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.
[Illustration]
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.