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diff --git a/old/56159-0.txt b/old/56159-0.txt deleted file mode 100644 index f22c5da..0000000 --- a/old/56159-0.txt +++ /dev/null @@ -1,11177 +0,0 @@ -Project Gutenberg's Auroræ: Their Characters and Spectra, by J. Rand Capron - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Auroræ: Their Characters and Spectra - -Author: J. Rand Capron - -Release Date: December 10, 2017 [EBook #56159] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK AURORAE: CHARACTERS, SPECTRA *** - - - - -Produced by Chris Curnow and the Online Distributed -Proofreading Team at http://www.pgdp.net (This file was -produced from images generously made available by The -Internet Archive) - - - - - - - - - - - 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. - - - - -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 -spectrum of fluorescence showed two bright bands having their centres at -6470 and 5800, and their limits towards the blue end at 6320 and 5710. - -[Sidenote: Sea phosphorescence, a continuous spectrum.] - -These instances of course cannot be connected with the Aurora except -as showing the spectrum region and lines of fluorescence. The sea -phosphorescence, according to Professor Piazzi Smyth, has a continuous -spectrum extending from somewhat below E to near F (Plate V. fig. 3). - -[Sidenote: Ångström finds the sky almost phosphorescent.] - -Ångström, on the occasion of the starry night when he found traces of -the green line in all parts of the heavens, speaks of the sky as being -“almost phosphorescent.” - -[Sidenote: Author of article in Encyc. Brit. suggests that the -phosphorescent or fluorescent light may be due to chemical action. -Herschel’s observation of phosphorescence in Geissler and “garland” -tubes.] - -The author of the Aurora article in the Encyc. Brit. suggests that the -phosphorescent or fluorescent light attributed to the Aurora may be -due to chemical action. He also questions Ångström’s assumption that -water-vapour is absent in the higher atmosphere, and thinks that it and -other bodies may, by electric repulsion, be carried above the level they -would attain by gravity. He then continues that if discharges take place -between the small sensible particles of water or ice in the form of cirri -(as Silbermann has shown to be likely) surface decomposition would ensue, -and it is highly probable the nascent gases would combine with emission -of light. He adds “that it has been almost proved that in the case of -hydrogen phosphide the very characteristic spectrum (light?) produced -by its combustion is due neither to the elements nor to the products of -combustion, but to some peculiar action at the instant of combination; -and it is quite possible that under such circumstances as above described -water might also give an entirely new spectrum.” Professor Herschel has -referred to the phosphorescent light which remains glowing in Geissler -tubes after the spark has passed, and to the fact that one of the globes -of a “garland” tube which was heated did not shine after the spark had -passed, apparently because of the action of heat on the ozone to which -the phosphorescence might be due. (See experiments on Mr. Browning’s -bulbed tube, Part III. Chap. XV.) - - -_Aurora and Ozone._ - -[Sidenote: Aurora and Ozone. Smells of sulphur during Auroræ attributed -to ozone.] - -Accounts are given by travellers in Norway of their being enveloped -in the Aurora, and perceiving a strong smell of sulphur, which was -attributed to the presence of ozone. M. Paul Rollier, the aëronaut, -descended on a mountain in Norway 1300 metres high, and saw brilliant -rays of the Aurora across a thin mist which glowed with a remarkable -light. To his astonishment, an incomprehensible muttering caught his ear; -when this ceased he perceived a very strong smell of sulphur, almost -suffocating him (‘Arctic Manual,’ p. 726). - -[Sidenote: Question whether the oxygen of the air may be changed into -ozone.] - -In the case of the Aurora, the question naturally arises whether the -oxygen of the air may be changed into ozone, perhaps also whether the -nitrogen may not be modified in some similar manner. - -[Sidenote: Ozone destroyed by heat.] - -The absorption spectra of oxygen, and of the same gas in its form of -ozone, may possibly differ; but this can hardly happen in the case of -incandescent oxygen, for ozone is at once destroyed by heat at 300°, and -slowly at 100°, and must be partially at least destroyed by the heat of -the discharge. If any lines were due to ozone in such a spectrum, we -should expect they would be weakened by heat and brightened by cold. - -[Sidenote: Ozone in a large bell-receiver not manifested in spectrum.] - -In the case of a continued discharge in a large exhausted bell-receiver, -the presence of ozone in considerable quantities was manifested to us by -its odour when the receiver was removed from the pump; but the spectrum -of the stream of light did not appear to differ from that in Geissler -tubes. - -[Sidenote: Professor Dewar demonstrates that ozone is condensed oxygen.] - -In a course of lectures at the Royal Institution in March 1878, on the -Chemistry of the Organic World, Prof. Dewar appears to have demonstrated, -by Prof. Andrews’ apparatus, that ozone is really condensed oxygen, and, -further, that during this condensation heat is absorbed, which is evolved -during the decomposition or re-expansion. - -[Sidenote: Refers to the silent discharge between the atmosphere and the -earth.] - -He also exhibited the oxidizing power of ozone in its action on mercury, -and commented on its similar action upon organic matter in forming -nitrates, and on its remarkable bleaching properties, but added there -was as yet no proof of its combining with free nitrogen. That peroxide -of hydrogen accompanies the formation of ozone by the slow combustion of -phosphorus, and that this peroxide acts with ozone in decomposing organic -bodies, though in an inexplicable manner, the Professor considered to be -proved. He also referred to the silent discharge probably perpetually -going on between the upper and lower strata of the atmosphere, and also -between these and the earth, accounting, as the Professor considered, for -some of the chemical actions whereby nitrogenous compounds are formed in -the soil. - -[Sidenote: No spectrum of ozone obtained.] - -As far as I am aware, no information as to a possible spectrum of ozone, -or a modification of the oxygen or other spectra by its presence, has, up -to the present time, been obtained[8]. - -[Sidenote: Suggestion to subject electric discharge to influence of cold.] - -It has been suggested by Mr. Procter and myself that the electric -discharge in an exhausted moist tube, if subjected to a considerable -degree of cold, might produce a modification of the air-spectrum, perhaps -even a spectrum analogous to that of the Aurora. - -For some further notes on this subject see Appendix D (Aurora and Ozone). - - -_Polarization of the Aurora Light._ - -[Sidenote: Polarization of the Aurora light. Mr. Ranyard found none.] - -In ‘Nature,’ vol. vii. p. 201, is contained an account of observations -of the polarization of the zodiacal light and of the Aurora, by Mr. A. -Cowper Ranyard, who, using both a double-image prism and a Savart on the -great Aurora of February 4th, 1872, detected no trace of polarization. He -also examined a smaller one of 10th November, 1871, with a like result. - -[Sidenote: Prof. Alexander found strong polarization in latitude 60°.] - -Mr. Fleming (who refers to these observations) remarks that the only -other account he had met with was contained in Prof. Stephen Alexander’s -Report on his Expedition to Labrador, given in Appendix 21 of the U.S. -Coast Survey Report for 1860, p. 30. Professor Alexander found strong -polarization with a Savart’s polariscope, and thought that the dark parts -of the Aurora gave the strongest polarization. This was in latitude -about 60°, at the beginning of July, and near midnight. It is not stated -whether there was twilight or air-polarization at the time, nor is the -plane of polarization given. - -[Sidenote: Mr. Shroeder found no polarization.] - -The question naturally arises, especially as the darkest parts of the -Aurora are usually situated low down near the horizon, whether the -polarization in the latter case did not proceed from the atmosphere and -not from the Aurora itself. Mr. Shroeder found no traces of polarization -in the Aurora of February 4th, 1872. Further examinations of the Aurora -with some delicate form of polariscope would seem very desirable. - -[Sidenote: Polarization not found in the zodiacal light; except faint -traces by Mr. Burton.] - -The evidence of polarization in the case of the zodiacal light seems also -almost entirely negative—Mr. Ranyard pointing out observations of his -own, of Captain Tupman, and of Mr. Lockyer with this result. Mr. Burton, -using a Savart set so as to give a black centre when the bands were -parallel to the plane of polarization, believed he detected faint traces -of polarization in the brightest parts of the zodiacal light (as seen in -Sicily), the bands being black-centred when their direction coincided -with the axis of the cone of light. Mr. Burton saw no trace of bands when -examining the slight remaining twilight apart from the zodiacal light. -Mr. Ranyard was not able to confirm Mr. Burton’s observations on the same -evening and with the same instrument. - - -_Number of Auroræ._ - -[Sidenote: Number of Auroræ. Sir John Franklin’s observations.] - -Sir John Franklin saw in the Arctic Regions, in the years 1819, 1820, -1821, 1822:—In the month of September two Auroræ, in October three, in -November three, in December two, in January five, in February seven, in -March sixteen, in April fifteen, and in May eleven. - -[Sidenote: Periodicity as to days not established.] - -Periodicity as to days seems to have no certain law; and though certain -days in February and March are marked as those of fine returning -displays, they must be looked on as accidental. - -[Sidenote: Maxima and minima.] - -Two well-marked annual maxima seem to occur in March and October (the -latter the greater), and two minima in June and January, the greater in -June (Encyc. Brit.). The 4th of February, 1872, and same day 1874, are, -however, curious instances of a recurring remarkable display. - -[Sidenote: Kæmtz’s table.] - -A table by Kæmtz, showing the number of Auroras in each month of the -year, with the maxima and minima as above stated, will be found on Plate -V. fig. 5. - -[Sidenote: Dr. Hayes’s observations in winter of 1860-61.] - -Dr. Hayes has observed that in the winter of 1860-61 (when the ten or -eleven years’ inequality was at its maximum) only three Auroræ were seen -and recorded, and they were feeble and short in duration. - -[Sidenote: Captain Maguire’s observations at Point Barrow as to number -and time of appearances.] - -Captain Maguire, at Point Barrow (1852-54), reports that the Aurora was -seen six days out of seven, and on 1079 occasions, being nearly one third -of the hourly observations. It was seldom seen between 9 A.M. and 5 P.M., -not at all between 10 A.M. and 4 P.M. It increased regularly and rapidly -from 5 P.M. until 1 A.M., and then diminished in the same way until 9 A.M. - -The winters of 1877 and 1878 and the springs of 1878 and 1879 have been -singularly deficient in Auroræ. I have seen none at Guildown. - - -_Duration of Aurora._ - -[Sidenote: Duration of Aurora. Sometimes a few minutes; at other times -the whole night or even days.] - -In the article in the ‘Edinb. Encyc.’ before referred to some remarks -are made on the duration of the Aurora. Sometimes it is formed and -disappears in the course of a few minutes. At other times it lasts for -hours or during the whole night, or even for two or three days together. -Musschenbroek observed one in 1734 which he considered to have lasted ten -days and nights successively, and another in 1735 which lasted from the -22nd to the 31st March. - -[Sidenote: Auroræ may run on into the day without being noticed.] - -With respect to Captain Maguire’s observations (_antè_) it may be -remarked that Auroræ may doubtless frequently run on into and through the -day without their being noticed (instances, however, are known of Auroræ -seen in daylight); and hence it is difficult to judge of the limit of -duration of a particular Aurora unless indications are sought for during -the day (by the shapes of clouds, action of the magnet, &c.) as well as -during the night. Probably Auroræ seen during successive nights may be -parts of a continuous discharge. - - -_The Travelling of Auroræ._ - -[Sidenote: Travelling of Auroræ. Donati’s investigations.] - -Donati undertook to study the Aurora with reference to the mode of its -extension; and he arrived at the result that the Aurora of February 4, -1872, was not observed in different regions of the earth in the same -physical moment; _but everywhere at the same local hour_, as in the case -of celestial phenomena, which do not share in the earth’s rotation. - -[Sidenote: Questions sent to Italian Consuls.] - -The Minister of Foreign Affairs sent a circular to all Italian Consuls, -asking them the necessary questions; and in reply received reports from -forty-two places in our hemisphere and from four in the southern, the -places embracing in one latitude the considerable extent of 240 degrees -of longitude. - -An epitome of the tables (in which the results are divided into three -zones) is as follows:— - -[Sidenote: Table of results.] - - +---------+--------------------+----------+------------+------------+ - | Zone. | Mean longitude | No. of | Mean hour | Mean hour | - | | of zone. | stations.| of maximum.| of end. | - +---------+--------------------+----------+------------+------------+ - | Eastern | 2 hrs. 5 mins. E. | 9 | 9½ hrs. | 12¼ hrs. | - | Middle | 0 hr. 20 mins. E. | 17 | 8½ hrs. | 11½ hrs. | - | Western | 5 hrs. 38 mins. W. | 13 | 8¾ hrs. | 9¾ hrs. | - +---------+--------------------+----------+------------+------------+ - -[Sidenote: Extensions of the Aurora. The Aurora passed through four -periods. First period of origin, light weak. Second period, increase of -intensity. Third period, continuous brightness. Fourth period, decrease.] - -Donati summed up the facts:—That the light phenomena of this Aurora began -to show themselves in the extreme east of the southern hemisphere in -Eden and Melbourne; shortly after, they were observed in the east of our -hemisphere in China (but not in Japan); from China the Aurora passed over -the whole of Asia and Europe, and crossed the Atlantic and the American -Continent as far as California. It was invisible in Central and South -America. During these immense extensions it passed through four periods. -In the first (called by Donati the period of origin) the light of the -Aurora was pretty weak, and spread from Shanghai to Bombay; in the second -period, during which it passed on from Bombay to Taganrog, it acquired a -sudden increase of intensity; in the third period (called by Donati the -normal) the Aurora passed over Europe from east to west with regularity -and a continuous brightness; the fourth period, that of decrease, -was observed in America. The Aurora had a tendency to end earlier in -reference to the local hour in the western stations than in the eastern. -The acceleration on an average of the end of the phenomenon was twenty -minutes for every hour of longitude. - -[Sidenote: Donati’s conclusions. Explanation of mode of propagation of -same Aurora.] - -Donati concluded that these facts were not reconcilable with the theory -of the Aurora depending on meteorological and electro-magnetic phenomena -of the globe. Since, too, we have not a yearly, but a ten-yearly period -of the Aurora, which coincides with that of sun-spots and terrestrial -magnetism, Donati supposed that the cosmic causes of the polar lights -were electro-magnetic currents between the sun and the earth. This would -explain the mode of propagation of the Aurora of 4th February. Conceive -an electric current going from the earth to the sun, or _vice versâ_; -certain phenomena of the Aurora could only be observed in those parts -of the atmosphere which have a determinate position or direction with -reference to this current; and consequently these phenomena would be -successively visible on the different meridians, as these meridians, -by reason of the earth’s rotation, assume the same position to the -current. For the Aurora to be visible certain meteorological and telluric -circumstances must, however, doubtless work together with the cosmical -cause. - - -_Geographical Distribution of Auroræ (Fritz and Loomis)._ - -[Sidenote: Geographical Distribution of Auroræ. Prof. Fritz’s and Prof. -Loomis’s line of frequency.] - -Professors Fritz and Loomis have investigated this subject; and -Petermann’s ‘Mittheilungen,’ vol. xx. (1874), contains a paper by the -former, from which it appears that the northern limit of Auroræ chosen -by Professor Loomis nearly coincided, except in England, with a line of -frequency in Professor Fritz’s paper. This line nearly passes through -Toronto, Manchester, and St. Petersburg. Professor Loomis places it -as far north as Edinburgh. On a line across Behring’s Straits, and -coming down below 60° N. in America and the Atlantic, and just north of -the Hebrides, to Dröntheim, and including the most northern points of -Siberia, the frequency is represented by 100. - -[Sidenote: Within this another zone of greatest frequency and intensity.] - -Within this is another zone of greatest frequency and intensity, which -passes just south of Point Barrow, in lat. 72° N., on the northern coast -of America, and by the Great Bear Lake to Hudson’s Bay, where it reaches -a latitude of 60°, then on to Nain, on the coast of Labrador, and to the -south of Cape Farewell; then bending sharper to the northward, it passes -between Iceland and the Faroe Islands, near to the North Cape, on by -the northern ice-sea to Nova-Zembla and Cape Tschejuskin, and on just -to the north of the Siberian coast to the south of Kellett Land, thence -returning to Point Barrow. - -[Sidenote: Lines on which annually nearly the same number of Auroræ are -seen.] - -More or less parallel with this line are the lines on which annually -nearly the same number of Auroræ are seen. The line for one Aurora -annually went from Bordeaux, through Switzerland, past Krakau, south of -Moscow and Tobolsk, to the northern end of Lake Baikal, on to the Sea of -Ochotsk and to the Southern Aleutes, thence through Northern California -to the mouth of the Mississippi and to Bordeaux. The line for five Auroræ -annually went from Brest through Belgium, Stettin, Wologda, between -Tobolsk and Beresow, parallel to the previous line to Ochotsk, and on to -Brest, &c. Almost exactly with the line of greatest frequency coincides -the line forming the boundary of the direction of visibility of the -Northern Light towards the Pole or towards the Equator; while northwards -of this line the Polar Light is seen in the direction towards the -Equator; and from all stations the Northern Lights are seen in directions -which are pretty much normal to that curve and the entire system of -isochasms. - -[Sidenote: Assumed connexion between Aurora and ice-formation.] - -Professor Fritz has remarked that the curves of greater frequency tend -towards the region of atmospheric pressure, and also that they bear some -relation to the limit of perpetual ice—tending most southward where, as -in North America, the ice limit comes further south. He also endeavours -to show a connexion between the periods of maximum of Auroræ and those -of ice-formation, and considers ice to be an important local cause -influencing their distribution. These being most frequently seen over -open water in the Arctic regions, has been referred to as noticed by -Franklin and others. - - -_Extent and principal Zone of the Aurora._ - -[Sidenote: Extent and principal zone of Aurora. M. Moberg’s Finland -observations (1846-55) compared by Prof. Fritz with those in other -regions.] - -The Finland observations, published by M. Moberg in his ‘Polarlichter -Katalogue’ of Northern Lights in the years 1846-55, numbering 1100, have -been compared by Prof. Fritz, in his paper in the ‘Wochenschrift für -Astronomie,’ with the auroral phenomena of the same period in all other -regions. The Table shows that of 2035 days of the months August to April -on which Northern Lights were seen, 1107 days were those of Northern -Lights for Finland. On 794 they were visible simultaneously in America, -and mostly also in Europe; on 101 days in Europe only, and on 212 days -in Finland only. On 958 days Northern Lights were visible in Europe and -America which were not visible in Finland. All these numbers refer only -to the months August to April, as in the remaining months the brightness -of the night in Finland makes such observations impossible. - -The conclusion is arrived at that a large portion of Auroræ have no very -great extension, or that the causes producing the phenomena must often be -of a very local character; while in another portion of the phenomena the -extent, or the regions of simultaneous appearance are very considerable. - -[Sidenote: Number limited to Finland only small.] - -The number limited to Finland, for which hitherto corresponding -observations from other lands are wanting, is very small—212, or 19 per -cent. of the whole number seen in Finland. With the increase of frequency -of the phenomena at the time of maximum, the number observed in Finland -and America on the same day increases; while those observed in Finland -and Europe only, or in Finland only, decreased, in accordance with the -known law that with the frequency the intensity and extent also increase. - -[Sidenote: One third of Auroræ seen in America and Europe simultaneously.] - -Between 1826 and 1855, of 2878 days on which, in America, the Northern -Lights were seen, there were 1065 on which they were also visible in -Europe; so that at least every third day of Auroræ was common to both -these portions of the globe. In the years 1846 to 1855, and 1868 to 1872, -there were in the first period 657 Northern-Light days common to America -and Europe out of 1691, and in the second 397 out of 715. - -[Sidenote: Local occurrence of the Aurora not in favour of its assumed -cosmical nature.] - -The comparison by Prof. Fritz of M. Moberg’s Finland observations has -been lately reviewed in ‘Nature’ (March 8, 1878) and the result arrived -at that, “After ten years, in spite of the vastly accumulated material of -careful observations, there appears no necessity to change Herr Fritz’s -system of curves in any essential detail; indeed certain parts of the -same, which were at first only based on probability and supposition -(the part of the principal zone between the north of Norway and Nishen -Kolynisk as an instance), we now know with perfect certainty to be -correct.” It has been remarked that the local occurrence of Auroræ is -not in accordance with the hypothesis of the phenomenon being one of a -cosmical nature. - -The winter of 1870 was remarkable for brilliant displays; and the -displays of October 24th and 25th, 1870, were remarkably brilliant in -England and in America also, and the Aurora Australis was seen on the -same days at Madras. These displays were seen in England and America -in the daytime as patches or coronæ of white light, with streamers -stretching upward from them. - - - - -CHAPTER VI. - -THE AURORA IN CONNEXION WITH OTHER PHENOMENA. - - -_Auroræ and Clouds._ - -[Sidenote: Auroræ and clouds. Dr. Richardson’s observations. Aurora -constantly accompanied by or immediately precedes the formation of -cirro-stratus.] - -Dr. Richardson (‘Sir John Franklin’s Narrative’), so long ago as the -years 1819-1822, made many recorded observations on the connexion of -clouds with the Aurora Borealis in the Polar regions. Some of these are -alluded to in Chapter V., section “Height of the Aurora,” for the purpose -of showing the moderate distance he found it to be above the earth; and -his inference is there mentioned, “that the Aurora Borealis is constantly -accompanied by or immediately precedes the formation of one or other -of the various kinds of cirro-stratus.” On the 13th November and 18th -December, 1820, the connexion of an Aurora with a cloud intermediate -between cirrus and cirro-stratus is mentioned. It is, however, also -mentioned that the most vivid coruscations of the Aurora were observed -when there were only a few attenuated shoots of cirro-stratus floating in -the air, or when that cloud was so rare that its existence was only known -by the production of a halo round the moon. (An instance of attenuated -streaks of cirro-stratus in connexion with an auroral arc will be found -in the Aurora seen at Guildown on the 4th February 1874, a sketch of -which is reproduced on Plate VI. fig. 1.) - -[Sidenote: Polarity discerned in cirro-stratus clouds.] - -Dr. Richardson goes on to express his opinion that he, on some occasions, -discerned a polarity in the masses of clouds belonging to a certain kind -of cirro-stratus (approaching cirrus), by which their long diameters, -having all the same direction, were made to cross the magnetic meridian -nearly at right angles. - -[Sidenote: Apparent polarity of Aurora might perhaps be ascribed to the -clouds themselves.] - -Dr. Richardson further suggests that if it should be thereafter proved -that the Aurora depends upon the existence of certain clouds, its -apparent polarity might perhaps be ascribed to the clouds themselves -which emit the light; or, in other words, the clouds might assume their -peculiar arrangement through the operation of one cause (magnetism, for -instance), while the emission of light might be produced by another—a -change in their internal constitution perhaps connected with a motion of -the electric fluid. - -Dr. Richardson further remarks that, generally speaking, the Aurora -appeared in small detached masses for some time before it assumed that -convergency towards the opposite parts of the horizon which produced the -arched form. - -[Sidenote: Sir John Franklin’s observations.] - -Sir John Franklin says in his Polar expeditions he often perceived the -clouds in the daytime disposed in streams and arches such as the Aurora -assumes. - -[Sidenote: Dr. Low’s.] - -Dr. Low (‘Nature,’ iv. p. 121) considers he witnessed a complete display -of auroral motions in cirrus cloud, and considers all clouds subject -to magnetic or diamagnetic polarization; he states that when the lines -converge towards the magnetic pole fine weather follows, and when at -right angles to it wet and stormy. - -[Sidenote: M. Silbermann’s observations, 15th April, 1869. Cirrus clouds -took the place of the Aurora.] - -In the Encyc. Brit. edition 9, article “Aurora Polaris,” after referring -to the evidence of Franklin, Richardson, and Low, M. Silbermann (‘Comptes -Rendus,’ lxviii. p. 1051) is quoted in detail for observed connexion -between the Aurora and cirrus cloud. 