
height of the Aurora could be calculated.

[Sidenote: Professor Newton’s method of calculating height.]

Professor H. A. Newton (Sil. Journ. of Science, 2nd ser. vol. xxix. p.
286) has proposed a method of calculating the height of Auroræ by one
observation of altitude and amplitude of an arch. It assumes that the
auroral arches are arcs of circles, of which the centre is the magnetic
axis of the earth, or at least that they are nearly parallel to the
earth’s surface, and probably also to the narrow belt or ring surrounding
the magnetic and astronomical poles. Professor Newton finds that, _d_
being the distance from the observer to the centre of curvature of the
nearest part of this belt (for England, situated about 75° N. lat., 50°
W. long.), _h_ the apparent altitude of the arch, 2_a_ its amplitude on
the horizon, _x_ its height, R the earth’s radius, and _c_ the distance
of the observer from the ends of the arch:—

  sin φ   = sin _d_ cos _a_ cosec(_d_ + _h_)   (1)
  tan _c_ = _z_ sin _h_ sin φ sec ²φ           (2)
  _x_     = R - (sec _c_ - 1)                  (3)

[Sidenote: Gave a height from 33 to 281 miles, and a mean of 130 miles.]

This method with 28 Auroræ gave a height from 33 to 281 miles and a mean
of 130 miles.

Galle has suggested (Pogg. Ann. cxlvi. p. 133) that the height of Auroræ
might be calculated from the amount of divergence between the apparent
altitude of the auroral corona and that indicated by the dipping-needle,
a principle which has been adopted in Prof. Heis’s apparatus before
described. The results do not differ materially from Professor Newton’s.

The conclusions to be arrived at from the foregoing instances and
opinions are certainly very puzzling. The terrestrial character of some
Auroræ seems well established. The height to which these phenomena _may_
ascend is left almost a matter of conjecture, and further observations
are very desirable.

_Phosphorescence._

[Sidenote: Phosphorescence. Phosphorescent bands. Storm-clouds which
threw out cirri. Shone with a sort of phosphorescence. Storm-cloud
surrounded by glories of a phosphorescent whiteness.]

In the voyage of the ‘Hansa’ (‘Recent Polar Voyages,’ p. 420), on the
9th September, 1869, at 10 P.M., Aurora gleams appeared in the west,
shooting towards the south. “Radiant sheaves and phosphorescent bands
mounted towards the zenith,” but the phantasmagoria quickly vanished.
M. Silbermann (‘Comptes Rendus,’ lxviii. p. 1120) mentions storm-clouds
which threw out tufts of cirri from their tops, which extended over
the sky, and resolved into, first, fine, and afterwards more abundant
rain. (I saw a fine day example of this on the Lago di Guarda, ending
in a copious discharge of rain attended with loud thunder and vivid
lightning.) Usually the fibres were sinuous; but in much rarer cases they
became perfectly rectilinear and surrounded the cloud like a glory, and
occasionally shone _with a sort of phosphorescence_. On the night of 6th
September, 1865, at 11 P.M., a stormy cloud was observed in the N.N.W.,
and lightning was seen in the dark cumulous mass. Around this mass
extended _glories of a phosphorescent whiteness_, which melted away into
the darkness of the starry sky. Round the cloud was a corona, and outside
this two fainter coronæ. After the cloud had sunk below the horizon the
glories were still visible.

[Sidenote: Sabine’s luminous cloud at Loch Scavaig, Skye. Other
observations of luminous clouds.]

Sabine mentions a cloud frequently enveloping Loch Scavaig, in Skye, as
being at night perfectly self-luminous, and that he saw rays, similar to
those of the Aurora, but produced in the cloud itself. Sabine also refers
to luminous clouds mentioned in Gilbert’s Annals, and to observations by
Beccaria, Deluc, the Abbé Rozier, Nicholson, and Colla; and to luminous
mists as observed by Dr. Verdeil at Lausanne in 1753, and by Dr. Robinson
in Ireland.

[Sidenote: Aurora at Melville Island.]

He also describes (Parry’s First Voyage) an Aurora seen at Melville
Island, and says the light was estimated as equal to that of the moon
when a week old. Besides the pale light, _which resembled the combustion
of phosphorus_, a slight tinge of red was noticed when the Aurora was
most vivid; but no other colours. This Aurora was repeatedly seen _on the
following day_.

[Sidenote: Procter suspects Aurora is formed in a mist. M’Clintock:
Aurora is never visible in a perfectly clear atmosphere.]