15th April, 1869, at 11h 16m, an -Aurora appeared and disappeared; but it seemed as if the columns were -still visible, and it soon became obvious that fan-like cirrus clouds, -with their point of divergence in the north, had taken the place of the -Aurora. Between 1 and 2 A.M. the clouds had passed the zenith, and let -fall a little fine frozen rain. At 4 A.M. the cirrus of the false Aurora -was still visible, but deformed towards the top, and presenting a flaky -aspect. The cirrus never appeared to replace the Aurora either from right -or left, but to substitute itself for it like the changes of a dioramic -view. - -[Sidenote: Payer thinks the transition of Aurora into clouds not proved.] - -Payer, in his ‘Austrian Arctic Voyages,’ thinks that the occurrence of -the Aurora during the day (i. e. _light clouds with its characteristic -movement_) had been rather imagined than actually observed, and that the -transition of white clouds into auroral forms at night has never been -satisfactorily proved. He, however, mentions the mist-like appearance of -the Aurora. - -[Sidenote: Dr. Allnatt’s observations, 4th February, 1872, at Frant. -Aurora passed into cirro-stratus.] - -Dr. Allnatt observed the splendid Aurora of 4th February, 1872, at -Frant, and noticed the weird and wonderful appearance of the phenomena. -At 6 P.M. the Aurora commenced by the S.W. portion of the heavens being -tinged with a bright carmine hue, and in a short time the whole visible -hemisphere was lighted up. A dark elliptical cloud extending from S. to -S.E. and S.W. sent up volumes of coloured radii. At 7 the Aurora had -passed the zenith, and a dark, broken, rugged cloud some 8° E. of zenith -was surrounded by electric light of all hues. At 7.40 the Aurora began to -wane, and passed into a homogeneous cirro-stratus of sufficient density -to obscure the stars, disappearing at 7.45. - -[Sidenote: Later, cirro-stratus was transformed into luminous cumulus.] - -At a later hour of the night the canopy of cirro-stratus had separated -and was transformed into luminous masses of radiant cumulus; so that, as -Dr. Allnatt observes, there were called in requisition almost all the -most prominent cloud-modifications during the progress of the phenomena. -The succession of formation, transformation, and reformation from Aurora -to cloud and from cloud to Aurora was, Dr. Allnatt concluded, conclusive -of the theory before advanced of the electric origin of the recurrent -rayed cloud-modifications in the place of the magnetic meridian, over -which so much mystery had been cast. - - -_Aurora and Thunder-storms._ - -[Sidenote: Aurora and thunder-storms. Silbermann’s theory.] - -Silbermann asserts that Auroræ are produced by the same general phenomena -as thunder-storms, and concludes that the Auroræ of 1859 and 1869 assumed -the character of thunder-storms which, instead of bursting in thunder, -had been drawn into the upper parts of the atmosphere, and their vapour -being crystallized in tiny prisms by the intense cold, the electricity -became luminous in flowing over these icy particles. - -[Sidenote: Prof. Piazzi Smyth on monthly frequency of Auroræ and storms.] - -Professor Piazzi Smyth has observed that the monthly frequency of Auroræ -varies inversely with that of thunder-storms. His Table of comparisons is -as follows:— - -[Sidenote: His table of observations.] - - Month. Lightning. Auroræ. - - January 24·0 29·7 - February 14·4 42·5 - March 7·0 35·0 - April 15·4 27·5 - May 37·4 4·8 - June 48·0 0·0 - July 55·2 0·5 - August 38·4 12·6 - September 22·4 36·6 - October 20·8 49·4 - November 15·0 32·4 - December 15·0 28·8 - ---- ---- - Mean of whole year 24·0 20·1 - -[Sidenote: Silbermann’s observations 15th April, 1869. 30th April, 1865.] - -Silbermann, on 15th April, 1869, observed a fall of rain (tiny crystals -of ice) on the disappearance of an Aurora and its change into cloud -forms (see section, “Auroræ and Clouds,” p. 53). He also observed a rain -of little sparkling ice-prisms on 30th April, 1865, at Paris, the city -being then enveloped in a cirrus of vertical fibres similar to that which -frequently accompanies the Aurora. - -On the occasion of the Aurora seen by me at Guildown, 4th February, 1872, -rain fell immediately succeeding the formation of the corona. - -The falling of rain as an immediate sequence of an Aurora seems, however, -to be rather the exception than the rule; but possibly this may vary with -the character of the Aurora itself—whether it be of the crimson class, -passing into cloud and accompanied with much electric disturbance, or of -the more quiescent white. - -[Sidenote: A falling barometer observed to follow Auroræ.] - -A falling barometer following a display of Auroræ has been noticed by Sir -John Franklin and others; and in some cases (notably one in Sicily before -referred to) storms and floods have accompanied this. - -[Sidenote: Professor Christison’s observations.] - -In a paper read before the Royal Society of Edinburgh in 1868, Prof. -Christison mentioned, as a fact of importance to agriculturists, that the -first great Aurora after autumn is well advanced, and following a period -of fine weather, is a sign of a great storm of rain and wind in the -forenoon of the second day afterwards. - -Mr. C. L. Prince, in his ‘Climate of Uckfield,’ p. 218, remarks that -displays of Auroræ are almost invariably followed by very stormy weather, -after an interval of from 10 to 14 days. - - -_Aurora and the Magnetic Needle._ - -[Sidenote: Aurora and the magnetic needle. Sir John Franklin’s -observations. Motion communicated to the needle was neither sudden -nor vibratory. Return of needle to its former position very gradual. -Different positions of the Aurora had considerable influence on the -direction of the needle. Needle disturbed when Aurora not visible. -Quiescent yellow Aurora produced no perceptible effect on needle. Return -of needle more speedy after formation of a second arch. Slow when -disturbance was considerable.] - -Sir John Franklin, in his ‘Narrative’ (before referred to), gives -Lieutenant Robert Hood, R.N., the credit of being “the first who -satisfactorily proved, by his observations at Cumberland House (before -mentioned), the important fact of the action of the Aurora upon the -compass-needle,” and also “to have proved the Aurora to be an electrical -phenomenon, or at least that it induces a certain unusual state of -electricity in the atmosphere.” Sir John Franklin then mentions that -the motion communicated to the needle was neither sudden nor vibratory. -Sometimes it was simultaneous with the formation of arches, prolongation -of beams, or certain other changes of form or of activity of the Aurora. -But generally the effect of these phenomena upon the needle was not -visible immediately; but in about half an hour or an hour the needle had -obtained its maximum of deviation. From this its return to its former -position was very gradual, seldom regaining it before the following -morning, and frequently not until the afternoon, unless it was expedited -by another arch of the Aurora operating in a direction different from -the former one. The magnetic needle in the open air was disturbed by the -Aurora whenever it approached the zenith. Its motion was not vibratory -(as observed by Mr. Dalton), perhaps owing to the weight of the card. -It moved slowly to the E. or W. of the magnetic meridian, and seldom -recovered its original direction in less than eight or nine hours. The -greatest extent of its aberration was 45´. The arches of the Aurora were -remarked commonly to traverse the sky nearly at right angles to the -magnetic meridian; but deviation was not rare, and it was considered -that the different positions of the Aurora had considerable influence on -the direction of the needle. When an arch was nearly at right angles to -the magnetic meridian, the motion of the needle was towards the W. This -motion was greater when the extremity of the arch approached from the -west towards the magnetic north. A westerly motion also took place when -the extremity of an arch was in the true north, or about 36° to the west -of the magnetic north. The motion of the needle was towards the east -when the same end of an arch originated to the southward of the magnetic -west, and when of course its opposite extremity approached nearer to -the magnetic north. In one case only a complete arch was formed in the -magnetic meridian. In another the beam shot up from the magnetic north to -the zenith. In both these cases the needle moved towards the west. The -needle was most disturbed on February 13, 1821, at a time when an Aurora -was distinctly seen passing between a stratum of clouds and the earth. -Sometimes the needle deviated though no Aurora was visible; but it was -uncertain whether there might not have been a concealed Aurora at the -time. Clouds were sometimes observed during the day to assume the form of -the Aurora, and deviations of the needle were occasionally remarked at -such times. An Aurora sometimes approached the zenith without producing -any change of position of the needle; while at other times a considerable -alteration took place, though the beams or arches did not come near the -zenith. The Aurora was frequently seen without producing a perceptible -effect on the needle. At such times it was generally an arch or a -horizontal stream of dense yellowish light with little or no internal -motion. The disturbance of the needle was not always proportionate to the -agitation of the Aurora, but was always greater when the quick motion -and vivid light were observed to take place in a hazy atmosphere. In a -few instances the needle commenced at the instant a beam started from -the horizon upwards; and its return was according to circumstances. If -an arch formed immediately afterwards, having its extremities placed on -opposite sides of the magnetic north and south to the former one, the -return of the needle was more speedy, and it generally went beyond the -point from which it first started. When the disturbance was considerable, -it seldom regained its usual position before 3 or 4 P.M. on the following -day. On one occasion only the needle had a quick vibratory motion -(between 343° 50´ and 344° 40´). The disturbance produced by the Aurora -was so great that no accurate deductions as to diurnal variation could be -made. - -[Sidenote: Magnetic observations on board the ‘Tegetchoff.’] - -Payer, in his ‘New Lands within the Arctic Circle’ (vol. i. pp. 327, -328), gives the result of the magnetic observations on board the Austrian -ship ‘Tegetchoff’ in the years 1872-74, made by means of a magnetic -theodolite, a dipping-needle, and three variation instruments. The -extraordinary disturbances of the needle rendered the determination of -exact mean values for the magnetic constants impossible. The following -were the principal results of these observations:— - -[Sidenote: Disturbances great.] - -(1) The magnetic disturbances were of extraordinary magnitude and -frequency. - -[Sidenote: Greater as the rays were rapid. Quiescent arches exercised no -influence.] - -(2) They were closely connected with the Aurora, and they were greater -as the motion of the rays was more rapid and fitful and the prismatic -colours more intense. Quiescent and regular arches, without changing rays -or streamers, exercised mostly no influence on the needle. - -[Sidenote: Declination-needle, effects on.] - -(3) In all the disturbances the declination-needle moved towards the -east, and the horizontal intensity decreased while the inclination -increased. - -Sir John Franklin sums up his information as to the needle to much -the same effect, viz. that brilliant and active coruscations cause a -deflection almost invariably if they appear through a hazy atmosphere -and if the prismatic colours are exhibited in the beams or arches. On -the contrary, when the air is clear and the Aurora presents a steady -dense light of a yellow colour and without motion, the needle is often -unaffected by its appearance. - -[Sidenote: Parry’s experience.] - -Parry (Third Voyage) found his variation-needle (extremely light and -delicately suspended) in no instance affected by the Auroræ; but he seems -to have principally met with the quiescent form of that phenomenon. - -M. Lottin, the French savant (whose description of an Auroral display has -been given in Chapter II.), observed in the North Sea, between September -1838 and April 1839, while the sun was below the horizon, 150 Auroræ. -During this period 64 were visible, “besides many which a cloudy sky -concealed, but the presence of which was indicated by the disturbances -they produced upon the magnetic needle” (Lardner’s ‘Museum of Science and -Art,’ vol. x. p. 189). - -[Sidenote: Grand displays accompanied by motion of needle to the west.] - -It has been remarked by some observers that grand displays of the Aurora -are frequently preceded or accompanied by an extraordinary motion of the -needle to the westward. - -Captain Maguire found at Point Barrow (1852-54) that the appearance of -the Aurora in the south was connected with the motion of the magnet to -the east of the magnetic north, and if in the north to the west of the -same. - -[Sidenote: Solar disturbances and Aurora.] - -On an occasion in 1859 great solar disturbances were observed, the -Greenwich magnets were much disturbed, and a fine Aurora was visible. - -[Sidenote: Cipoletti’s observation.] - -Cipoletti, of Florence, remarks on the strong magnetic disturbances -at Vienna and Munich during the Auroræ of 4th February, 1872, and 4th -February, 1874. - -[Sidenote: Dr. Thompson concludes that cylinders of Aurora cannot be -doubted to be magnets.] - -Dr. Thompson, in his ‘Annals of Philosophy,’ vol. iv. p. 431 (1814), -mentions as an authenticated fact that during the prevalence of the -Aurora the magnetic needle was frequently observed to become unsteady, -and (p. 432) concludes that cylinders of Aurora cannot be doubted to be -magnets. The only three bodies capable of assuming magnetic properties -are iron, nickel, and cobalt. When meteors are considered, it is not -altogether extravagant to conjecture that bodies similar in their nature -to some of the solid bodies which constitute our globe may exist in some -unknown state in the atmosphere. - -During the Aurora of 13th May, 1869, the declination at Greenwich varied -1° 25´, while the vertical force experienced four successive maxima, and -the greatest oscillation amounted to 0·04 of the total mean value. The -horizontal force varied only 0·014 of its mean value. - -During the Aurora of 15th April, 1869, the declination at Stonyhurst -varied 2° 23´ 14″ in nine minutes. - -[Illustration: Plate IX.] - - -_Auroræ, Magnetic Disturbances, and Sun-spots._ - -[Sidenote: Auroræ, magnetic disturbances, and sun-spots in Italy.] - -Auroræ were frequent in Italy in April 1871. On the 10th a remarkable one -was seen, with declinometer deflected towards the east, and 63 sun-spots -were counted. On the morning of the 10th the deflection continued, and at -midday 97 sun-spots were counted. - -On the 18th a brilliant Aurora lasted to 10 o’clock at night. From this -time till the 23rd the Aurora appeared constantly, giving a reddish -tinge in the north and north-west. A brilliant display took place on -the evening of the 23rd. On the evenings when the Aurora appeared the -magnetometers were disturbed throughout Italy, and ended by a violent -agitation during the whole of the 24th. Sun-spots were observed at Rome, -Palermo, and Moncalieri, but the greater number on the days of the -Auroræ. A brilliant display at Moncalieri on June 18 was accompanied by -very violent magnetic disturbance. - -[Sidenote: Proctor’s sun-spots and Aurora.] - -September 25, 1870, Mr. Proctor counted 102 spots on the solar disk; and -on the night of the 24th and morning of the 25th an Aurora of unwonted -magnificence was visible at various stations in England, France, and -Germany. - -[Sidenote: Sun-spots and the magnet. 11 years’ period. Schwabe’s sun-spot -period.] - -With respect to sun-spots and the magnet, the frequency of magnetic -storms, causing oscillation of the needle, gradually increased from a -minimum in 1843 to a maximum in 1848, giving a variation of something -near 11 years altogether. Schwabe observed the sun-spots for 24 years, -and found they had a regular maximum and minimum every five years, and -that the years 1843 and 1848 were minimum and maximum years coinciding -with the magnetic variation at those periods. - -[Sidenote: Prof. Loomis considers connexion established between magnetic -declination, auroral displays, and sun-spots.] - -Professor Loomis (‘American Journal of Science,’ vol. v. April 1873) -considers that a comparison between the mean daily range of the magnetic -declination and the number of Auroras observed in each year, and also -with the extent of the black spots on the surface of the sun, establishes -a connexion between these phenomena, and indicates that auroral displays -(at least in the middle latitudes of Europe and America) are subject -to a law of periodicity, that their grandest displays are repeated at -intervals of about 60 years, and that there are also other fluctuations, -less distinctly marked, which succeed each other at an average interval -of about 10 or 11 years, the times of maxima corresponding quite -remarkably with the maxima of solar spots. - -[Sidenote: Illustrative table of coincidences.] - -An illustration of the result of these observations is given on Plate -IX. fig. 2. The curves are in close correspondence, and the coincidence -at the times of maximum and minimum is remarkable. The auroral maximum -generally occurs a little later than the magnetic maximum; and the -connexion between the auroral and magnetic curves appears somewhat more -intimate than between the auroral and sun-spot curves. - -[Sidenote: Prof. Loomis considers a sun-spot a solar disturbance -affecting the earth’s magnetism.] - -Professor Loomis contends “that the black spot is a result of a -disturbance of the sun’s surface, which is accompanied by an emanation -of some influence from the sun, which is almost instantly felt upon the -earth in an unusual disturbance of the earth’s magnetism, and a flow of -electricity, developing the auroral light in the upper regions of the -earth’s atmosphere.” - -[Sidenote: Carrington and Hodgson’s observations of bright spots on -the sun, accompanied by magnetic disturbance at Kew, and followed by -wide-spread Auroræ.] - -This connexion between the sun’s spots and the earth’s magnetism has been -considered as proved; and one instance at least of an intense disturbance -and outbreak of the sun’s surface having been observed simultaneously -with the occurrence of a terrestrial magnetic storm is a matter of -record. This will be found detailed in the ‘Monthly Notices of the Royal -Astronomical Society,’ vol. xx. pp. 13 and 15, and is so interesting -in its character that it may be briefly referred to here. Mr. R. C. -Carrington, September 1, 1859, 11h 18m, while observing and drawing a -group of solar spots, saw suddenly two patches of intense bright light -break out in the middle of the group. The brilliancy was fully equal to -that of direct sunlight. Seeing the outbreak was on the increase, Mr. -Carrington left the telescope, to call some one to witness it. On his -return within sixty seconds it was nearly concluded. The spots travelled -from their first position, and vanished as two rapidly fading dots of -white light. In five minutes the two spots traversed a space of about -35,000 miles. Mr. Carrington found no change in the group itself. His -impression was that the phenomena took place at an elevation considerably -above the general surface of the sun, and above and over the great group -of spots on which it was seen projected. It broke out at 11h 18m, and -vanished at 11h 23m. Mr. R. Hodgson independently on the same day, and -at close upon the same time, saw a very brilliant star of light, much -brighter than the sun’s surface, most dazzling to the protected eye, -illuminating the upper edges of the adjacent spots and streaks. The rays -extended in all directions, and the centre might be compared to α Lyræ -when seen in a large telescope. It lasted for some five minutes. - -At the very moment of this solar disturbance the instruments at Kew -indicated a _magnetic storm_; and Proctor, in his volume on the Sun, -page 206, details how this magnetic storm was accompanied by very -widely-spread indications of electrical