Mr Procter, in a letter to me, suspects that the Aurora is generally
formed in a sort of “mist or imperfect vapour;” and this mist or
imperfect vapour seems in many instances to form part of the Aurora, and
to partake of its self-luminous character. M’Clintock does not imagine
that the Aurora is ever visible in a perfectly clear atmosphere. He has
often observed it just silvering or rendering luminous the upper edge of
low fog or cloud-banks, and with a few vertical rays feebly vibrating.

[Sidenote: Aurora of Feb. 4, 1874. Illuminated fog-cloud. Capt.
Oliver’s meteor-cloud. Auroral display, 24th Oct., 1870. Streamers of
phosphorescent cloud.]

An instance of apparent phosphorescence is supplied by the Aurora of the
4th February, 1874 (_antè_), when a bright cloud of light was seen which
gave the impression of an “_illuminated fog-cloud_.” Captain S. P. Oliver
saw at Buncrana, Co. Donegal, on February 4, 1874, what he describes as
a meteor-cloud, viz. “a broad band of silvery white and luminous cloud.”
This appearance, as described by another correspondent, was evidently
an imperfectly formed (perhaps actually forming) Auroral arc. The great
Auroral display of the 24th of October, 1870, as seen by me, included,
according to my notes made at the time, “streamers of opaque white
phosphorescent cloud, very different from the more common transparent
Auroral diverging streams of light.”

[Sidenote: Aurora of Feb. 4, 1872, at Frant. Radii of phosphorescent
light.]

Describing the Aurora of February 4, 1872, at Frant, Dr. Allnatt
says:—“At a later hour of the night the canopy of cirro-stratus had
separated, and was transformed into luminous masses of radiant cumulus.
At 10.40 the Aurora reappeared in the N., and sent luminous radii of
white _phosphorescent_ light from the periphery of a segment of a
perfectly circular arch”[7].

[Sidenote: The author’s description of same Aurora. Masses of
phosphorescent vapour.]

Again, February 4th, 1872, as described by me, the first signs of the
Aurora were (in dull daylight) a lurid tinge upon the clouds, which
suggested the reflection of a distant fire; while scattered among
these, “torn and broken masses of white vapour having a phosphorescent
appearance” reminded me of a similar observation in October 1870.

[Sidenote: Day Auroræ must have a phosphorescent glow. Ångström considers
yellow-green line due to fluorescence or phosphorescence. Oxygen and some
of its compounds phosphorescent.]

The day Auroræ, which are elsewhere described, and are not very uncommon,
could, we may presume, hardly be seen without the presence of some
phosphorescent glow. Professor Ångström, in his Aurora Memoir (discussed
elsewhere), in discussing the yellow-green line, considers the only
probable explanation to be that it owes its origin to fluorescence
or phosphorescence. He says that some fluorescence is produced by
the ultra-violet rays; and adds, “an electric discharge may easily
be imagined, which, though in itself of feeble light, may be rich in
ultra-violet light, and therefore in a condition to cause a sufficiently
strong fluorescent light.” And he refers to the fact that oxygen and some
of its compounds are phosphorescent.

[Sidenote: A phosphoretted hydrogen spectrum-band is close to
yellow-green auroral line. Phosphorescent or fluorescent after-glow of
electric discharge.]

In the examination of certain spectra connected with the Aurora,
detailed in Part II., I have shown that the bright edge of one of the
phosphoretted hydrogen bands is in close proximity to the yellow-green
Auroral line. I have also referred to the peculiar brightening by
reduction of temperature of one of the bands in the red end of the
spectrum of phosphoretted hydrogen, so that from almost invisible
it became bright, and to the peculiar brightening of a line in the
yellow-green in certain “Aurora” and phosphorescent tubes. It has
also been observed that the electric discharge has a phosphorescent
or fluorescent after-glow (isolated, I believe, by Faraday). It seems
difficult to avoid in some way connecting all these circumstances with
the yellow-green line of the Aurora, if not also with the line in the red.

[Sidenote: Sorby’s experiments on fluorescence and absorption.
Bonelleine, spectrum of. Coloured layer of fungi. Spectrum of
_Oscillatoriæ_.]

Mr. Sorby, in his experiments on the connexion between fluorescence and
absorption (‘Monthly Microscopical Journal’), found in the spectrum of a
solution in alcohol of a strongly fluorescent substance called bonelleine
(the green colouring-matter found in the _Aurelia Bonellia-viridis_) two
bright bands, the one red and the other green, with centres respectively
at 6430 and 5880, and their limits towards the blue end at 6320 and 5820.
On adding an acid the red band changed its place to 6140. The superficial
membranous coloured layer of the fungi _Russula nitida_ and _vesca_ in
alcohol gave an absorption band with centre at 5540, while the spectrum
of fluorescence extended to 4400. A solution of _Oscillatoriæ_ in water
gave a spectrum of absorption with bands at 6200 and 5690; while the