disturbance in many parts of -the globe. Vivid Auroræ were seen not only in both hemispheres, but in -latitudes and places where they are seldom witnessed. Rome, Cuba, and -the West Indies, the tropics within 18° of the equator, and even South -America and Australia, are thus referred to for displays. At Melbourne, -on the night of September 2nd, the greatest Aurora ever seen there made -its appearance. - -It was observed, too, that magnetic communication was at the same time -disturbed all over the earth. Strong currents, continually changing -their direction, swept along the telegraphic wires. At Washington and -Philadelphia the signal-clerks received severe shocks, and the wires had -to give up work. At a station in Norway the transmitting apparatus was -set fire to; and at Boston, in North America, a flame of fire followed -the pen of Baine’s electric telegraph. - -[Sidenote: Mr. John Allan Broun’s magnetic oscillation-curves; showing -that the sun’s magnetic action has lately become more constant. In -diagram, curves gradually flatten.] - -In an interesting communication to ‘Nature’ (January 3rd, 1878), entitled -“The Sun’s Magnetic Action at the Present Time,” Mr. John Allan Broun has -contributed some magnetic oscillation-curves, deduced from observations -made in the Trevandrum Observatory (nearly on the magnetic equator), -by which, if confirmed by other observations, it would appear that the -sun’s magnetic action has lately become gradually more constant. The -curves are three in number,—no. 1 for the years 1855-58, no. 2 for the -years 1865-68, no. 3 for the years 1874-77. In no. 1 curve the minimum -is very clearly marked by two points corresponding to April 1 and May 1, -1856, and there is little difference in the rapidity with which the curve -descends to and ascends from the minimum. In no. 2 curve the epoch of -minimum is by no means so well marked; it occurs between the points for -April 1 and September 1, 1866. There is also a considerable difference -in the rapidity of variation in the descending and ascending branches -of the curve. The descent is nearly as rapid as in curve no. 1; but the -ascent is very much slower. In curve no. 3 the lowest point is that for -December 1, 1875; but it is even now, with points a year and a half -later, difficult to say whether this is the minimum or not, the point -for January 1, 1877, being only 0·02 (two hundredths of a minute of arc) -higher. In this curve the change of range in diurnal oscillation is quite -insignificant from November 1, 1874, to April 1, 1877, an interval of -three years and five months. In the diagram given by Mr. Broun the curves -show themselves gradually flattening, no. 3 being almost a straight line. - -[Sidenote: Mr. Broun never found an Aurora without a corresponding -irregularity in the declination-needle.] - -Mr. Broun remarks upon the report of Sir George Nares as to the -insignificant nature of the Auroræ seen in the Arctic Expedition in the -winter of 1875-76, and the accompanying statement that, as far could be -discovered, they were totally unconnected with any magnetic or electric -disturbance; and states, as the result of his own experience in the south -of Scotland, that several of the Auroræ observed by him were of the very -faintest kind, “were traces” which he could never have remarked had he -not been warned by very slight magnetic irregularities to examine the sky -with the greatest attention. Again, in no case had he seen the faintest -trace of an Aurora without finding at the same time a corresponding -irregularity in the movement of the force or declination-magnet. - -[Sidenote: Prof. Piazzi Smyth comments on variance in the cycles.] - -Prof. Piazzi Smyth, commenting on this article, makes the inquiry how -the sun-spot cycle and the terrestrial magnetic oscillation cycle can -be considered as agreeing, the sun-spot cycle, according to Prof. Wolf, -being 11·111 years, and the magnetic cycle 10·5 years according to Mr. -Broun. - -[Sidenote: M. Faye’s remarks to a similar effect.] - -Another correspondent writes and quotes M. Faye, in ‘La Météorologie -Cosmique,’ for the remark, “La période des taches portée à 11 ans ·1 par -M. Wolf n’étant pas égale à celle des variations magnétiques (10 ans -·45), ces deux phénomènes n’ont aucun rapport entre eux.” - -[Sidenote: Mr. Broun’s rejoinder and explanation.] - -Mr. Broun, in a further letter, rejoins that if we could accept Dr. -Wolf’s view we should find that the mean duration of a cycle for _both_ -phenomena since 1787 would be 11·94 years, while the sun-spot results for -eight cycles determined by Dr. Wolf during eighty years before 1787 give -10·23 or, if we take nine cycles, 10·43 years for the mean duration. It -is by mixing these two very different means that the Zurich philosopher -finds 11·1 years, a mean which Mr. Broun considers can evidently have -no weight given to it. On the other hand, if Dr. Wolf is in error (as -Mr. Broun believes he is) as to the existence of a maximum in 1797, -the mean durations for the eighty years after and for the eighty years -before 1787 agree as nearly as the accuracy of the determinations for the -beginning of the eighteenth century will permit. Mr. Broun then repeats -his conviction that the sun-spot maxima and minima are really synchronous -with those of the magnetic diurnal observations. - -[Sidenote: Mr. Jenkins’s explanation of Prof. Loomis’s chart.] - -Mr. B. G. Jenkins, in a letter to ‘Nature,’ refers Prof. Smyth to Prof. -Loomis’s chart of magnetic oscillations given in Prof. Balfour Stewart’s -paper in ‘Nature’ (vol. xvi. p. 10), for the purpose of showing that -there are exactly seven minimum periods from 1787 to 1871, the mean -of which is twelve years, the mean of the seven corresponding maximum -periods being 11·8 years. The true magnetic declination-period is, then, -the mean of these, viz. 11·9 years. In exactly the same manner he finds -that the mean period of sun-spots is 11·9 years. - -[Sidenote: Jupiter’s suspected connexion with sun-spots.] - -The auroral displays also have the same period. Mr. Jenkins also refers -to Wolf, De La Rue, Stewart, and Loewy, as having stated their belief -that Jupiter is the chief cause in the production of sun-spots, and draws -attention to the period of 11·9 years as being Jupiter’s anomalistic -year, or the time which elapses between two perihelion passages. - -[Sidenote: Infrequency of Auroræ and absence of sun-spots in 1876-78.] - -The infrequency of Auroræ during the years 1876-78, and a corresponding -comparative absence of sun-spots, may be added to the evidence on the -subject. I have seen no account of important Auroræ during the years -mentioned, and day after day has recently (1878) passed with a perfectly -clean sun-disk. - - -_Aurora and Electricity._ - -[Sidenote: Aurora and electricity. Sir John Franklin’s experience with -electrometer.] - -Sir John Franklin failed to get indications of electricity connected -with the Aurora with a pith-ball electrometer; but with another form of -electrometer specially constructed for the purpose he seems to have got -some, though not very strong or regular, indications of repulsion between -the needle of the instrument and the conductor when Auroræ were seen. He -does not decide whether the electricity was received from or summoned -into action by the Aurora. - -[Sidenote: Parry’s experience.] - -Parry, at Fort Bowen, with a gold leaf electroscope connected with a -chain attached by glass rods to the skysail mast-head, 115 feet above -sea-level, found no effect. - -[Sidenote: Dr. Allnatt’s experience, February 4, 1872.] - -Dr. Allnatt, at Frant, during the display of 4th February, 1872, -found the earth’s electricity so powerful that the gold leaves of the -electrometer remained divergent for a considerable time. - -[Sidenote: M’Clintock found electroscope affected in Baffin’s Bay and -Port Kennedy.] - -M’Clintock observes that on six occasions of Aurora in Baffin’s Bay, the -electroscope was strongly affected, and on three occasions of Aurora at -Port Kennedy. The electricity was always positive. - -Dec. 18.—Dr. Walker called him to see the electroscope. The charge was at -first weak, but afterwards strong enough to keep the leaves diverged. Dr. -Walker found two periods of minimum electrical disturbance about 9 P.M. -and noon. - -[Sidenote: Electric currents in telegraphic wires during Auroræ.] - -Electric currents have been reported as produced in telegraph wires -during Auroræ. Though transient they are said to be often very powerful, -and to interrupt the ordinary signals. Loomis (Sillim. Journ. vol. -xxxii.) mentions cases where wires have been ignited, brilliant flashes -produced, and combustible materials kindled by their discharge. - -Here, too, we may note the account of electric phenomena in the case of -the Aurora Australis described (_antè_, p. 28) by Mr. Proctor. - -[Sidenote: Mr. George Draper’s report as to disturbed condition of the -Indian Submarine and other cables during Aurora of February 4, 1872.] - -Mr. George Draper, of the British-Indian Submarine Telegraph Company, -speaking of the Aurora of February 4, 1872 (and writing to the ‘Times’ -under date February 5th), states that the Aurora visible in London -was also visible at Bombay, Suez, and Malta, and that the Company’s -electrician at Suez reported that the earth-currents there were equal -to 170 cells (Daniell’s battery), and that sparks came from the cable. -The electrical disturbances lasted until midnight, and interrupted the -working of both sections of the British Indian Cable between Suez and -Aden, and Aden and Bombay. For some days previously the signals on the -British Indian cables had been much interfered with by electrical and -atmospheric disturbances. - -At Malta there was a severe storm on the morning of the 4th, so that it -was necessary to join the cable to earth for some hours, and the Aurora -was very large and brilliant there. - -The electrical disturbances on the cables in the Mediterranean and on -those between Lisbon and Gibraltar, and Gibraltar and the Guadiana, were -also very great. The signals on the land line between London and the -Land’s End were interrupted for several hours on the night of the 4th by -atmospheric currents. Similar effects accompanied the displays of Oct. 24 -and 25, 1870. - - -_Aurora and Meteoric Dust._ - -[Sidenote: Aurora and meteoric dust. Theories of Dr. Zeyfuss and M. -Gröneman.] - -A theory has been propounded independently by Dr. Zeyfuss and by M. -Gröneman, of Gröningen, according to which the light of the Aurora is -caused by clouds of ferruginous meteoric dust ignited by friction with -the atmosphere. Gröneman shows that these might be arranged along the -magnetic curves by the action of the earth’s magnetic force during their -descent, and that their influence might produce the observed magnetic -disturbances. - -[Sidenote: Ferruginous dust in the Polar Regions.] - -The arches might be accounted for by the effects of perspective; and the -iron spectrum shows correspondence with some of the lines of the Aurora. -Ferruginous particles have been found in the dust of the Polar regions -according to Professor Nordenskiöld, but whether derived from stellar -space or from volcanic eruption is uncertain. A difficulty has been -suggested that while meteors are more frequent in the morning, or on the -face of the earth which is directed forward on its orbit, the reverse -prevails in the case of Auroræ. Gröneman meets this by supposing that in -the first case the velocity may be too great to allow of arrangement by -the earth’s magnetic force. He accounts for the infrequency of the Aurora -in equatorial regions by the weakness of the earth’s magnetic force, and -the fact that when it does occur the columns must be parallel to the -earth’s surface. - -[Sidenote: Baumhauer’s proposition.] - -Baumhauer (Compt. Rend. vol. lxxiv. p. 678) advances, as regards Polar -Auroræ, the proposition, that not only solid masses large and small, but -clouds of “uncondensed” (meteoric) matter probably enter our atmosphere. - -If from our knowledge of the meteoric stones which fall to the earth’s -surface we may draw any conclusion respecting the chemical constitution -of these clouds of matter, it would appear that they may contain a -considerable portion of the magnetic minerals iron and nickel. Let -such a cloud approach our earth, regarded as a great magnet, it would -be attracted towards the Pole, and, penetrating our atmosphere, the -particles which have not been oxidized, and are in a state of extremely -fine division, would by their oxidation generate light and heat, -producing the polar Auroræ. Baumhauer suggests it would be interesting -in support of this theory to detect in the soil of polar areas the -presence of nickel. The presence of iron and nickel in meteoric masses -in considerable quantities is frequent; and cases are also on record by -Eversmann of hailstones containing crystals of a compound of iron and -sulphur, by Pictet of hailstones containing nuclei which proved to be -iron, and by Cozari of hailstones containing nuclei of an ashy-grey -colour, the larger ones of which were attracted by the magnet, and found -to contain iron and nickel. Nickel was found by Reichenbach in parts of -Austria on hills consisting of beds of sandstone and limestone, and quite -free from metallic veins. - -[Sidenote: Mr. Lefroy’s description of a phenomenon ascribed by him to -streams of cosmic dust.] - -Mr. J. W. N. Lefroy, in ‘Nature,’ describes a phenomenon seen by him -at Fremantle, West Australia, in the month of May, which he designates -“A Lunar Rainbow, or an Intra-lunar convergence of Streams of slightly -illuminated Cosmic Dust?” - -It lasted about three quarters of an hour, and consisted of one grand -central feather, of very bright white cloud, springing out of the horizon -at W.N.W., and crossing the meridian at about 20° north of the zenith, -with a width of 7° to 8°. - -On either side of this was a system of seven or eight minor beams of -light, extending from the W. to the E. horizon, subtending a chord common -to themselves and to the main stream, and converging towards the axis -of the central stream so as to intersect it at a point about 30° or -40° below the western horizon, at which the whole system subtended an -azimuth of about 20°. Near the zenith, where its transverse section was a -maximum, that section subtended an angle of about 40°. - -The idea strongly suggested itself to Mr. Lefroy of converging streams of -infinitely minute particles of matter passing through space at a distance -from the earth at which its aerial envelope may have still a density -sufficient by its resistance to give cosmic dust passing through it that -illumination which it possessed. In about twenty minutes the streams of -light had attained their maximum brightness. Their apparent figure was -that of a nearly circular (slightly flattened) arc of an amplitude of 15° -or 20°, as viewed from the middle point of its chord. - -The brightness and the convergence of the streams were both more marked -towards the western horizon than the eastern. This same phenomenon was -described in the ‘South-Australian Register’ as a beautiful lunar rainbow -visible in the western heavens. - -Mr. Lefroy and other observers concurred in the impression that the minor -lateral streams on the N. side of the main one intervened between the -earth and the moon, and that one or more of them in their slow vibrations -swept the surface of the moon and sensibly obscured its light. There can -be hardly any question that the phenomenon observed was in fact an Aurora. - -[Sidenote: Suggestion as to collecting iron and nickel particles from the -atmosphere.] - -It may be a question whether iron and nickel particles of meteoric origin -do not ordinarily exist in the atmosphere in a greater degree than we -suspect, and might be detected if special means, such as magnets, plates -of glass covered with glycerine, &c., were adopted for the purpose of -collecting and examining the cosmic dust. Larger gatherings than usual -of iron and nickel particles during the presence of Auroræ would be in -support of Mr. Lefroy’s theory. - - -_The Aurora and the Planets Venus and Jupiter._ - -[Sidenote: The Aurora and planets Venus and Jupiter. The planet Venus’s -halo during Aurora.] - -During a brilliant Aurora seen at Sunderland, February 8, 1817 (‘Annals -of Philosophy,’ p. 250), about 8 o’clock, Venus was about 8° above the -horizon, and displayed a very peculiar appearance. Her rays passed -through a thin mist or cloud, probably electric, of a deep yellow tint. -Her apparent magnitude seemed increased, and a halo was formed round her -as sometimes appears round the moon in moist weather; but the stars that -were in that part of the heavens shone with their accustomed brilliancy. - -[Sidenote: Dr. Miles’s observation of Venus during an Aurora.] - -The Rev. T. W. Webb, in his ‘Celestial Objects’ (1859), p. 43, quoting -from the Philosophical Transactions, mentions that, “January 23rd, -1749-50, there was a splendid Aurora Borealis about 6 P.M. The Rev. Dr. -Miles, at Tooting, had been showing Jupiter and Venus to some friends -with one of Short’s reflectors, greatest power 200, when a small red -cloud of the Aurora appeared, rising up from the S.W. (as one of a deeper -red had done before), which proceeded in a line with the planets and soon -surrounded both. Venus appearing still in full lustre, he viewed her -again with the telescope without altering the focus, and saw her much -more distinctly than ever he had done upon any occasion. His friends -were of the same opinion. They all saw her spots plain (resembling those -in the moon), which he had never seen before, and this while the cloud -seemed to surround it as much as ever.” - -I think this effect might perhaps have arisen from the Aurora acting as -a screen, and removing the glare with which so bright an object as Venus -is always accompanied; but the case is a singular one, and one would be -glad of further experience. I suggested observations on this head during -Sir Geo. Nares’s Arctic Expedition; but the suggestion, for some reason -of which I am not aware, was not included in the official instructions -issued. - -[Sidenote: Brightness of stars during Auroræ.] - -Remarks are frequent of the brightness of stars as seen through Auroræ. -Payer, of the Austrian Expedition, remarks that falling stars passed -through the Aurora without producing any perceptible effect or undergoing -any change. - -[Sidenote: Aurora of Oct. 24, 1870, and Jupiter.] - -A grand display of the Aurora took place 24th October, 1870. About -this time the belts of Jupiter were observed to be highly coloured. As -observed by me on the night of November 2, 1870, at 9 P.M., with an -8¼-in. Browning reflector, achromatic eyepieces 144, 305, and 450, the -equatorial zone was of a distinctly dark ochre colour, deepening to -red-brown as it approached the lower (N.) edge. Two thin belts above were -slate-purple, and a darker belt below was of a deep purple colour. - -[Sidenote: According to Lassell and others, Jupiter’s belts exhibit the -brightest colours at period of Auroræ.] - -Lassell, Proctor, and others have reported Jupiter’s belts to exhibit the -brightest colours at the period of Auroræ. Mr. Browning gives a drawing -of Jupiter as seen on January 31, 1870 (a year noted for Auroræ), with -the belts brightly coloured. The finest view of Jupiter I ever had was -on the 8th February, 1872 (a fine Aurora was on the 4th), when, with the -8¼-inch Browning reflector, I saw the whole surface of the planet (by -glimpses) cloud-mottled. The equatorial belt was, however, then slightly -tinted only. In Dr. Miles’s observation (p. 66) he does not seem to have -noticed the colouring of Jupiter’s belts. - -[Sidenote: Infrequency of Auroræ and lightness in tint of Jupiter’s -belts.] - -The three past years, 1876, 1877, and 1878, have been distinguished by -the infrequency of Auroræ; and Jupiter’s equatorial zone and belts have -been mainly reported of light tints. - -The subject apparently deserves more attention than it has hitherto -received. - - -_The Aurora and the Zodiacal Light._ - -[Sidenote: The Aurora and the Zodiacal Light. Ångström’s observation on -spectrum.] - -Ångström in 1867 found the spectrum of the Zodiacal Light to be -monochromatic, consisting of a single line in the green, to which he -assigned approximately the position 1259 on Kirchhoff’s scale, the same -that he had determined for the green line of the Aurora Borealis; and -Respighi, on the Red Sea, on the evening of the 11th and the morning of -the 12th January 1872, perceived in the Zodiacal Light not only this -green line, but near it, towards the blue, a band or zone of apparently -continuous spectrum. - -[Sidenote: Respighi’s at Campidoglio.] - -At the Observatory of the Royal University of Campidoglio, February 5th, -1872, Respighi, at 7 P.M., was able to discern the same spectrum; and on -directing the spectroscope to other points he found that this spectrum -showed itself in all parts of the heavens from the horizon to the zenith, -more or less defined in different parts, but everywhere as bright as in -the Zodiacal Light. The Observatory Assistant, Dr. di Legge, likewise -observed this spectrum distinctly in various parts of the heavens. -Respighi’s observations corroborating Ångström’s in 1867, appeared to him -to demonstrate the identity of the Zodiacal Light with the Aurora, and to -establish the identity of their origin. - -[Sidenote: Pringle thinks the Aurora may be considered as allied to the -Zodiacal Light.] - -Pringle, in a letter to ‘Nature’ from South Canara, October 3, 1871, -alludes to the Aurora as being considered by many allied to the Zodiacal -Light, and does not think the evidence then hitherto adduced against the -theory at all conclusive. He says:—“Assume the auroral light to consist -of solid particles of matter, planet dust, shining by reflected light, -and it is not difficult to imagine the Aurora playing amongst these tiny -worlds, each of which would have its own small magnetic system swayed -like our own by the monster magnet the sun.” - -[Sidenote: Phosphorescence of sky when Zodiacal Light has been seen -bright.] - -He notices he has never found it to have a decided outline, nor traced -it east or west to 180° from the sun. He also refers to others having -noticed that when the Zodiacal Light has been seen unusually bright, a -“phosphorescence” of the sky was everywhere visible. - -[Sidenote: Pringle failed to find bright lines or bands in the Zodiacal -Light.] - -He does not seem at that time to have examined the matter -spectroscopically; and on June 23, 1872, he writes again, pointing out -the peculiarity in Respighi’s observation that the green line was seen -everywhere as bright as in the Zodiacal Light, and suggesting that it -was due to a concealed Aurora present at the time of Ångström’s and -Respighi’s observations. He further states he had examined the Zodiacal -Light with a Browning 5-prism spectroscope (I presume a compound -direct-vision form is meant) since the last December, and, brilliant -as the phenomenon had frequently been, failed to detect the slightest -appearance of bright lines or bands. A faint diffuse spectrum about as -intense as that of a bright portion of the Milky Way was all he had -obtained. - -[Sidenote: Prof. Piazzi Smyth confirms this.] - -Professor Piazzi Smyth, in the clear sky of Italy, and with an instrument -specially designed for showing faint spectra, found no lines or bands, -but only a faint continuous spectrum extending from about midway between -D and E in the solar spectrum to nearly F (see Plate V. fig. 3, in which -the continuous spectrum is graphically shown, white on a black ground). - -[Sidenote: Colour of the Zodiacal Light.] - -It may here be mentioned that the Zodiacal Light is usually described as, -in these latitudes, of a golden yellow or pale lemon tinge. - -[Sidenote: Rev. Mr. Webb’s observation, February 2, 1862. He found no -green line of the Aurora.] - -On one occasion, however, it has been described as not having this -tinge, but rather resembling the light of the Milky Way, but brighter. -On another occasion I saw the whole cone of a crimson hue without any -mixture of yellow. The Rev. Mr. Webb thought that a display seen at -Hardwick Vicarage, February 2nd, 1862, showed a ruddy tinge not unlike -the commencement of a crimson Aurora—“it was certainly redder or yellower -than the galaxy.” He examined it with a pocket spectroscope which would -show distinctly the green line of the Aurora (probably Browning’s -miniature), but nothing of the kind was visible, nor could any thing be -traced beyond a slight increase of general light, which, on closing the -slit, was extinguished long before the auroral band would have become -imperceptible. - -[Sidenote: A. W. Wright’s observations and conclusions.] - -A. W. Wright examined the Zodiacal Light with a Duboscq single-prism -spectroscope, the telescope and collimator having a clear aperture of -2·4 centimetres, magnifying-power of telescope 9 diameters. Special -precautions were taken about the observations, and the conclusions -arrived at were:— - -(1) The spectrum of the Zodiacal Light is continuous, and is sensibly the -same as that of faint sunlight or twilight. - -(2) No bright line or band can be recognized as belonging to this -spectrum. - -(3) There is no evidence of any connexion between the Zodiacal Light and -the Polar Aurora. - -[Sidenote: Polarization of Zodiacal Light. Burton’s observation confirmed -by Wright and Tacchini.] - -The Polarization of the Zodiacal Light has been already referred to under -the head of “Polarization of the Aurora:” but it may be here noted that -Mr. Burton’s observation of polarization of the light there mentioned -has been confirmed by Wright and Tacchini, and the presence of reflected -sunlight established. In this respect it differs from the Aurora, in -which no trace of polarization has hitherto been detected; and looking at -this, and at the weight of evidence in the spectroscopic observations, -the theory of a connexion between the Aurora and the Zodiacal Light must, -as the matter stands, be given up. - - - - -CHAPTER VII. - -AURORA-LIKE PATCHES ON THE PARTIALLY-ECLIPSED MOON. - - -[Sidenote: Aurora-like patches on the partially-eclipsed moon, Feb. 27, -1877.] - -In anticipation of the total eclipse of the Moon on the 27th February, -1877, several articles appeared in the leading journals of the day -describing, for the public benefit, the appearances which might be -expected during the occurrence of the phenomenon. - -[Sidenote: Formerly it was thought the moon was illuminated by auroral -light.] - -Among these was one by Mr. R. A. Proctor, in which the following passage -occurs:—“That dull, or occasionally glowing red colour, shown by the moon -when she is fully and even deeply immersed in the shadow of the earth, -is a phenomenon whose explanation is not without interest. Formerly it -was thought that the moon possessed an inherent light, or _perhaps was -illuminated by auroral light_, which only became discernible at the time -of total eclipse. Indeed even Sir W. Herschel fell into the mistake of -supposing this the only available explanation, having miscalculated the -efficiency of the true cause.” - -[Sidenote: Author’s notes of the eclipse. Colour-tints described. A -crimson-scarlet tint reminded author of an auroral glow.] - -This passage was only pointed out to me by a friend after the eclipse -had actually taken place, and I had sent him some notes of what I then -saw. My notes on the occasion comprised, amongst others, the following -remarks:—“The tints of colour also during partial eclipse, owing, no -doubt, to the moon’s considerable altitude, were singularly bright and -well contrasted. Silver-grey, dusky copper-red, and the same tint clearer -and brighter were ranged side by side with a lovely jewel effect. _We -noticed also at times a crimson-scarlet tint, deeper and less mixed with -yellow than the copper colour._ This last tint reminded me much of a -_crimson glow common to the Aurora_, and which I also once distinctly -remarked (of course in a weaker degree) in the zodiacal light” (_antè_, -p. 68). - -[Sidenote: Eclipse, Aug. 23-24, 1877. Sky clear, but eclipsed moon misty -and indistinct until total obscuration. Succession of colours.] - -On the occasion of the eclipse of August 23-24, 1877, we were favoured -at Guildown, in common with many other places, by a singularly clear sky -during the progress of the moon’s obscuration and subsequent clearing. -In the early part of the evening, however, the moon, from some cause -(possibly atmospheric vapour), seemed to have, as the earth’s shadow -advanced on its disk, an unexpectedly misty and indistinct appearance, -which lasted up to and including total obscuration. Golden yellow, yellow -copper, dull copper, ruddy copper, and dull red were successively the -principal colours observed at different times and at various portions of -the moon’s surface. - -[Sidenote: As shadow passed off, indistinctness gave way to a sharpness -of the moon’s features as seen through shadow. Two patches of crimson -light described.] - -After referring to some spectroscopic appearances, my notes then ran on -thus:—“As the shadow began to pass off, and the bright sharp crescent -of the illuminated portion of the moon to appear, the general aspect of -the moon’s disk seemed to me to greatly change. The certain amount of -indistinctness noticeable during approach and continuance of totality, -gave way to a considerable sharpness of the moon’s features as seen -through the shadow. The shadowed part glowed with a richer copper tint, -on which were seen dark, almost black, spots and patches.” Then follows a -description of these; and the notes continue:—“Two features here struck -me—the one a continuation of the upper limb of the illuminated crescent, -so that it seemed to form a bead of light just on the centre of the upper -edge of the moon; the other _two patches of crimson light_, similar to -those I described as having been seen in the last total eclipse. One -of these, quite a small one, was just under the elongated bead before -described; the other, a much larger and more diffused one, was seen -towards the south-west limb of the moon, about midway between it and the -centre. The spots or patches were of a decidedly crimson-red, in contrast -to the ordinary copper-red of the disk, and were noticed by my friend as -well as by myself.” - -[Sidenote: Patches well seen in field-glass; lost in small refractor. -They gradually deepened in tint.] - -These were eye observations. The patches were quite well seen (but not -so brightly as with the eye) with a double achromatic field-glass. -With a 3¼-inch Cooke refractor and low power, they seemed lost in the -general moon tint; but they were then diminishing in brightness. From -a comparison of my two sketches, the patches seem to have gradually -deepened in tint, and we considered them to have disappeared in a like -gradual manner. - -[Sidenote: Two sketches taken.] - -My first sketch was taken shortly after end of total phase; the second -about ten minutes later. I have reproduced the original sketches in -preference to any drawing prepared from them (Plate IV. figs. 2 and 3). - -The patches did not last long, but were lost as the shadow swept off -the moon. I saw nothing of the sort during the approach of or pending -totality, nor until a small crescent of the moon began to appear behind -the shadow. - -I have looked for other accounts of these patches, but cannot find any. -Most observers have described the deeper colour of the shadowed moon by -the word “copper.” Some extend this colour to red; but there is probably -much in the state of the atmosphere affecting this. - -[Sidenote: Dec. 3, 1703, moon’s colour described.] - -At Avignon, December 3, 1703, the moon appeared, pending eclipse, -“extraordinarily illuminated and of a very bright red,” while other and -different features were seen at Montpelier. - -On March 19, 1848, observers in England, Ireland, and Belgium described -the moon’s disk as “intensely bright coppery red.” On the occasion of -August 23-24, 1877, before mentioned, an article in one of the public -papers described the moon’s disk, during totality, as of a “dull copper -colour.” - -[Sidenote: Mr. Keye’s observation.] - -Mr. Henry Keye, in the Engadine, at a height of 4500 feet above -sea-level, and with the purest air, saw the partially covered moon -(before totality) as a “dull copper colour.” - -[Sidenote: Prof. Pritchard’s. M. Faye’s. Dr. Allnatt’s at Frant.] - -Prof. Pritchard, writing from the Oxford University Observatory, says -that at 12h 10m (about the time my sketches were taken) there was a -good deal of light on the moon’s following limb, and the colour was -“more red than copper,” and apparently redder than it had been at a -similar distance of time before totality. Mons. Faye reported to the -French Academy of Sciences that “a striking phenomenon not previously -noticed was that the reddish tinge, resembling that of a fine sunset, -was deepest at the margin of the disk, a circumstance which he could not -explain.” Dr. Allnatt, writing from Frant, says:—“At totality the moon’s -disk presented a most extraordinary appearance: the western limb was -comparatively transparent, but the main body appeared as though enveloped -in a semi-opaque clot of coagulated blood, through which the lunar -features were dimly visible.” - -[Sidenote: Observations as to the patches.] - -The observations of Prof. Pritchard and Mons. Faye point more immediately -to redness; and this is the nearest approach I can find to the patches I -noticed. These patches do not seem to me easy of explanation. They could -not well be colours or details due to the actual surface of the moon -itself. The moon, we are aware, has only a certain portion of the visible -disk slightly tinted. The Mare Serenitatis is certainly of a slight green -tinge; and to the Palus Somni and certain other districts is attributed a -pale red or pink; but these tints could hardly have sufficed to produce -the effect seen, as the patches were conspicuous for a bright and decided -colour. The positions, moreover, did not correspond; while the ease -with which other details of the surface were seen at the time would, if -the tints had arisen from the surface itself, probably have enabled the -circumstance to be detected. - -[Sidenote: Refraction of sun’s rays not a satisfactory explanation.] - -The refraction of the sun’s rays by passage through the earth’s -atmosphere is, too, not a satisfactory explanation. This, as judged by -the appearance of the covered moon immediately before and at totality, -gives a disk of shadow deeper in tone in the centre and lightening -towards the edges, but in other respects fairly uniform, so that the -whole disk seems to partake of the same tint and its graduations; and -this is what might have been expected under the circumstances. The -patches, on the other hand, were quite local. - -[Sidenote: Question of lunar atmosphere.] - -The theory of the moon’s possessing no atmosphere whatever is now very -generally, but perhaps too readily, received (mainly upon the evidence -of the spectroscopic observations of occulted stars[9]), as there still -seems a reasonable doubt whether our satellite may not possess an -atmosphere, possibly rarefied, but yet sufficiently dense to permit of -the formation of cloud or vapour. - -[Sidenote: Instance of patch of vapour or cloud on moon’s surface.] - -A curious case, in which a patch of vapour or cloud was supposed to be -detected on the moon’s surface, is reported by the Rev. J. B. Emmett in -a communication to the ‘Annals of Philosophy’ (New Series, vol. xii. p. -81). It is dated “Great Ouseburn, near Boroughbridge, July 5, 1826,” -the observation being made with “the greatest care with a very fine -telescope.” - -On the 12th April 8h, while observing the part of the moon called Palus -Mœotis by Nevelius, with an excellent Newtonian reflector of 6 inches -aperture, at a particular part of the Palus, which he minutely describes, -he saw, with powers 70 and 130, “a very conspicuous spot wholly enveloped -in black nebulous matter, which, as if carried forward by a current of -air, extended itself in an easterly direction, inclining a little towards -the south, rather beyond the margin of Mœotis.” April 13th 8h to 9h, the -cloudy appearance was reduced both in extent and intensity, and the spot -from which it seemed to issue had become more distinctly visible. On -April 17th scarcely a trace of the nebulous matter remained; but so long -after as June 10th 8h “a little blackness” remained about the spot. Mr. -Emmett suggested “smoke of a volcano or cloudy matter.” A copy of the -drawing annexed to the paper is given on Plate X. fig. 10 (black patch on -moon). If this observation was (as it certainly appears to be) critical -and exact, there must have been a disturbance of the moon’s surface, -indicating some sort of cloud- or vapour-supporting atmosphere; and -probably, for the purposes of Auroræ, an atmosphere of a very rarefied -condition would suffice[10]. - -[Sidenote: Prof. Alexander’s evidence in favour of a lunar atmosphere.] - -According to the ‘New York Tribune,’ at a recent semi-annual meeting of -the American Academy of Sciences, Professor Alexander “brought forward a -variety of evidence tending to indicate some envelope like an atmosphere -for the moon. The evidence was principally drawn from observations -during eclipses. The explanations usually offered for the bright band -seen around the moon at such times was fully considered, and shown to -be inadequate, though good as far as they would apply. The ruddy band -of light is much too broad to be the sun’s chromosphere. It was most -apparent in those instances where the moon was nearest the earth. It -would best be accounted for by supposing an atmosphere to the moon, a -thin remnant of ancient nebulosity, comparable to that which accompanies -the earth and gives rise to the appearance of the Aurora Borealis.” Is -it not, however, possible that the appearance might have arisen from -Auroræ in action within the region of the earth’s own atmosphere during -the passage of the sun’s rays through it at the time of the eclipse? -The whole subject is difficult of explanation, and should be one of the -points for attention on the occasion of the next total Lunar eclipse. It -seemed to me appropriate for introduction into the present history of the -Aurora, whatever its solution may ultimately be. - -[Sidenote: Mars and Jupiter.] - -In the case of Mars and Jupiter, whose atmospheres are sufficiently -recognized, red- and scarlet-tinted patches are frequently noticed. In -Mars this is generally attributed to the geological character of the -surface of the planet itself; but I have observed on Mars’s surface -during the recent opposition a local rosy tint of a more diffused and -indefinite character; and in the case of Jupiter the appearances seem -almost always connected with the clouds’ belts, as distinguished from the -regions lying nearer to the planet’s surface. - -[Sidenote: Prof. Dorna’s “Lunar Aurora.”] - -Professor Dorna, of Turin, ascribed a flickering light seen on the -reddened disk of the moon during the Lunar eclipse of February 1877 -to the action of a _Lunar Aurora_, holding that the refraction of the -sun’s rays within the cone of the earth’s shadow was not an adequate -explanation (‘L’Opinione Nazionale,’ March 3, 1877). - -[Sidenote: Spectroscopic observations bearing on the subject. Mr. -Christie’s observations at Greenwich.] - -The spectroscope might have afforded some information on the question; -but my own telescopes (8¼ and 3¼ in.) were not of sufficient aperture -to give a sensible spectrum of a portion of the moon’s eclipsed -surface, and my observations were chiefly made on the entire disk -with hand-spectroscopes without a slit. Mr. Christie, at the Royal -Observatory, Greenwich, made a set of observations during totality, and -also during subsequent partial phase, with a single-prism spectroscope. -During totality a strong absorption band was seen in the yellow, and -the red and blue ends of the spectrum were completely cut off, while -the orange was greatly reduced in intensity. The yellow and green -were comparatively bright, and seemed to constitute the whole visible -spectrum. The absorption band became narrowed as the end of the total -phase approached, and during partial phase was reduced to a mere line. -The red end of the spectrum was cut off by a dark band commencing about -halfway from D to C, in which a black line was suspected. The bands -observed were characteristic of the spectrum of light which has passed -through a thick stratum of air. In the description of the spectrum of the -Aurora in Part II., it will be seen that the conspicuous red and green -lines of the Aurora are either coincident with, or very close to, some of -these atmospheric lines. It does not appear that Mr. Christie examined -the crimson patches specifically, nor that he saw bright lines on any -part of the moon’s eclipsed disk. - -[Sidenote: Mr. Pratt’s notes of Lunar Eclipse, August 23, 1877.] - -Mr. Henry Pratt has also kindly handed me for use his notes of the Lunar -eclipse of August 23, 1877, as seen at Brighton on a splendid night. They -were made as the phenomenon progressed, are 58 in number, and in many -instances only a few minutes, or even seconds, apart. A selection of -them is here given:—9h 13m 50s, first contact of shadow. 9h 30m, shadow -very dark; no details of disk easily seen. 9h 40m, first appearance of -red. 9h 50m, _red_ all over disk, except margin bluish and S. part green -tint. 10h 2m, _a sudden brightening of whole disk_, in strong contrast to -two minutes previously. 10h 15m, _E. limb much darker_. 10h 35m, _south -pole decidedly brightest_. 10h 44m, _S.E. limb much brighter_. 10h 48m, -_whole disk much darker_. 10h 51m, _S.E. limb brightening again_. 11h 1m, -_N.E. limb brightening_. 11h 3m, _N.E. limb has darkened and brightened -three times during last two minutes_. 11h 20m, N. pole has _darkened_. -11h 21m, N. pole has _brightened_. 11h 24m 30s, N. pole darker _red_. 11h -35m, N. pole _bright_. 11h 35m 30s, same _dark_ and _red_. 11h 42m, N.E. -limb especially bright for a few seconds, and then _reddened_ and shaded -again. 11h 49m, _S. pole reddened_. 12h 1m, _S.W. limb reddest part; S. -pole red; N. pole paler red_. 12h 3m 50s, first appearance of E. limb -(my first sketch was made shortly after this, and my second about ten -minutes later). 12h 21m, a bright patch on N.N.W. separated from N. pole. -12h 24m, _S.W. region is reddest part of eclipse_. 12h 40m, _redness_ of -shadow fading out. - -With a small Browning star-spectroscope Mr. Pratt saw the red and blue -ends of the spectrum cut off, but nothing else. Mr. Pratt adds that the -_red_ colour was not an effect of contrast or an optical delusion in any -way, as was proved by using at times a limited field containing only the -red portion under examination. In reference to the curious brightening -and darkening of the disk, and the change from time to time of local -colour, he says that with much experience he has seen nothing of the -same marked character on other occasions, and that “the whole matter was -at the time astonishing to me, but none the less real.” The local red -patches seen by me seem also to have been observed by Mr. Pratt. - -[Sidenote: Mr. Pratt’s observation on the floor of Plato.] - -As an addition to the instances of Tycho, Picard, &c., mentioned in the -note on p. 73, Mr. Pratt has also sent me his notes of some observations -by him, of “local obscuration of the floor of Plato.” As somewhat -condensed, they are as follows:—1872, July 16. While in other parts of -the floor spots and streaks were well visible, “the N.W. portion was -in such a hazy condition that nothing could be defined upon it.” 1873, -Nov. 1. 27 light streaks seen (7 new): the brightness of the streaks was -in excess of their usual character, as compared with the craterlets; -“an _obliteration_ or _invisibility_ of _all_ the light streaks in the -neighbourhood of craterlet no. 1 was very noticeable;” and also “a -similar obliteration of the N. end of the streak called the Sector, near -craterlet 3.” 1874, January 1. 18 light streaks seen, including 3 new, -“some of which outshone other longer known ones. This was curious; for -had they been as bright within the last two years as on this occasion -I must have noticed them.” Mr. Pratt points out, as worthy of remark, -that some months previous to November 1st, 1873, neither craterlets -nor streaks on the floor of Plato “had maintained their previous -characteristic brightness,”—a fact which he thinks ought to be considered -together with the outbreak of brilliancy of both orders on that day, as -well as the apparently sudden existence of new ones. - -[Sidenote: Observation by Mr. Hirst of a dark shade on the moon.] - -The ‘Observatory,’ March 1, 1879, p. 375, contains an account, by Mr. H. -C. Russell, of some Astronomical Experiments made on the Blue Mountains, -near Sydney, N. S. W. Among these it is noticed that on 21st October, -1878, at 9 A.M., when looking at the moon, Mr. Hirst found that a large -part of it was covered with a dark shade, quite as dark as the shadow -of the earth during an eclipse of the moon. Its outline was generally -circular, and fainter near the edges. Conspicuous bright lunar objects -could be seen through it; but it quite obliterated the view of about half -the moon’s terminator, while those parts of the terminator not in the -shadow were distinctly seen. - -No change in the position of the shade could be detected after three -hours’ watching. The observation is made, “One could hardly resist the -conviction that it was a shadow; yet it could not be the shadow of any -known body. If produced by a comet, it must be one of more than ordinary -density, although dark bodies have been seen crossing the sun which were -doubtless comets.” The diameter of the shadow from the part of it seen on -the moon was estimated at about three quarters that of the moon[11]. - - - - -CHAPTER VIII. - -AURORA AND THE SOLAR CORONA. - - -[Sidenote: Aurora and the solar corona. Mr. Norman Lockyer’s ‘Solar -Physics.’] - -Mr. Norman Lockyer, in his ‘Solar Physics,’ a work of 666 pages, gives -but little space to the Aurora. The index comprises:—“Aurora Borealis, -connexion with sun-spots, pp. 82-102.” “Affirmed coincidence of spectrum -with that of the corona, pp. 244, 256.” - -[Sidenote: Extracts from as to Aurora’s connexion with sun-spots and with -solar corona.] - -Page 82. After referring to Gen. Sabine as having shown that there are -occasional disturbances in the magnetic state of the earth, and that -these disturbances have a periodical variation, coinciding in period -and epoch with the variation in frequency and magnitude of the solar -spots as observed by Schwabe, the author proceeds to state, “and the -same philosopher has given us reason to conclude that there is a similar -coincidence between the outburst of solar spots and of the Aurora -Borealis.” - -Page 102. “We have also shown that sun-spots or solar disturbances appear -to be accompanied by disturbances of the earth’s magnetism, and these -again by auroral displays.” - -[Sidenote: Evidence of American observers on nature of the corona -considered.] - -Page 243. “What, then, is the evidence furnished by the American -observers on the nature of the corona (solar)? It is bizarre and puzzling -to the last degree. The most definite statement on the subject is -that it is nothing more nor less than a _permanent Solar Aurora_! the -announcement being founded on the fact that three bright lines remained -visible after the image of a prominence had been moved away from the -slit, and that one (if not all) of these lines is coincident with a line -(or lines) noticed in the spectrum of the Aurora Borealis by Professor -Winlock.” Mr. Lockyer then adds, that amongst the lines he had observed -up to that time, some forty in number, this line was among those which he -had most frequently recorded, and was, in fact, the first iron line which -made its appearance in the part of the spectrum he generally studied, -when the iron vapour is thrown into the chromosphere. - -[Sidenote: Mr. Lockyer’s conclusion adverse to the question being -settled.] - -Hence he thought he should always see it if the Aurora were a permanent -solar corona, and gave out this as its brightest line, and on this ground -alone should hesitate to regard the question as settled. - -[Sidenote: Prof. Young’s communication to ‘Nature.’] - -Page 256 is an extract from a communication by Prof. Young to ‘Nature,’ -March 24, 1870, in which the Professor refers to the bright line 1474 as -being always visible with proper management. He also thinks it probable -that this line coincides with the Aurora line reported by Prof. Winlock -at 1550 of Dr. Huggins’s scale, though he is by no means sure of it. He -had only himself seen it thrice, and then not long enough to complete a -measurement. He was only sure that its position lay between 1460 and 1490 -of Kirchhoff. - -[Sidenote: He does not abandon his hypothesis, it having other elements -of probability.] - -For this reason he did not abandon the hypothesis, which appeared to have -other elements of probability, in the general appearance of the corona, -the necessity of immense electrical disturbances in the solar atmosphere -as the result of the powerful vertical currents known to exist there, -as well as the curious responsiveness of our terrestrial magnets to -solar storms; yet he did not feel in a position to urge it strongly, but -rather awaited developments. Father Secchi was disposed to think the line -hydrogen, while Mr. Lockyer still believed it to be iron. - -[Sidenote: Dr. Schellen reviews the subject in eclipse of 1869.] - -Dr. Schellen, in his ‘Spectrum Analysis,’ treats the matter more in -detail. Referring to the eclipse of 1869 as confirming the previous -observations that the coronal spectrum was free from dark lines, he -points out that Pickering, Harkness, Young, and others were agreed that -with the extinction of the last rays of the sun all the Fraunhofer lines -disappeared at once from the spectrum. He further says:— - -[Sidenote: Young observed three bright lines in the spectrum of the -corona. Coincidence of these lines with three bright lines observed by -Winlock in the Aurora. Corona self-luminous, and probably of a gaseous -nature. Corona supposed to be a permanent polar light existing in the -sun. Polar light in the sun attributed to electricity. Dr. Schellen -thinks nature of the corona still a problem.] - -“The small instruments employed by Pickering and Harkness, with a large -field of view, exhibited a spectrum obtained at once from the corona, -the prominences, and the sky in the neighbourhood of the sun. These -instruments showed during totality a faint continuous spectrum free -from dark lines, but crossed by two or three bright lines. Young, with -a spectroscope of five prisms, observed the three bright lines in the -spectrum of the corona, and deduced the following positions according to -Kirchhoff’s scale:—1250 ± 20, 1350 ± 20, and 1474. It had been already -explained why the last and brightest of these lines was thought to belong -to the corona and not to that of the prominences, and it seemed probable -that the other two lines belonged also to the light of the corona, from -the fact that they were both wanting in the spectrum of the prominences -when observed without an eclipse. But what invested these three lines -with a peculiar interest was the circumstance that they appeared to -coincide exactly with the first three of the five bright lines observed -by Professor Winlock in the spectrum of the Aurora Borealis. These lines -of the Aurora were determined by Winlock according to Huggins’s scale; -and if these be reduced to Kirchhoff’s scale, the positions of the lines -would be 1247, 1351, and 1473, while the lines observed by Young were -1250, 1350, and 1474.” Dr. Schellen then points out that if it be borne -in mind that Young found the positions of the two fainter lines more by -estimation than by measurement, the coincidence between the bright lines -of the corona and those of the Aurora would be found very remarkable. -The brightest of the lines, 1474, was the reversal of a strongly marked -Fraunhofer line, ascribed by Kirchhoff and Ångström to the vapour of -iron. Dr. Schellen then details Professor Pickering’s observations with -the polariscope, showing that the corona must be self-luminous, and that -from the bright lines seen in its spectrum it is probably of a gaseous -nature, and forms a widely diffused atmosphere round the sun; and then -adds, “It has been supposed, from the coincidence of the three bright -lines of the corona with those of the Aurora Borealis, that the corona -is a permanent polar light existing in the sun analogous to that of our -earth.” Dr. Schellen here adds:—“Lockyer, however, justly urges against -this theory the fact that although the brightest of these three lines, -which is due to the vapour of iron, is very often present among the -great number of bright lines occasionally seen in the spectrum of the -prominences, it is by no means constantly visible, which ought to be the -case if the corona were a permanent polar light in the sun.” (Professor -Young’s answer to this, on the ground of line 1474 being always visible, -has been already given.) “A yet bolder theory is the ascription of such -a polar light in the sun to the influence of electricity, which has been -proved, it is well known, by the relation of the magnetic needle, and -the disturbance of the electric current in the telegraph wires, to play -an important part in the phenomenon of the Aurora Borealis;” and Dr. -Schellen then concludes with an opinion that the nature of the corona was -still a problem[12]. - -[Sidenote: Various places of wave-length assigned to these lines.] - -On reference to the ‘American Journal of Science,’ vol. xlviii. pp. 123 -and 404, it seems that the auroral observations before referred to were -made on 15th April, 1869, by C. S. Pierce, with “an ordinary chemical -spectroscope, with the collimator pointed directly to the heavens,” -and were reported by Winlock. The lines were 1280, 1400, and 1550 of -Huggins’s scale, and were reduced to Kirchhoff’s scale by Young. These -lines have had all sorts of places of wave-length assigned to them -by different writers. Proctor gives 5570, 5400, 5200; Pickering and -Alvan Clarke, 5320 (assumed to be 5316, coronal line); Barker, 5170, -5200, 5020; Backhouse, 5320, 4640, and 4310. In my ‘Aurora Spectrum,’ -Plate XII., I have assigned two, with a?, to 5320 (Alvan Clarke) and -5020 (Barker). The third might perhaps be placed at 4640 (Backhouse and -Winlock). - -[Sidenote: Doubts raised as to closeness of the observations for the -purpose of comparison.] - -The coincidences relied on in the foregoing observations depend, of -course, upon (1) the accuracy of the observations themselves, and -(2) the subsequent reduction of the lines for comparison. Assuming -the correctness of the latter, what have we as to the former? Two of -Professor Young’s positions of coronal lines, as stated, seem to have far -too much of the ± element to make them sufficiently accurate. Pierce’s -auroral observation does not state how the lines were positioned. As -they _all_ end with a cypher, the suspicion naturally arises that the -measurements did not extend beyond the first three places of the figures, -and, if so, could not be used for accurate comparison. The auroral lines, -too, are generally rather wide and nebulous, and not easy of comparison -with sharper ones. - - - - -CHAPTER IX. - -SUPPOSED CAUSES OF THE AURORA. - - -[Sidenote: Supposed causes of the Aurora. Sulphurous vapours. Magnetic -effluvia.] - -At first the Aurora was described to be sulphurous vapours issuing from -the earth; and Musschenbroek pointed out that certain chemical mixtures -sent forth a phosphorescent vapour, in some respects resembling the -Aurora. Dr. Halley originally proposed a similar theory, but ultimately -concluded that the Aurora might be occasioned by the circulation of the -magnetic effluvia of the earth from one pole to another. - -[Sidenote: Zodiacal light.] - -M. de Mairan, in 1721, in a treatise, ascribed the Aurora to the impulse -of the zodiacal light upon the atmosphere of the earth. - -[Sidenote: Luminous particles of our atmosphere.] - -Euler combated this theory, and ascribed the Aurora to the luminous -particles of our atmosphere driven beyond its limits by the light of the -sun, and sometimes ascending to the height of several thousand miles. - -[Sidenote: Electric fluid _in vacuo_ resembles Aurora.] - -Mr. Hawksbee very early showed that the electric fluid assumes, _in -vacuo_ or in highly rarefied atmosphere, an appearance resembling the -Aurora. Mr. Canton contrived an imitation of the Aurora by means of -electricity transmitted through the Torricellian vacuum in a long glass -tube, and showed that such a tube would continue to display strong -flashes of light for 24 hours and longer without fresh excitation. - -[Sidenote: Experiment with electrical machine and exhausted receiver.] - -In the ‘Edinburgh Encyclopædia,’ date 1830, is mentioned an experiment -in which an electrical machine and air-pump are so disposed that strong -sparks pass from the machine to the receiver of the air-pump. - -[Sidenote: Dr. Franklin’s theory.] - -As the exhaustion proceeds the electricity forces itself through the -receiver in a visible stream, at first of a deep purple colour; “but as -the exhaustion advances it changes to blue, and at length to an intense -white, _with which the whole receiver becomes completely filled_.” [It -will be noticed that this experiment bears a close resemblance to Prof. -Ångström’s exhausted flask referred to later in treating of the spectrum -of the Aurora.] - -Dr. Franklin gave a different form to the electric theory of the Aurora, -supposing that the electricity which is concerned in the phenomenon -passes into the Polar regions from the immense quantities of vapour -raised into the atmosphere between the tropics (Exper. and Observ. 1769, -p. 43). - -[Sidenote: Mr. Kirwan’s theory.] - -Mr. Kirwan (Irish Trans. 1788) supposed that the light of the Aurora -Borealis and Australis was occasioned by the combustion of inflammable -air kindled by electricity. - -[Sidenote: Mons. Monge’s.] - -Mons. Monge proposed the theory that the Auroræ were merely clouds -illuminated by the sun’s light falling upon them after numerous -reflections from other clouds placed at different distances in the -heavens (Leçons de Physique par Prejoulz, 1805, p. 237). - -[Sidenote: Mons. Libes’.] - -Mons. Libes propounded a theory that the electric fluid, passing through -a mixture of azote and oxygen, produced nitric acid, nitrous acid or -nitrous gas, and that these substances, acted upon by the solar rays, -would exhibit those red and volatile vapours which form the Aurora -Borealis (Traité de Physique, ou Dictionnaire de Physique, par Libes; -Rozier’s Journal, June 1790, February 1791, and vol. xxxviii. p. 191). - -[Sidenote: Mr. Dalton’s.] - -Mr. Dalton considered the Aurora a magnetic phenomenon whose beams were -governed by the magnetism of the earth. He observed that the luminous -arches were always perpendicular to the magnetic meridian (Dalton’s -Meteorological Observations and Essays, 1793, pp. 54, 153). - -[Sidenote: Abbé Bertholon’s.] - -The Abbé Bertholon ascribed the Aurora Borealis to a phosphorico-electric -light (Encyc. Méthod. art. Auroræ). - -[Sidenote: Dr. Thompson concluded the arches to be an optical deception.] - -Dr. Thompson (Annals of Philosophy, vol. iv. p. 429), from the -observations of Mr. Cavendish and Mr. Dalton, concluded there was no -doubt that the arched appearance of the Aurora was merely an optical -deception, and that in reality it consisted of a great number of straight -cylinders parallel to each other and to the dipping-needle at the place -where they were seen. - -[Sidenote: Artificial Auroræ produced in exhausted tubes.] - -With many of us (at least it was so in my own case) our first viewed -Auroræ have been artificial ones, devised by electricians and having -their locus at the Royal Polytechnic in Regent Street or in some -scientific lecture-room. The effects in these cases are produced in tubes -nearly exhausted by means of an air-pump, and then illuminated by some -form of electric or galvanic current. - -[Sidenote: Tubes described.] - -In one instance the tube is usually of the form shown on Plate X. fig. -9, supported on a base with a brass ball electrode at the lower end, and -a pointed wire at the upper. In another case the tube is of the form -shown on same Plate, fig. 8. After exhaustion it is permanently closed, -the current passing through it by means of the platinum-wire electrodes -introduced into each end of the tube. The first form of tube is usually -excited by a frictional plate machine; the second by a galvanic current -from a Grove or bichromate battery, which, by the aid of a Ruhmkorff -coil, has had its character changed from quantity to intensity. In each -instance, upon connexion with the source supply of the electric current, -a very similar effect is produced. - -[Sidenote: Effects described.] - -Brilliant streams of rose-coloured light pass between the electrodes, -sometimes as a single luminous misty band, sometimes in divided vibrating -sprays or streams, and sometimes in a flaky column of striæ. - -All this, before the spectroscope took its part in the investigation, we -were content to accept as a very fair and probable explanation of the -Aurora accompanied by a mimic representation of the phenomenon. - -These appearances may, of course, be produced at will in tubes having -electrodes; but it is, moreover, possible to produce them, though with -less effect, in certain other forms of tube having no such direct -communication with the external electric machine. - -One electrode only may be connected with the coil or electrical machine. -The appearance is then a faint representation of what happens when the -current entirely passes (but see experiments with a single wire detailed -in Part III.). - -[Sidenote: Tube without electrodes.] - -In the case of an exhausted tube having no electrodes, the wires from -the coil may be made into a little helix and placed at each end of the -tube, and the induced currents within will show themselves in flashes and -streams of light, varying in colour and tint according to the gaseous or -other contents of the tube. - -[Sidenote: Tube excited by friction.] - -In some cases the ordinary forms of galvanic or electrical machine for -supplying the current of electricity may be dispensed with. A long -straight tube exhausted and closed at each end, and without electrodes, -Plate X. fig. 6, being slightly warmed and then excited by friction with -the dry hand or a piece of flannel, silk handkerchief, or the like, is -soon filled with the most brilliant flashes of light playing in the -interior, and when once thoroughly charged needs but little further -excitation to keep up the effect. - -[Sidenote: Geissler’s mercury tube.] - -Geissler has introduced a form of tube in which electricity in its form -of flashes and glow of light is produced by the friction of mercury. The -outer tube is strong, and contains within it a smaller tube of uranium -glass with balls blown upon it (Plate X. fig. 7). The tubes are exhausted -and a small quantity of mercury is introduced which has access to both -surfaces of the inner tube, as well as to the inner surface of the outer -tube. Upon the tube being reversed end for end or shaken, the mercury -runs up or down the tube and causes a very considerable display of -whitish light. - -The before-described tubes are also referred to, and their spectra -described, in the section “On the comparison of some tube and other -Spectra with the Aurora” (Part II.). - -The aura or brush from the electrical machine has been considered as -resembling the Aurora, while the hissing and crackling accompanying it -has been supposed to corroborate the reports of similar noises having -been heard during an auroral display. - -[Sidenote: Prof. Lemström’s instrument to demonstrate the nature of -Auroræ.] - -Prof. Lemström, of the University of Helsingfors, has devised an -instrument for the purpose of demonstrating that Auroræ are produced by -electrical currents passing through the atmosphere. An illustration of -this instrument (for which I am indebted to the Editor of ‘Nature’) is -introduced (fig. 1). - -The instrument was exhibited at the recent Scientific Loan Collection at -South Kensington, and a full description of it, together with an essay by -Prof. Lemström, “On the Theory of the Polar Light,” will be found in the -third edition of the Official Catalogue, p. 386. no. 1751. The apparatus -is intended to show that an electric current passing from an insulated -body does not produce light in air of normal pressure; but as it rises to -the rarefied air in the Geissler tubes a phenomenon very like the real -Polar Light is produced. - -[Illustration: Fig. 1.] - -A is an electrical machine, the negative pole being connected with a -copper sphere and the positive with the earth. - -_s s´_ is of ebonite as well as R R _d_, so that B is quite insulated as -the earth is in space. B is surrounded by the atmosphere. _a´ a´ a´ a´ -a´ a´_ are a series of Geissler tubes with copper ends above and below. -All the upper ends are connected with a wire which goes to the earth; -consequently a current runs in the direction of the arrows through the -air, and the Geissler tubes become luminous when the electrical machine -is set into operation. - -The Geissler tubes represent the upper part of the atmosphere which -becomes luminous when the Aurora Borealis is observed in the northern -hemisphere. The phenomena produced by the Lemström apparatus are -considered consistent with the theory advocated by Swedish observers that -electrical currents emanating from the earth and penetrating into the -upper regions produce Auroræ in both hemispheres. The experiment differs -from the apparatus of M. de la Rive, who placed his current _in vacuo_, -and did not show the property of ordinary atmospheric air, in allowing to -pass unobserved, at the pressure of 760 millims., a stream of electricity -which illuminates a rarefied atmosphere. - -[Sidenote: M. de la Rive’s apparatus described.] - -De la Rive’s apparatus was also exhibited at the same time, and will be -found described at p. 385 of the Catalogue, No. 1749. A large sphere of -wood represented the earth, and iron cylinders the two extremities of the -terrestrial magnetic axis. These penetrated into two globes filled with -rarefied air, simulating the higher regions of the Polar atmosphere. The -electric discharge turned around a point situate in the prolongation of -the axis, in a different direction at either pole, when the two cylinders -were charged by means of a horseshoe electro-magnet, in accordance with -observations on the rotation of the rays of the Aurora. - -[Sidenote: De la Rive’s magnet in an electric egg.] - -De la Rive placed an electro-magnet in an electric egg. As soon as the -magnet was set in action the discharge which had before filled the egg -was concentrated into a defined band of light, which rotated steadily -round the magnet. - -[Sidenote: Gassiot’s experiment with 400 Grove cells and exhausted -receiver between poles of magnet.] - -Gassiot describes an experiment with his great Grove’s battery of 400 -cells, in which an exhausted receiver was placed between the poles of the -large electro-magnet of the Royal Institution. - -“On now exciting the magnet with a battery of 10 cells, effulgent strata -were drawn out from the positive pole, and passed along the under or -upper surface of the receiver according to the direction of the current. - -“On making the circuit of the magnet and breaking it immediately, the -luminous strata rushed from the positive pole and then retreated, cloud -following cloud with a deliberate motion, and appearing as if swallowed -up by the positive electrode.” Mr. Marsh considered this bore a very -considerable resemblance to the conduct of the auroral arches, which -almost invariably drift slowly southward. - -[Sidenote: Mr. Marsh considered the Aurora an electric discharge between -the magnetic poles of the earth.] - -He considered it probable that the Aurora was essentially an electric -discharge between the magnetic poles of the earth, leaving the immediate -vicinity of the north magnetic pole in the form of clouds of electrified -matter, which floated southward, bright streams of electricity suddenly -shooting forth in magnetic curves corresponding to the points from -which they originated, and then bending southward and downward until -they reached corresponding points in the southern magnetic hemisphere, -and forming pathways by which the electric currents passed to their -destination; and, further, that the magnetism of the earth caused these -currents and electrified matter composing the arch to revolve round the -magnetic pole of the earth, giving them their observed motion from east -to west or from west to east. - -[Sidenote: Varley’s observation on a glow-discharge _in vacuo_. Spark -surrounded by an aura which could be separated.] - -Varley showed that when a glow-discharge in a vacuum tube is brought -within the field of a powerful magnet, the magnetic curves are -illuminated beyond the electrodes between which the discharge is taking -place, as well as in the path of the current, and also thought that this -illumination was caused by moving particles of matter, as it deflected a -balanced plate of talc on which it was caused to infringe. It has also -been shown that in electrical discharges in air at ordinary pressure, -while the spark itself was unaffected by the magnet, it was surrounded by -a luminous cloud or aura which was driven into the magnetic curve, and -which might also be separated from the spark by blowing upon it. - -Most of the foregoing interesting results and experiments will be found -repeated and verified in Part III. - - -_Prof. Lemström’s Theory._ - -[Sidenote: Prof. Lemström’s theory.] - -Prof. Lemström thinks that terrestrial magnetism plays only a -comparatively secondary part in the phenomena of the Polar Light, this -part consisting essentially in a direct action upon the rays. - -That the experiments of M. de la Rive do not all furnish the proof that -the rays of the light are really united under this influence. - -[Sidenote: Character of the Polar Light.] - -That the Polar Light considered as an electrical discharge gives the -following results:— - -(1) An electric current arising from the discharge itself, which takes -place slowly. - -(2) Rays of light consisting of an infinite number of sparks, each spark -giving rise to two induction currents going in opposite directions. - -(3) A galvanic current going in an opposite direction to that of the -discharge, and having its origin in the electromotive force discovered -by M. Edlund in the electric spark. That these currents require a closed -circuit; but this is not necessary in the case of the Aurora, as the -earth and rarefied air of the upper regions are immense reservoirs of -electricity producing the same effect as if the circuit were closed. That -permanent moisture in the air, a good conductor of electricity, is the -cause of a slow and continuous discharge assuming the form of an Aurora, -instead of suddenly producing lightning as in equatorial regions and mean -latitudes. - -[Sidenote: Polar Light due to electric discharges only.] - -He sums up, that the electric discharges which take place in the Polar -regions between the positive electricity of the atmosphere and the -negative electricity of the earth are the essential and unique cause -of the formation of the Polar Light, light the existence of which is -independent of terrestrial magnetism, which contributes only to give to -the Polar Light a certain direction, and in some cases to give it motion. - -This Prof. Lemström maintains contrary to those who believe they see -in terrestrial magnetism, or rather in the induction currents, what is -capable of developing the origin of the Polar Light. - - -_Theories of MM. Becquerel and De la Rive._ - -[Sidenote: Theories of MM. Becquerel and De la Rive.] - -M. Becquerel’s theory is that solar spots are cavities by which hydrogen -and other substances escape from the sun’s protosphere. That the hydrogen -takes with it positive electricity which spreads into planetary space, -even to the earth’s atmosphere and the earth itself, always diminishing -in intensity because of the bad conducting-power of the successive layers -of air and of the earth’s crust. That would then only be negative, as -being less positive than the air. The diffusion of electricity through -planetary space would be limited by the diffusion of matter, since it -cannot spread in a vacuum. That gaseous matter extends further than the -limits usually assigned to the earth’s atmosphere, is proved by the -observation of Auroræ at heights of 100 and 200 kilometres, where some -gaseous matter must exist. M. de la Rive agrees with M. Becquerel as to -the electric origin of the Aurora, but considers the earth is charged -with negative electricity and is the source of the positive atmospheric -electricity, the atmosphere becoming charged by the aqueous vapour rising -in tropical seas. The action of the sun he considers is an indirect one, -varying with the state of the sun’s surface, as shown by coincidences in -the periods of Aurora and sun-spots. - - -_M. Planté’s Electric Experiments._ - -[Sidenote: M. Planté’s experiments. Effects produced resembling Auroræ.] - -M. Planté has performed some experiments with a very considerable series -of secondary batteries. By inserting the positive electrode after the -negative in a vessel of salt water, luminous and other effects were -observed which were considered to have a strong resemblance to those of -Auroræ. - -M. Planté advocates the theory that the imperfect vacuum of the upper -regions, acting like a large conductor, plays the part of the negative -electrode in his experiments, while the positive electricity flows -towards the planetary spaces, and not towards the ground, through the -mists and ice-clouds which float above the Poles. - -[Sidenote: M. Planté’s experiments producing a corona, an arc, or a -sinuous line.] - -In an article in ‘Nature,’ March 14, 1878, a further account is given -of M. Planté’s experiments, under the head of “Polar Auroræ;” and it is -stated that, in these experiments, the electric current, in presence -of aqueous vapour, yielded a series of results altogether analogous to -the various phases of Polar Auroræ. If the positive electrode of the -secondary battery was brought into contact with the sides of a vessel -of salt water, there was observed, according to the distance of the -film (electrode?) from the liquid, either a corona formed of luminous -particles arranged in a circle round the electrode (fig. 2, p. 90), an -arc bordered with a fringe of brilliant rays (fig. 3), or a sinuous line -which rapidly folded and refolded on itself (fig. 4). This undulatory -movement, in particular, formed a complete analogy with what had been -compared in Auroræ to the undulations of a serpent, or to those of -drapery agitated by the wind. The rustling noise accompanying the -experiment was analogous to that sometimes said to accompany Auroræ, and -was caused by the luminous electric discharge penetrating the moisture. -As in Auroræ, magnetic perturbations were produced by bringing a needle -near the circuit, the deviation increasing with the development of the -arch. - -The Auroræ were produced by positive electricity, the negative electrode -producing nothing similar. - -Illustrations of these miniature Auroræ are given in ‘Nature,’ and -reproduced on p. 90. No mention of any spectroscopic observations is made. - -[Sidenote: Mr. Holden’s views.] - -In a communication to the Metropolitan Scientific Association -(‘Observatory,’ March 1, 1879, p. 389), Mr. A. P. Holden, after -supporting the theory of a connexion between the waxing and waning of the -solar corona and sun-spots, adopts Mr. F. Pratt’s hypothesis “that the -Aurora is simply light filmy cirrus cloud, first deposited at the base -of a vast upper body of highly rarefied vapour, and illuminated by the -free electricity escaping in the condensation through the very rarefied -medium above, towards the north or south. The Aurora would, according to -this theory, have its origin in a vast electrical storm, resulting from -a violent condensation of vapour which causes a flow of electricity from -the pole to restore equilibrium.” The Aurora would thus, in Mr. Holden’s -opinion, “depend on storm phenomena of an intense character; and the -frequency of Auroræ at the sun-spot maxima would indicate the connexion -of the latter with the weather.” - -[Illustration: Fig. 2. The corona. - -Fig. 3. The arc and rays. - -Fig. 4. The sinuous line.] - - - - -PART II. - -THE SPECTRUM OF THE AURORA. - - - - -CHAPTER X. - -SPECTROSCOPE ADAPTED FOR THE AURORA. - - -[Sidenote: Must be of moderate dispersion, with ready mode of measuring -line-positions.] - -Any form of spectroscope of moderate dispersion will suffice for -observations of the spectrum of the Aurora; but, for sake of convenience, -a hand or direct-vision spectroscope is to be preferred, and it is -desirable also to have some quick and ready mode of measuring the -position of the lines while the Aurora lasts. - -[Sidenote: Mr. Browning’s instrument described.] - -Mr. John Browning arranged for me a form of instrument which I have -found very convenient for observations by hand of the Aurora-lines, and -also, when fixed on a stand, for tube and chemical investigations. A -representation of this instrument is given on Plate X. fig. 1. A brass -tube carries a large compound (5) direct-vision prism (shown dark in the -drawing). An arrangement is made so that a second prism can at will be -slipped into the tube (shown in outline in the drawing). With one prism -and a fine slit the D lines are widely separated, and the field of view -extends at one glance from near C to near G. When the second prism is -inserted and used in combination, the nickel line can be seen between the -two D lines, and the instrument may be used for solar work. A photograph -of the sun’s spectrum, taken with one prism only, shows a great number -of the dark solar lines and many of the bright ones, ascribed by Prof. -Draper to oxygen and nitrogen. - -[Illustration: Plate X.] - -[Sidenote: Diaphragm micrometer described. Mode of use of the micrometer.] - -The collimator and observing telescope are respectively 6 inches in -length, and carry achromatic lenses of one inch aperture. The telescope -traverses the field so that the extremities of the spectrum may be -observed. The dispersion of the instrument was ascertained by a set -of observations of the principal solar and some metallic lines, made -with an excellent filar micrometer. For the Auroral observations, Dr. -Vogel has described an instrument (see Appendix E) in which the usual -spider’s-web wires are replaced by a needle-point, as being easily seen -upon a faint spectrum. Illuminated wires may also be used; but I was led -ultimately to employ, in preference, a diaphragm micrometer which the -spectrum itself illuminates, as being adapted for speedy, yet fairly -accurate, observations. It was made in this manner:—A card was first of -all prepared (Plate X. fig. 2), and within a circle described on this, -a scale was drawn of moderately wide white spaces, with black divisions -between, short and long, so as to read off easily by eye. The upper half -of the circle was then entirely filled in with black; and from the card -as thus prepared a reduced negative photograph was made. In this the -spaces and lower half of the circle were opaque, and the upper half of -the circle and the lines between the spaces were transparent (Plate X. -fig. 4). This photograph was about the size of a shilling (fig. 3, same -Plate). It was mounted carefully in Canada balsam, with a thin glass -cover, and then placed in the focus of the eyepiece. In use, the spectrum -is brought upon the scale so that the upper half shows above the scale -without any interruption at all; while the lower half illuminates the -scale and renders the divisions visible, showing the spectrum-lines -falling either upon them or the spaces between. The photographed scale -was next enlarged to a considerable size and printed upon faintly ruled -paper; and the enlargement was so arranged as to comprise five of the -faint ruled lines between each division of the scale. Each of these faint -lines in turn represented a certain portion of the spectrum as read off -with the filar micrometer; so that the scales as constructed with the -filar micrometer and with the photographed micrometer corresponded for -all parts of the spectrum included in the field of the eyepiece. - -[Sidenote: Advantage of the method.] - -One of the photographed enlargements being laid on the table under the -spectroscope, the observed lines were marked off with ease and accuracy -upon it; and as the photograph was an exact copy of the scale, any want -of exactitude in the divisions was of no moment. - -One great advantage of this method was, that all the lines seen could -be recorded at one time and with all in view, and without the risk of -slight shift in the instrument, which frequently happens when lines are -read off seriatim. - -I found this plan most effective for the rapid and correct recording of -a faint and evanescent spectrum, and it gave close results when compared -with traversing-micrometer measured spectra. The records, too, admitted -of subsequent examination at leisure. - -[Sidenote: Double-slit plate arrangement.] - -Mr. Browning subsequently constructed for me a double-slit plate (lately -in the Scientific Loan Collection at South Kensington) for the same -instrument (Plate X. fig. 5). The lower half of the plate is fixed. The -upper half traverses the lower by the aid of a micrometer-screw. The slit -is widened or closed at pleasure by loosening the small screws by which -the jaw-plates are attached. A scale is engraved on the fixed lower half -of the plate for an approximate measurement, while the division of the -micrometer-screw-head completes it. - -In use, one half of the spectrum slides along the other, and a bright -line in the upper spectrum is selected as an index. The distances between -the lines of the lower half of the spectrum are read off by means of the -bright line above. This form of micrometer was suggested by Mr. Procter -(in ‘Nature’) many years ago as a substitute for a more complicated -apparatus by Zöllner. Other instruments on a similar principle have been -lately introduced, but for Aurora purposes I prefer a fixed scale. - -[Sidenote: Photographed spectrum suggested.] - -In ‘Photographed Spectra’ I have pointed out that we shall probably -obtain no spectrum of the Aurora to be absolutely depended upon for -comparison with other spectra until we succeed in a photographed one. -From experiments made with a special prism of the Rutherfurd form, -constructed for me by Mr. Browning (with which many gas-spectra have -been already photographed), I see no reason, should an unusually bright -Aurora favour us with a visit, why its spectrum may not be recorded in a -permanent form, and with lines sufficiently well marked to be compared -with other spectra. Rapid dry plates would be especially useful for such -a purpose, and some Auroræ, if wanting in brilliancy, would doubtless -compensate by their period of endurance. - -[Sidenote: Mr. Hilger’s half-prism spectroscope.] - -Mr. Adam Hilger has also made for me one of his “half-prism” -spectroscopes, in which considerable dispersion is obtained with but very -little loss of light. This instrument has a simple and rapid micrometer -arrangement, with a bright line as an index. I have (for want of Auroræ) -had no opportunity of trying it, but I doubt not it is well adapted for -such a purpose. - - -_Spectrum of the Aurora described._ - -[Sidenote: Lines or bands and continuous spectrum.] - -The spectrum of the Aurora consists of a set of lines or bands upon a -dark ground at each extremity of the spectrum, but with more or less of -faint continuous spectrum towards the centre. The extreme range of the -spectrum, as observed up to the present time, is from “_a_” (between C -and D) in the red to “_h_” (hydrogen) in the violet. - -[Sidenote: Lines nine in number.] - -The lines have been classified and arranged by Lemström and others as -nine in number, but I believe not more than seven have ever been seen -simultaneously. - -The author of the article “Aurora Polaris,” in the ‘Encyclopædia -Britannica,’ classes the lines as nine, and gives a table with the -following results (to these I have added Herr Vogel’s lines, for the -purpose of identification and comparison):— - -[Sidenote: Table from Encyc. Brit.] - - +------+-------------+------+--------+-------+ - |No. of| Number of | Mean |Probable|Vogel’s| - |line. |observations.| W.L. | error. |lines. | - +------+-------------+------+--------+-------+ - | 1. | 5 | 6303 | ± 8·1 | 6297 | - | 2. | 10 | 5569 | ± 2·9 | 5569 | - | 3. | 4 | 5342 | ±16 | 5390 | - | 4. | 6 | 5214 | ± 5·4 | 5233 | - | 5. | 4 | 5161 | ± 9·7 | 5189 | - | 6. | 6 | 4984 | ±11 | 5004 | - | 7. | 4 | 4823 | ± 9·3 | | - | 8. | 8 | 4667 | ± 9·8 | 4663 | - | 9. | 8 | 4299 | ± 9·3 | | - +------+-------------+------+--------+-------+ - -The probable errors are large, and it is a question whether any thing is -gained by thus endeavouring to average the lines. - -[Sidenote: Ångström’s line. Zöllner’s line in the red. Other Lines of the -spectrum.] - -The principal and brightest line, in the yellow-green, is generally -called “Ångström’s,” and his (probably the first) measurement of its -position at 5567 adopted. This was in the winter of 1867-68, and he saw -in addition, by widening the slit, traces of three very feeble bands -situated near to F. Zöllner is credited with the first observation of -the line in the red. These two lines are generally described as with -similar characteristics, and in about the same respective positions, by -all observers, and have never been remarked to spread into bands. The -other lines in the spectrum are difficult to position, owing to the many -discordant observations of them. They seem also variable in intensity -as well as in number (sometimes even in the same Aurora), and are not -unfrequently observed to have their places supplied by bands. - -[Sidenote: Second German expedition observations. Austro-Hungarian.] - -The spectroscope was used in the second German expedition, but only the -one brightest line seen—Dr. Börgen stating he had never seen a trace of -the weak lines in the blue and red, which were observed so distinctly -with the same spectroscope on 25th October, 1870, after the return of the -expedition. Lieutenant Weyprecht used a small spectroscope during the -Austro-Hungarian Expedition, and saw only the well-known yellow-green -line. - -[Sidenote: Swedish expedition, 1868. Lemström’s observations.] - -In the Swedish Expedition, 1868, Lemström mentions that in the -Aurora spectrum there are nine lines (he does not say he saw them -simultaneously), which he considers to agree with lines belonging to the -air-gases. He also thinks the Aurora could be referred to three distinct -types, depending on the character of the discharge. - -[Sidenote: Spectrum or Aurora seen at Tronsa.] - -At Tronsa an Aurora was seen October 21st, 1868, which commenced in the -north and became very brilliant. The spectroscope showed:— - -1. A yellow line at 74·9. - -2. A very clear line in the blue at 65·90. - -3. Two lines of hair’s breadth, with very pronounced (horizontal?) striæ -on the side of the yellow, one at 125 and the other about 105. - -[I presume the striæ were really vertical, and that the explanation -intended to convey that these lines shaded off towards the yellow. From -a comparison of the figures they must have been in the red, and are the -only instance recorded of two auroral lines in that region. They are -subsequently spoken of as “shaded rays.”—J. R. C.] - -[Sidenote: MM. Wijkander and Parent’s observations.] - -M. Auguste Wijkander and Lieut. Parent, of the Swedish Expedition in -1872-73, under Professor Nordenskiöld, used a direct-vision spectroscope, -with a micrometer-screw movement of the prisms, the reading being -afterwards reduced to wave-lengths upon Ångström’s line-values. - -The following Table gives the results, with Dr. Vogel’s lines added for -the sake of comparison:— - - +------+------------------------+-----------------------+-------+------+ - | |Observations, Wijkander.| Observations, Parent. | | | - | |-------+-----+----------+-------+-----+---------|Mean of| | - |Lines.| | | Probable | | |Probable | both. |Vogel.| - | |Number.|W.L. | error. |Number.|W.L. | error. | | | - +------+-------+-----+----------+-------+-----+---------+-------+------+ - | .. | .. | .. | .. | .. | .. | .. | .. | 6297 | - | .. | .. | .. | .. | .. | .. | .. | .. | 5569 | - | (1) | 5 | 5359| ±3 | .. | .. | .. | 5359 | 5390 | - | (2) | 6 | 5289| ±5 | 3 | 5280| ± 1 | 5286 | .. | - | (3) | 6 | 5239| ±4 | 2 | 5207| ±11 | 5231 | 5233 | - | .. | .. | .. | .. | .. | .. | .. | .. | 5189 | - | (4) | 5 | 4996| ±9 | .. | .. | .. | 4996 | 5004 | - | (5) | 1 | 4871| .. | 1 | 4873| .. | 4872 | .. | - | (6) | 8 | 4692| ±2 | 10 | 4708| ± 5 | 4701 | 4663 | - | (7) | 1 | 4366| .. | .. | .. | .. | 4366 | .. | - | (8) | 4 | 4280| ±3 | 3 | 4286| ±16 | 4284 | .. | - +------+-------+-----+----------+-------+-----+---------+-------+------+ - -The brightest line in all Auroræ, 5567, was intentionally not included in -the Tables. The red line was not seen. Nos. 5 and 7 were only seen once, -and not in the same Aurora. - -[Sidenote: Spectrum of Aurora of October 24th, 1870.] - -The Aurora of October 24th, 1870, came at a time when spectroscopes of a -direct-vision form were being introduced, and a number of observations -were communicated at the time to ‘Nature.’ - -[Sidenote: T. F.’s observations. W. B. Gibbs’s observation. Elger’s -observation.] - -A correspondent, T. F., writing from Torquay, saw, with a direct-vision -spectroscope, one strong red line near C, one strong pale yellow line -near D, one paler near F, and a still paler one beyond, with a faint -continuous spectrum from about D to beyond F. The C line was very -conspicuous and the brightest of the whole. It was intermediate in -position and colour to the red lines of the lithium and calcium spectra. -Plainly there were two spectra superposed, for while the red portions of -the Aurora showed the four lines with a faint continuous spectrum, the -greenish portions showed only one line near D on a faint ground. W. B. -Gibbs saw, in London, only two bright lines, one a greenish grey, situate -about the middle of the spectrum, and the other a red line very much like -C (hydrogen). Thomas G. Elger, at Bedford, on the 24th and 25th, saw:—(1) -a broad and well-defined red band near C; (2) a bright white band near D -(same as Ångström’s W.L. 5567), on 25th visible in every part of the sky; -(3) a faint and rather nebulous line, roughly estimated to be near F; (4) -a very faint line about halfway between 2 and 3. The red band was absent -from the spectrum of the white rays of the Aurora, but the other lines -were seen. - -[Sidenote: J. R. Capron’s observation.] - -With a small Browning direct-vision spectroscope on the 24th, I found -no continuous spectrum, but two bright lines, one in the green (like -that from the nebulæ, but more intense, and considerably flickering), -the other in the red (like the lithium line, but rather duskier: Plate -V. fig. 6). The latter was only well seen when the display was at its -height; it could, however, be faintly traced wherever the rose tint of -the Aurora extended. The line in the green was well seen in all parts of -the sky, but was specially bright in the Auroral patches of white light. - -[Sidenote: Mr. Browning’s observation. Alvan Clarke’s, jun., -observations.] - -Mr. Browning also saw the red line, but found comparison difficult. On -the evening of the 24th October, Mr. Alvan Clarke, jun., at Boston, -used a chemical spectroscope of the ordinary form, with one prism and a -photographed scale illuminated with a lamp. Four Auroral lines were seen -at points of his scale numbered 61, 68, 80, and 98. These were reduced to -wave-lengths by Professor Pickering, with the following results:— - - +-----+---------+--------+-------+-------------------+--------+ - |Line.| Reading | Wave- |Assumed| Comments. |Probable| - | |on scale.|lengths.| line. | | error. | - |-----+---------+--------+-------+-------------------+--------| - | 1. | 61 | 5690 | 5570 |Common Aurora-line.| -20 | - | 2. | 68 | 5320 | 5316 | Corona line. | + 1 | - | 3. | 80 | 4850 | 4860 | F, hydrogen. | - 3 | - | 4. | 98 | 4350 | 4340 | G, hydrogen. | + 6 | - +-----+---------+--------+-------+-------------------+--------+ - -[61 is evidently wrong, and was probably a mistake for 63.] - -[Sidenote: G. F. Barker’s observations.] - -George F. Barker, observing at New Haven (U.S.A.), saw, on November 9th, -a crimson and white Aurora, which he examined with a single glass-prism -spectroscope, by Duboscq, of Paris. The line positions were obtained by -an illuminated millimetre scale. In the white Aurora were four lines (the -red one being absent); in the red Aurora five. The wave-lengths of the -Aurora-lines were run out as follows:— - - (1.) Between C and D, 6230 (Zöllner’s 6270). - (2.) ” D and E, 5620 (Ångström’s 5570). - (3.) ” E and _b_, 5170 (Winlock’s 5200). - (4.) ” _b_ and F, 5020. - (5.) ” F and G, 4820 (Alvan Clarke’s, jun., 4850). - -[Sidenote: Spectrum of Aurora of Feb. 4, 1872. Prof. Piazzi Smyth’s -observations.] - -Mr. Procter’s Aurora-lines will be found noticed in connexion with the -spectrum of oxygen; and Lord Lindsay’s lines, with a comparison scale -drawing, are separately described further on in this Chapter. The Aurora -of February 4th, 1872, had many observers; some of whom communicated -at the time spectroscopic notes. Professor Piazzi Smyth minutely -describes the display as seen in Edinburgh, and saw “Ångström’s green -Aurora-line perpetually over citron acetylene[13] at W.L. 5579, and the -red Aurora-line between lithium _a_ and sodium _a_, but nearer to the -latter, say at W.L. 6370.” Extremely faint greenish and bluish lines also -appeared at W.L. 5300, 5100, and 4900 nearly. - -[Sidenote: Rev. T. W. Webb’s observations.] - -The Rev. T. W. Webb, with a very fine slit, saw the green Auroral line -even in the light reflected from white paper. With a wider slit he saw a -crimson band in the brighter patches of that hue, and beyond an extent of -greenish or bluish light, which he suspected to be composed of contiguous -bands. - -[Sidenote: R. J. Friswell’s observations.] - -R. J. Friswell, coming up the Channel at 9.40, with a Hoffman’s -direct-vision spectroscope (the observing telescope removed), saw the -green line, a crimson line near C, and faint traces of structure in the -blue and violet. - -[Sidenote: The Rev. S. J. Perry’s observations.] - -The Rev. S. J. Perry observed at Stonyhurst four lines, and, on examining -one of the curved streamers, found the red line even more strongly marked -than the green. A magnetic storm was observed to be at its height from 4 -to 9 P.M. of the same day. - -[Sidenote: J. R. Capron’s observations.] - -With a Browning 7-prism direct-vision spectroscope I saw the green line -in all parts of the Aurora, attended with a peculiar flickering movement. -I did not see the other lines. - -[Sidenote: His catalogue of lines up to Nov. 9, 1872.] - -In a letter to ‘Nature,’ dated November 9th, 1872, I catalogued the lines -observed up to that date as follows:— - -1. A line in the red between C and D. W.L., Ångström, 6279. - -2. A line (the principal one of the Aurora) in the yellow-green, between -D and E. W.L., Ångström, 5567. - -3. A line in the green, near E (corona line?). W.L., Alvan Clarke, jun., -and Backhouse, 5320. - -4. A faint line in the green, at or near _b_. W.L., Barker, 5170. - -5. A faint line or band in the green, between _b_ and F. W.L., Barker, -5020 (chromospheric?). - -6. A line in the green-blue, at or near F. W.L., Alvan Clarke, jun., 4850. - -7. A line in the indigo, at or near G. W.L., Alvan Clarke, jun., 4350. - -8. The continuous spectrum from about D to beyond F. - -[Sidenote: Dr. H.C. Vogel’s observations of Auroral lines. Spectrum -described.] - -Dr. H. C. Vogel, formerly of the Bothkamp Observatory, near Kiel, -and since of the Astrophysical Observatory, Potsdam, made several -observations of the Auroral lines, October 25th, 1870. Besides the bright -line between D and E, he found several other fainter lines stretching -towards the blue end of the spectrum on a dimly-lighted ground. February -11th, 1871, he observed the same set of lines, and an average of six -readings gave 5572 as the W.L. of the Ångström line. February 12th gave -5576 as Dr. Vogel’s reading, and 5569 as Dr. Lohse’s. April 9th gave -5569, and April 14th 5569. The Aurora of April 9th, 1871, was exceedingly -brilliant, so that micrometer measurements of the lines were taken. The -spectrum consisted of one line in the red, five in the green, and a -somewhat indistinct broad line or band in the blue. The lines are thus -described:— - -[Sidenote: Table of lines.] - -Table of Dr. Vogel’s lines. Aurora, April 9th, 1871. - - +-----+--------+------------------------------------------------+ - |W.L. |Probable| Remarks. | - | | error. | | - +-----+--------+------------------------------------------------+ - |6297 | 14 |Very bright stripe. } | - | | | } | - |5569 | 2 |Brightest line of the spectrum, } | - | | | became noticeably fainter at } On a faintly | - | | | appearance of the red line. } lighted | - | | | } ground. | - |5390 | .. |Extremely faint line; } | - | | | unreliable observation. } | - | | | } | - |5233 | 4 |Moderately bright. } | - | | | | - |5189 | 9 |This line was very bright when the red line | - | | | appeared at the same time; otherwise equal | - | | | in brilliancy with the preceding one. | - | | | | - |5004 | 3 |Very bright line. | - | | | | - |4694}| |{Broad band of light, somewhat less brilliant | - |4663}| 3 |{ in the middle; very faint in those parts of | - |4629}| |{ the Aurora in which the red line appeared. | - +-----+--------+------------------------------------------------+ - -A translation of Dr. Vogel’s interesting paper will be found printed _in -extenso_ in Appendix E, and his lithographed drawings of the spectrum in -the green and red portions of the Aurora respectively on Plate VI. figs. -2 and 3. The observations of April 9th by Dr. Vogel are probably, up to -the present time, the most exact of any one Aurora, and I have therefore -in most cases used them for comparison. - -[Sidenote: Mr. Backhouse’s catalogue of lines.] - -Mr. Backhouse, in a letter to ‘Nature,’ commenting upon my catalogue -of lines, gave the following as the latest determinations from his own -observations:— - - No. 1. Wave-length 6060 - 2. ” 5660 - 3. ” 5165 - 4. ” 5015 - 6. ” 4625 - 7. ” 4305 - -(6060 must be a mistake for 6260, and 5660 for 5560.—J. R. C.) Mr. -Backhouse never saw a line at 5320 again. He found the continuous -spectrum to reach from No. 2 to No. 7, being brightest from a little -beyond No. 2 to No. 6. This part of the spectrum did not give him so much -the idea of a true “continuous spectrum” as of a series of bright bands -too close to be distinguished. - -[Sidenote: Subsequent full catalogue of Auroral lines.] - -I have subsequently, in another section of this Chapter, added a full -catalogue of the Auroral lines, prepared by myself from the foregoing and -other sources and observations; and I also append to it a Plate [Plate -XII.], in which these lines are positioned and the wave-lengths and names -of observers are given. The numbers of the lines on the Plate correspond -with those in the catalogue. The solar spectrum and the spectrum of the -blue base of a candle-flame are added for purposes of comparison. [The -telluric bands in the solar spectrum are shown more distinctly than they -actually appear, and do not profess to give details.] - - -_Flickering of the Green Line._ - -[Sidenote: Flickering of the green line. Herschel’s observation. J. R. -Capron’s observation.] - -A. S. Herschel noticed this, April 9th (1871?). He says:—“A remarkable -circumstance connected with the appearance of the single line observed on -this occasion was the flickering and frequent changes with which it rose -and fell in brightness; apparently even more rapidly than the swiftly -travelling waves, or pulsations of light, that repeatedly passed over the -streamers, near the northern horizon, towards which the spectroscope was -directed.” In the spectrum of the Aurora of 20th October, 1870, I saw -and noted the green line as “considerably flickering;” and in the Aurora -of 4th February, 1872, I again saw and noted “the peculiar flickering” -I had remarked in 1870. I have not seen the peculiarity noted by other -observers. - - -_Mr. Backhouse’s graphical Spectra of four Auroræ._ - -[Sidenote: Mr. Backhouse’s graphical spectra of Auroræ.] - -Mr. Backhouse has been good enough to supply me with some details of -four several Auroræ seen by him at Sunderland, accompanied by drawings, -showing in a graphical way the spectrum of each display as seen with a -spectroscope with rather a wide slit and as drawn by eye. I have reduced -the four drawings to the same scale, and in this way they are extremely -interesting for comparison (Plate V. fig. 4). The line on the left in -each spectrum is Ångström’s bright Auroral line, and is supposed to be -considerably prolonged. The height of the lines denotes intensity. - -[Sidenote: April 18, 1873.] - -April 18th, 1873, was a bright Aurora. No. 3 is a faint band, which Mr. -Backhouse had not perceived before. No. 5 had not been visible lately, -and Mr. Backhouse thought it must belong to Auroræ of a different type -from those which had appeared latterly. - -[Sidenote: Feb. 4, 1874.] - -February 4th, 1874. In the spectrum of this Aurora Mr. Backhouse saw -seven lines, all that he had ever seen. (The red line, not shown in the -diagram, makes the seventh.) - -The spectrum is represented as seen between 6.50 and 7.5 P.M. Mr. -Backhouse had only once before seen No. 4, and it became quite invisible -between 7.45 and 7.55, though the other lines were as bright as before -and the red line had appeared. - -[Sidenote: Oct. 3, 1874.] - -October 3rd, 1874. This spectrum was examined, and diagram made between -10 and 10.25 P.M. Five lines only are indicated. - -It is mainly distinguished from the two preceding spectra by the -brightness of the continuous spectrum on which the lines 2, 3, and 4 lie, -and by the weakness of No. 5. - -[Sidenote: Oct. 4, 1874.] - -October 4th, 1874. Taken between 11.10 and 11.20 P.M.; distinguished, -like the last, by a considerable amount of continuous spectrum and by a -faint line (No. 3), not seen in the last spectrum, while No. 3 in the -last is missing in this spectrum. - -[Sidenote: Mr. Backhouse’s remarks as to comparative frequency of some of -the Auroral lines.] - -Mr. Backhouse, as to both these last spectra, remarks that the lines were -very variable in intensity, and sometimes some were visible and sometimes -others. They varied also in relative brightness in different parts of -the sky at the same time. Mr. Backhouse, in a communication to ‘Nature,’ -referring to a statement of Mr. Procter’s, that the bands of the Auroral -spectrum are seldom visible, except the bright line at 5570, says that -he always found two bands, “doubtless Winlock’s 4640 and 4310,” to be -invariably visible when the Aurora was bright enough to show them. Of -thirty-four Auroræ examined by Mr. Backhouse, fourteen showed the lines -4640 and 4310, and three others at least one of these, while eight showed -the red line. (Ångström only once saw this line.) In five Auroræ, all -more or less red, he saw a faint band, the wave-length of which he placed -at 5000 or 5100. He never saw the line 5320 (also Winlock’s coronal -line), unless it were once, probably from want of instrumental power. -With regard to these observations, I may say that with a Browning’s -miniature spectroscope I saw only two lines (the red and the green) in -the grand display of the 24th October, 1870; and with an instrument of -larger aperture the green line only on the 4th February, 1872; while I -saw the green line and three others towards the violet with the same -instrument during the Aurora of 4th February, 1874. (See description of -this Aurora, _antè_ p. 21, and drawing of spectrum, Plate VI. fig. 1 _a_.) - - -_Lord Lindsay’s Aurora-Spectrum, 21st October, 1870._ - -[Sidenote: Lord Lindsay’s Aurora of 21st Oct., 1870.] - -Lord Lindsay observed a fine Aurora at the Observatory at Dun Echt on the -night of the 21st October, 1870. It commenced about 9.30, reached its -maximum about 11, and faded away suddenly about 11.30 P.M. - -[Sidenote: Spectrum described.] - -A spectrum obtained in the north-west gave five bright lines with a -Browning’s direct-vision spectroscope—two strong, one medium, two very -faint. A tallow candle was used to obtain a comparison spectrum of sodium -and carburetted hydrogen. - -A drawing of the spectrum obtained is given on Plate XI. fig. 2. No. 1 is -a sharp well-formed line visible with a narrow slit. - -No. 2, a line very slightly more refrangible than F. The side towards D -is sharp and well defined, while on the other side it is nebulous. - -No. 3, slightly less refrangible than G, is a broad ill-defined band, -seen only with a wide slit. - -No. 4, a line near E, woolly at the edges, but rather sharp in the -centre. This, says Lord Lindsay, should be at or near the position of the -line 1474 of the solar corona. - -No. 5, a faint band, coincident with _b_, extending equally on both sides -of it. - -The lines are numbered in order of intensity. It is questionable, -from observations with instruments carrying a scale, whether the -line-positions are exact; but the description of their characters is -valuable. - -[Sidenote: Candle-spectrum.] - -As a candle blue-base spectrum is at times a ready and handy mode of -reference in Auroral observations (as was found in this instance), I -have, on Plate XI. fig. 5, given a representation of it as seen with my -Auroral spectroscope. Dr. Watts’s corresponding carbon-spectrum is added -on the lower margin. The numbers on the upper margin refer to my scale. - -[Illustration: Plate XI.] - - -_Spectrum of the Aurora Australis._ - -[Sidenote: Captain Maclear’s spectra of Aurora Australis.] - -Captain Maclear, on examining the streamer seen by him Feb. 9th, 1874 -(_antè_, p. 27), with the spectroscope, found three prominent lines in -the yellow-green, green, and blue or purple, but not the red line. In -the Aurora of March 3rd, 1874 (p. 27), he could trace four lines, three -bright and one rather faint. They must have been exceedingly bright to -show so plainly in full-moon light. - -[Sidenote: Instrument used, and mode of registering lines.] - -The spectroscope used was a Grubb single-prism with long collimator. A -needle-point in the eyepiece marked the position of the lines; and a -corresponding needle-point, carried on a frame by a screw movement in -concord with the point of the eyepiece, scratched the lines on a plate of -blackened glass. Two plates were taken. On the first were scratched the -auroral lines and the solar lines as seen in the moonlight; on the second -plate were scratched the auroral lines, the Solar lines from the moon, -and the carbon lines in a spirit-lamp. - -[Sidenote: Copies of the two spectra obtained. Discrepancy in the -spectra. Remarks on the spectra.] - -The next morning the solar lines were verified in sunlight. I subjoin -(Plate XI. fig. 3) copies of the two spectra as printed in ‘Nature,’ the -auroral lines being marked A, the solar lines by the usual designating -letters, and the carbon by _Car_. To these spectra I have added for -comparison Dr. Vogel’s spectrum of the Aurora Borealis. Captain Maclear -could not account for the different positions of the auroral lines in the -two plates; for the prism, as far as he was aware, was not moved during -the observations. As the solar lines are indicated in the same place in -both spectra, the case would seem one of actual change of position of the -auroral lines during observation. A comparison of the two spectra gives -the impression that the lower one is the same as the upper, except that -the dispersion is greater, the lines remaining relatively in position. -One does not, however, see how the dispersion could have so varied in a -single-prism instrument, and the position of the solar lines is adverse -to such an explanation. - -There is a suspicion that Auroræ are not always identical in position -of some of the lines; but the line in the green (considerably out of -place in the lower Australis spectrum) has always, within small limits, -the same position. It will be noticed how much further the Australis -spectrum runs into the violet than Vogel’s Borealis, the latter having no -lines much beyond F. - -The faint line (No. 2) mentioned by Captain Maclear possibly corresponds -with Dr. Vogel’s band. The absence of the four lines of the Aurora -Borealis in the green part of the spectrum of the Australis is peculiar; -and in this respect, too, the two Australis spectra agree. - -[Sidenote: Comparison of the lines.] - -The nearest approaches to Captain Maclear’s lines (of the upper spectrum) -which I can find are:— - - Line Corresponding line. - - 1. 5567, Ångström. - 2. (The faint line.) Vogel’s band, 4694-4629. - 3. I find no approximately corresponding line. - 4. 4350, Alvan Clarke. - -But the comparisons are not by any means close. Further observations of -the Australis spectrum are very desirable. - - -_Prof. Piazzi Smyth’s Aurora-Spectra._ - -[Sidenote: Prof. Piazzi Smyth’s chemical and auroral spectra.] - -Prof. Piazzi Smyth, in volume xiv. of the ‘Edinburgh Astronomical -Observations,’ 1870-77, has compared simultaneously the Aurora-spectrum -with the sets of bright lines seen in the blue base of flame—the lines of -potassium, lithium, sodium, thallium, and indium being also introduced -for comparison. The spectra are drawn as seen under small dispersion, -and will prove most useful in cases where an Aurora is not bright enough -to admit of the lines being measured by micrometer, and the eye and -comparison spectrum are obliged to be resorted to. - -[Illustration: Plate XII.] - - -_Author’s Catalogue of the Auroral Lines._ - -(See Plate XII.) - -1. W.L. 6297, Vogel. Very bright stripe; first noticed by Zöllner. Seen -only in red Auroræ; stands out on a dark ground, without other lines -near it. Character of line sharp and well defined; varies in colour from -dusky red to bright crimson. Intensity, Herschel, 0 to 4 or 8. According -to same, position coincident with atmospheric absorption-group “_a_” in -solar spectrum (between C and D). I confirm this position according to -my scale of solar lines, and a drawing of the coincidence (in which, -and in Plate XII., the absorption-lines are drawn too dark) is given on -Plate XIII. fig. 2. Herschel says this line coincides with a red band in -the negative glow-discharge, but its identity is doubtful. Its isolation -and want of adjacent lines seem to separate it from the air-spectrum and -gas-spectra in general. At the appearance of this line, 5569 (No. 2) -becomes noticeably fainter. When this line is bright, 5189 (No. 5) is -bright also (Vogel). - -If we propose to assign to this line, as well as to 5569, a -phosphorescent origin, it would be strongly confirmatory of such a theory -(in connexion with the phosphoretted-hydrogen spectrum) to find it -brighten at low temperatures. - -_Note._—Sir John Franklin says, in his ‘Polar Expeditions,’ that a low -state of temperature is favourable for the production of brilliant -coruscations. It was seldom witnessed that the Auroræ were much agitated, -or that the prismatic tints were very apparent, when the temperature was -above zero. - -2. Line in the yellow-green. Brightest of all lines in the -Aurora-spectrum. W.L. 5567, Ångström; 5569, Vogel. Intensity 25, -Herschel. To me more pale green than yellow, sometimes flickering -and changing in brightness (Herschel and Capron). Seen in all Auroræ -usually sharp and bright, but Procter has once recorded it nebulous. Its -character as to width, sharpness, and intensity, if carefully observed, -might indicate height and structure of Aurora. Becomes noticeably -fainter at appearance of red line (Vogel). Found by me to correspond in -position with a faint atmospheric absorption-band (see Plate XIII. fig. -2). According to Ångström and Herschel, arising from a phosphorescent -and fluorescent light, emitted when air is subjected to the action of -electrical discharge. - -3. Line in green near last. W.L. 5390. An extremely faint and unreliable -observation (Vogel). Seen only by him, unless Alvan Clarke’s 5320 -(coronal?) be the same. - -4. Line in green-blue. W.L. 5233, moderately bright (Vogel); 5200, -Winlock. Intensity, 2 or 0? to 6, Herschel. Coincides with line in the -negative glow according to same. Frequently observed. - -5. Line in green-blue. W.L. 5189. This line is very bright when the red -line appears at the same time; otherwise equal in brilliancy with No. -3 (Vogel); Winlock, 5200. Not so frequently observed as No. 3. Barker -gives a band extending from 5330 to 5200. Intensity of 5189, 0 to 8, -Herschel, who considers it coincident with a constant strong line in the -spark-discharge. - -6. Line in blue. W.L. 5004. Very bright line, Vogel; 5020, Barker -(coronal?). Intensity 2 or 0? to 8, Herschel. Coincides with line of -nitrogen in the nebulæ according to same. Barker gives a band extending -from 5050 to 4990. - -7. Line in the blue not found by Vogel in Aurora, April 9th, 1871. W.L. -4850, Alvan Clarke; 4820, Backhouse and Barker. Intensity, Herschel, of -4820-4870, 0 to 4? Herschel suspects this and No. 4 to be seen only in -Auroral streamers of low elevation. Barker gives a band extending from -4930 to 4850. - -8. 4694, 4663, 4629. Broad band of light, somewhat less bright in the -middle; very faint in those parts of the Aurora in which the red line -appears (Vogel). Intensity 3-6 (Herschel). A double band, consisting of -two lines, the first rather more frequently noted than the second in -Auroral spectra, agrees well in position with the principal band in the -negative glow-spectrum (same). Barker gives a band extending from 4740 to -4670; Backhouse and Winlock give a line at 4640, situate within the same. - -9. There seems a good deal of confusion about a fairly bright line -(intensity 0-6, Herschel) seen in most Auroræ (not, however, by Vogel, -April 9th, 1871), and situate somewhere near G in the solar spectrum. -Alvan Clarke places it at 4350, on the less refrangible side of G; -Backhouse and Barker at or very near to G; while Lemström and others -position it on the more refrangible side of G. Accurate observations, for -which a quartz spectroscope might be useful, are much wanted. Herschel -makes this line, at 4285, correspond with a strong band in the violet in -the negative glow-spectrum. - -Herschel also refers to an apparently additional line near the -hydrogen-line, or between G and H₁, in the solar spectrum, as mentioned -once by Lemström at Helsingfors. I am not aware of any other observation -of this line, which must be considerably beyond that at or near G, and -would probably be difficult to detect, except in instruments specially -adapted for examination of the violet end of the spectrum. - - -_Theories in relation to the Aurora and its Spectrum._ - -[Sidenote: Lemström’s.] - -Lemström (1):—That the Polar light is caused by an electric current -passing from the upper rarefied layers of the air to the earth, producing -light-phenomena that do not arise in the denser layers of the air. (2) -That there are nine rays (lines or bands) in the Aurora-spectrum, which -in all probability agree with lines which belong to the gases of the air. -(3) That the Aurora-spectrum can be referred to three distinct types, -which depend on the character of the discharge. - -[Sidenote: Vogel’s.] - -Dr. Vogel:—(1) That the Auroræ are electric discharges in rarefied-air -strata of very considerable thickness. (2) That the Aurora-spectrum is a -modification of the air-spectrum, involving the question of alteration of -the spectrum by conditions of temperature and pressure. - -[Sidenote: Ångström’s.] - -Ångström:—(1) seems to adopt the hypothesis that the Aurora has its final -cause in electrical discharges in the upper strata of the atmosphere, and -that these, whether disruptional or continuous, take place sometimes on -the outer boundary of the atmosphere, and sometimes near the surface of -the earth. - -(2) That the Aurora has two different spectra. - -(3) That the green line is due to fluorescence or phosphorescence, and -that there is no need to resort to Dr. Vogel’s variability of gas-spectra -according to circumstances of pressure and temperature. - -(4) That an agreement exists between the lines of the Aurora (except the -red and green before mentioned) and the lines or bands of the violet -light which proceed from the negative pole in dry air. - -[Sidenote: Zöllner’s remark as to temperature of Aurora and character of -spectrum.] - -Zöllner has pointed out that the temperature of the incandescent gas of -the Aurora must be exceedingly low, comparatively, and concludes that the -spectrum does not correspond with any known spectrum of the atmospheric -gases—only because, though a spectrum of our atmosphere, it is one of -another order, and one which we cannot produce artificially. - - - - -CHAPTER XI. - -THE COMPARISON OF SOME TUBE AND OTHER SPECTRA WITH THE SPECTRUM OF THE -AURORA. - -[In part from an Article in the ‘Philosophical Magazine’ for April 1875.] - - -[Sidenote: Testing Ångström’s Aurora theory. Battery and spectroscope -described. Vogel’s spectrum selected for comparison.] - -In order to test Professor Ångström’s theory of the Aurora, referred to -in the last Chapter, in an experimental way, I examined, in the winter of -1874, some tube and other spectra, not only for line-positions, but also -for general resemblance to an Aurora-spectrum. It did not seem desirable -to use powerful currents. A ½-inch-spark coil, worked by a quart -bichromate-cell, was found sufficient to illuminate the tubes steadily. -The spectroscope used was one made for me by Mr. Browning specifically -for Auroral purposes, and of the direct-vision form, being the same -instrument as is described _antè_, p. 91, and figured in Plate X. fig. 1. -The micrometer was the diaphragm one, also before described and figured -on same Plate, figs. 2, 3, and 4. I selected Dr. Vogel’s spectrum for -comparison, it being, so far as I am aware, the most accurately mapped, -with regard to wave-length, at one observation, of any Auroral spectrum. -It seemed an unsafe plan to attempt to obtain an average Aurora by -comparison of different observations made at various times by different -observers with all sorts of instruments—the difficulty, too, being -increased by the suspicion that the spectrum itself at times varies in -number and position as well as intensity of its lines. - -[Sidenote: Central part only of spectrum mapped.] - -In most cases the central part of the spectrum only (corresponding to -the central lines of the Aurora) was mapped, the red line in the Aurora -not being found to correspond with any prominent line in the gas-spectra -examined, and the Auroral line near solar G being so indefinitely fixed -as to render comparison almost valueless. (See Plate XIII. fig. 1.) - -Dr. Vogel’s spectrum does not comprise the line near G; but I have added -this (in an approximate place only) in order to complete the set of -lines. For drawing of Dr. Vogel’s spectrum, with its scales attached, see -Plate XIII. - -[Illustration: Plate XIII.] - - -_Hydrogen-tube._ - -[Sidenote: Hydrogen-tube. Colour of glow varied with intensity of -current.] - -This tube was one of Geissler’s and of rather small calibre. On -illumination the wide ends were easily lighted with a silver-grey -glow, having a considerable amount of stratification. The capillary -part glowed brilliantly with silver-white, bright green, and crimson -light, according to the intensity of the current. With the commutator -slowly working, white running into green and bright green were the main -features of the thread of light; on the current passing more rapidly, -the capillary thread became of an intense crimson, at the same time -apparently increasing in diameter (an effect probably due to irradiation). - -[Sidenote: Spectrum described.] - -The spectrum was very brilliant, consisting of the three bright lines -usually distinguished as Hα, Hβ, and Hγ, and a number of shaded bands -and fainter lines between these, with a bright continuous spectrum as a -background to the whole. - -[Sidenote: Lines α, β, and γ varied in intensity with colour as seen by -eye. Fainter lines or bands described.] - -The lines Hα, Hβ, and Hγ were found to vary in intensity with the -current, and in accordance with the colour of the light as seen by the -eye—a fact, as I think, not without bearing on the question of the -Aurora, the varying tints of which are so well known. The fainter lines -or bands were mostly stripes of pretty equal intensity throughout, and -all about the width of the Hβ line. I did not trace any marked degrading -on either side of the lines, though the edges were not uniformly so sharp -as Hα and Hβ. Some of the lines were found coincident in position with -lines of the air-spectrum. - -[Sidenote: Purity of subsidiary lines questioned.] - -It is a question whether these subsidiary lines are hydrogen, or are due -to some tube impurity. A photograph I have taken of this tube-spectrum -shows 17 lines in the part of the spectrum between F and H₂, some of -which are repeated in the hydrocarbon-tube spectra. - -[Sidenote: Coincidence of lines with Aurora-spectrum.] - -No principal line, and one subsidiary line only, actually coincide with -the Aurora-spectrum, this last being that to which Dr. Vogel assigns an -identical wave-length, viz. 5189. Other of the subsidiary lines, however, -fall somewhat near the Aurora-lines 5569, 5390, 5233, and 5004, two faint -lines also falling within the band 4694 to 4629. - -[Sidenote: Comparison of the lines.] - -The lines (adopting Dr. Vogel’s wave-lengths for the H lines) were, when -compared, as under:— - - {4694} - Aurora 5569 5390 5233 5189 5004 { to } band. - {4629} - Hydrogen 5555 5422 ... 5189 5008 4632 - -I remarked that a line (5596) described by Dr. Vogel as “very bright” in -his H spectrum does not appear in my tube, though in most other respects -our H spectra agree. - -[Sidenote: Effect of distance on the spectrum.] - -I thought this tube afforded a good opportunity for testing the effect -of distance upon the spectrum. The slit was made rather fine. At 6 -inches distance from it the line α in the blue-green (F solar) was very -bright. The lines marked β, γ, δ, ε, and ζ also survived, but were faint. -At 12 inches from the slit α and γ were alone seen, and at 24 inches -α stood by itself upon a dark ground. I also noticed that the red and -yellow parts of the spectrum first lost their light on the tube being -withdrawn from the slit; and this appeared to account for β disappearing -while γ survived. For drawing of the hydrogen-tube spectrum see Plate -XIV. spectrum 1. - -The question of effect of distance upon the spectroscopic appearance of -a glowing light, as tested for this and other tubes, seems an important -one. It may possibly account for the generally faint aspect of the lines -in the more refrangible part of the Auroral spectrum. - -[Illustration: Plate XIV.] - - -_Carbon- and Oxygen-tubes._ - -[Sidenote: Carbon- and oxygen-tubes. Tubes described. Carbon-tubes lighted -up. Spectra of the carbon-tubes described.] - -The following tube-observations were taken together, because my friend -Mr. Henry R. Procter (to whom I am indebted for many profitable hints and -suggestions in Auroral work) was disposed to regard the spectra found -in the carbon-tubes and in those marked “O” as identical, suggesting -that pure O, with the ordinary non-intensified discharge, gives only a -continuous spectrum; and that the O tubes are in fact generally lighted -up by a carbon-spectrum, as the result of impurity from accidental -causes. The tubes examined for the purpose of comparison were as -follows:—A coal-gas tube, a tube marked “C.A.,” three O tubes (two of, -I believe, London make, and the third from Geissler), and an OH₂ tube, -also from Geissler. The carbon-tubes were both brilliantly and steadily -lighted by the current. The C.A. tube glowed with a peculiar silvery-grey -green light in the capillary part, and with a grey glow, considerably -stratified, in the bulbs. The coal-gas tube illumination was whiter and -still more brilliant than the C.A., and with even finer stratification in -the bulbs. The spectra of both tubes were conspicuous for the same three -well-known principal bright lines or bands in the yellow, green, and blue -(with one fainter in the violet), all shading off towards the violet, and -in both cases with fainter intervening bands or lines. These last bands -or lines only partially coincided when the two tubes were compared. - -The spectra in both cases were rich and glowing, with a certain amount of -continuous spectrum between the lines; and the three principal bands or -lines showed well and distinctly their respective place-colours. - -[Sidenote: Tubes tested for distance.] - -_Tubes tested for distance._—In the case of the C.A. tube, at 18 inches -from the slit the continuous spectrum and fainter lines disappeared, -while the four principal lines still shone out, that in the green being -the strongest. At 24 inches the same lines were still visible, though -somewhat faintly. - -In the case of the coal-gas tube, at 24 inches the whole spectrum was -quite brilliant, the four principal lines being very bright and even -preserving their distinctive colours. The H line, near the line or band -in the blue, was also plainly seen. - -[Sidenote: O tubes lighted up.] - -The O tubes, when treated by the same current as the carbon-tubes, were -found to be all three identical in general features. The discharge -lighted up each of the tubes feebly and somewhat intermittently. Grey -in the bulbs and a faint but decidedly pinkish white in the capillary -part were the distinguishing light colours; while nothing could be more -marked than the difference in brilliancy between these and the preceding -carbon-tubes. - -[Sidenote: OH₂ tubes lighted up. O tubes spectra described.] - -The OH₂ tube presented very much the same character; but the discharge -occasionally varied from a pinkish white to a yellow colour, somewhat -like that which artists call brown-pink, and reminding one of the “golden -rays” in certain Auroræ. These O spectra presented, in common with the -carbon-tubes, three principal bright lines or bands in the yellow, green, -and blue, with a fainter one in the violet, all shading off towards -the violet. The bands, however, showed but very little trace of local -colour, and the whole spectrum had a faint and washed-out look, very -different from the carbon-spectra. (I certainly, by a little management, -subsequently succeeded in getting the same look to the C.A. spectrum; but -it was only by removing the tube to some distance from the slit, and thus -depriving the spectrum of very much of its brightness.) The hydrogen line -(solar F) was bright, more so than any of the O lines. - -The intensity of the three principal lines seemed to me to run in the -following order:— - - Yellow. Green. Blue. - - Coal-gas β α γ - Oxygen γ α β - -Between the lines γ and α in the Geissler O tube I found a rather bright -line, which I shall have occasion to refer to hereafter. - -At 12 inches distance from the slit the O spectrum lost nearly all -its light; the H line, and the three lines γ, α, and β, alone faintly -remaining, α being decidedly the brightest. At 24 inches no spectrum at -all was to be seen. - -[Sidenote: Comparison of spectra of coal-gas and O tubes.] - -I carefully compared together the three principal lines of the two -spectra of coal-gas and O by means of:— - -1st, the photographed micrometer before described; - -2nd, a comparison-prism on the slit plate; - -3rd, a piece of very fine brass foil cut as a pointer and fixed in the -focus of a positive eyepiece. - -The lines or bands in both tubes were found to be slightly nebulous -towards the less-refrangible end (where they were measured), and the O -tube was not bright under a moderately high power (positive eyepiece). -Subject to these remarks, the three principal lines in both tubes were -found to correspond in position within the limits of my instrument. The -spectra did not, however, I am bound to say, _look_ alike. - -[Sidenote: Dr. Vogel’s O spectrum reduced and compared.] - -Puzzled by these observations, it then occurred to me to reduce Dr. -Vogel’s spectrum of O, given in his memoir, to the same scale with my -own. This I did independently, and I then compared the result with my own -spectrum as mapped out. From the comparison, I judge that if my O tubes, -one and all, showed a carbon-spectrum, the learned Doctor’s tube must -have been subject to a similar infirmity, as the tubes all agreed in main -features. - -There is, however, one point to which I desire to draw attention, which -is this, that common to both the Doctor’s and my own Geissler spectrum I -found the before-mentioned rather bright line between γ and α. This line -I found no equivalent for in either of the carbon-tubes. For spectra of -coal-gas and oxygen-tubes, see Plate XIV. spectra 2, 3, & 4. - -[Sidenote: Tube- and flame-spectra of carbon do not correspond.] - -In comparing the spectra, it should be remembered that the tube- and -flame-spectra of carbon do not correspond. Compare, for instance, the -spectrum of coal-gas or CO₂ in tube, and the well-known lines or bands -in the blue base of a candle-flame. The sharper edge of the yellow -line or band of the carbon-tubes will be found about midway between -the two bright yellow candle-lines or bands. The first of the very -beautiful group of lines or bands in the green in the candle-flame -falls considerably behind the sharper edge of the green line or band -in the tube, while the third bright band in the tube, alone of the -three, corresponds with a very faint band in the candle-flame. A line -or band in the violet in the tube-spectrum finds no equivalent in the -candle-spectrum. For comparison of the carbon-tube and flame spectra (the -principal lines of the tube being alone shown), see Plate XVI. spectra 6 -& 7. - -[Sidenote: Prof. Piazzi Smyth’s measurements of the components of the -citron-band in a coal-gas flame.] - -_Note._—Prof. Piazzi Smyth has been good enough, at my instance, to -measure the components of the citron band of the carbo-hydrogen spectrum -(near Ångström’s Aurora-line), as seen in a coal-gas blowpipe-flame urged -with common air. - -The spectroscope used had prisms giving 22° of dispersion between A and -H, and the observing telescope magnified 10 times. The following is a -table of the results communicated to me by the Professor:— - - Reading of - Intensity. Micrometer. - - Reference line, lithium β 4 16·55 - ” sodium, α1 10 18·45 - ” ” α2 10 18·51 - - Citron band. Carbo-hydrogen. - - Line 1, exquisitely clear 6 21·28 - 2, ” 5 21·88 - 3, ” 3 22·44 - 4, faint but clear 2 22·95 - 5, faint 1 23·38 - 6, faint and hazy 1 23·70 - 7, doubtful ? 23·92 - Reference line, thallium α 10 25·08 - -[Sidenote: Comparison of Dr. Vogel’s O lines and Dr. Watts’s -carbon-lines.] - -From Dr. Watts’s ‘Index of Spectra’ I have extracted the three principal -carbon-tube bands or lines; and they compare with Dr. Vogel’s oxygen-tube -as under:— - - Yellow. Green. Blue. - Dr. Vogel’s oxygen-lines 5603 5189 4829 - Dr. Watts’s carbon-tube bands or lines 5602 5195 4834 - -Now these wave-length differences are so small that they raise a -presumption of the possibility of the spectra being identical. On the -other hand, assuming the spectra are not identical, the comparison tells -the other way, viz. that the differences are so minute as to escape -detection in instruments of moderate dispersion. With my own instrument -I found the O spectrum too faint to increase the dispersive power with -advantage. Considering the extremely different character of the two -discharges, the identity of all the O tubes, and the presence of the line -found between γ and α in the O spectrum, I think the two spectra are -independent, though I admit there is room for doubt. - -[Sidenote: O and CO₂ spectra photographed.] - -_Note._—Since this examination I have photographed both spectra side -by side (see ‘Photographed Spectra,’ Plate XXXI., text, pp. 69, 70). -The pictures include, of course, only the blue and violet parts of -the spectrum; but they are widely different in aspect, and show that, -photographically at least, in this part of the spectrum there is a -complete want of identity. Subsequent investigations, however, by -Schuster and others (detailed later in this Chapter), go to establish -that the principal lines shown in mine and Dr. Vogel’s tubes were due to -(probably hydrocarbon) impurity. The exception is the single line common -to mine and Dr. Vogel’s tubes, but absent from the coal-gas spectrum. -This line proves to be oxygen. Compare oxygen-tube spectra (Plate XIV. -spectra 3 and 4) with Schuster’s oxygen-tube spectrum (Plate XVIII. fig. -15). The line in question is found identical in the three tubes. - -The tube OH₂ was found to give the principal lines of the O and H spectra -combined on a faint continuous spectrum. - - -_Geissler Mercury-tube_ (Plate X. fig. 7) _and Barometer Mercurial -vacuum_. - -[Sidenote: Mercury- and barometer-tubes examined. Mercury-tube described. -Barometer-tube.] - -I next examined two vacuum-tubes of an entirely different character. The -one was a tube from Geissler of stout glass, some fifteen inches long, -without electrodes, and an inch across. Within this tube was a second -of uranium glass, with bulbs blown in it. In contact with both tubes a -quantity of fluid mercury ran loose (Plate X. fig. 7). Upon shaking this -tube with the hand brilliant flashes of blue-white light, like summer -lightning, flashed out. These were discernible (though faintly) even in -daylight. The fine terminal wires of the coil being wrapped round each -end of this tube, when the current passed, a bright and white induced -discharge, with a considerable amount of stratification, was seen in the -tube. The other tube was that of a mercurial siphon-barometer. This being -placed in a stand, one terminal wire was placed in the mercury in the -short leg of the siphon, while the other terminal was made into a little -coil and placed on the upper closed extremity of the barometer-tube. On -passing the current, the entire short space above the mercury was filled -with a grey-white light, not stratified, but showing a conspicuous bright -ring just above the level of the mercury. - -[Sidenote: Spectrum of both these tubes described.] - -Both these tubes, when examined with the spectroscope, showed four bright -rather uniform bands (the central one being the brightest), which I -assigned to the carbon-spectra (see Plate XIV. spectra 5 and 6). - -The Geissler tube was probably filled designedly with coal-gas. In the -case of the barometer-tube the spectrum must be assumed to be the result -of some carbon impurity. - -No lines of mercury could be detected in either case. - -An effort was made to examine the light of the Geissler mercury-tube as -excited by motion only, but the spectrum could not be kept in the field; -the four lines were, however, seen to flash out as the light passed -before the slit. - -[Illustration: Plate XV.] - - -_Air-tubes._ - -[Sidenote: Air-tube illuminated.] - -The first tube I examined was an ordinary Geissler tube charged with -rarefied air. The bulbs, on passing the discharge, were filled with the -well-known rose-tinged light like to the Aurora-streams. This in the -capillary part was condensed into a brighter and whiter thread, while the -platinum wire of the negative pole was surrounded by its characteristic -mauve or violet glow. - -[Sidenote: Spectrum described.] - -The spectrum, even with a weak current, was quite bright, and consisted -mainly of the nitrogen-lines and bands, with the lines Hα, Hβ, and Hγ, -and some of the intermediate lines of the H tube. - -The double line α was undoubtedly the brightest in the spectrum when -taken in the capillary part of the tube. After this followed β, and then -γ(H), δ, and ε. I was, however, uncertain as to the relative brightness -of the last three, and marked their intensities with hesitation. I tested -them several times independently with differing results, and suspected -them of variability with the current. - -The rest of the lines were very much of the same intensity. (For drawing -of spectrum of air-tube in capillary part see Plate XV. spectrum 1.) - - -_Violet [negative] Pole, same tube._ - -[Sidenote: Violet (negative) pole: spectrum described.] - -I next turned my attention to the violet or negative-pole glow; and here -a remarkable change took place in the spectrum, not only in the position -of the principal bands or lines, but in their relative intensity (see -Plate XV. spectrum 2). - -The double line α in the capillary part was replaced in the violet glow -by a shaded band of second intensity β, the sharp edge of which was -extended towards the red, and formed (except for some faint indications) -the limit of the spectrum in that direction. The somewhat faint line -next α in the capillary tube had its faint representative in the violet -pole; but the next two lines (capillary) were represented by the bright -band γ in the violet pole lying in a position between them. Next γ in -the violet pole came three faint lines, representing β, γ, and δ in the -capillary spectrum; and then the bright band α, which was the brightest -of the violet-pole group, and represented a medium-intensity band in the -capillary spectrum. After this was a faint band near α, representing -two rather bright ones in the capillary spectrum, this last being -succeeded by other bands in the violet. α, β, and γ in the violet pole -were examined carefully for relative brightness, and were, I believe, -correctly marked. - - -_Red [positive] Pole._ - -[Sidenote: Red (positive) pole: spectrum described.] - -The red [positive] pole was next examined, but presented no peculiar -features. It appeared as a fainter representation of the capillary -air-spectrum, with some few lines or bands absent, and (as will be seen -after) was also a fair representation of a diffused air-spectrum (see -Plate XV. spectrum 3). - -Examined for comparative intensity, at 24 inches from the slit, the whole -capillary air-spectrum showed faintly. The marked lines in the centre of -the spectrum generally retained their prominence; but after α I judged -ε next in brightness. On examining the violet pole at 12 inches from -the slit, the whole spectrum was faint and the bands α and β were alone -distinctly seen. - - -_Aurora (air)-tube._ (Plate XV. spectrum 4.) - -[Sidenote: Aurora-tube: discharge described. Spectrum described.] - -Next to the Geissler air-tube I examined an “aurora”-tube, about 15 -inches long and 1¼ inch across, with platinum terminals, and of the same -diameter throughout (Plate X. fig. 8). The discharge was of a rosy-red -colour, and the long flickering stream from pole to pole certainly -much reminded one optically of an auroral streamer. Spectroscopically -examined, the discharge presented a faint banded air-spectrum similar to -that of the positive pole (see Plate XV. spectrum 4); but the relative -intensity of the lines was somewhat altered, while a very bright line -in the green (seen also in the tube next described) was characteristic -of the spectrum, and in this respect distinguished it from the ordinary -air-spectrum. - - -_Phosphorescent tube._ - -[Sidenote: Phosphorescent tube described. Discharge described. Spectrum -described.] - -Following this last tube I examined one purchased as “phosphorescent.” -It was rather short (6½ inches), of equal calibre, and about the size of -the bulb of a Geissler tube. It was filled with a white powder (probably -one of the Becquerel compounds). On passing the current between the -electrodes, a bright rose-coloured stream appeared; and wherever this -was in contact with the powder, the tube glowed with a brilliant green -light. On stopping the current, the tube still continued to shine, but -with a fainter green glow, which gave only a continuous spectrum. When -examined in full glow, the tube-spectrum was also in the main continuous -and of a green tinge; but upon it were bright lines in the blue and -violet portions of the spectrum, while in the red, yellow, and green a -faint but distinct air-spectrum was seen; and with this was also found -the same bright line in the green which distinguished the “aurora”-tube. -[Five out of six of the lines in the blue and violet will be also found -in Schuster’s oxygen-tube, violet pole (Plate XVIII. fig. 15). The -air-spectrum probably arose from impurity.] - -[Illustration: Plate XVI.] - - -_Spark in Air._ - -[Sidenote: Spark in air: spectrum described.] - -I next took a ½-inch spark in air between platinum terminals (see Plate -XV. spectrum 6). The principal lines in this spectrum were the line α -(by far the brightest), corresponding to γ in the violet pole; next was -β, a line in the yellow, not appearing in the tube-spectrum, and then -other lines of less intensity. In the “aurora” and “phosphorescent” tubes -was found, as before mentioned, a line in the green prominent for its -brightness, and, indeed, in the “aurora”-tube the only one which survived -when it was moved away from the slit. This line also appeared in the -spark-spectrum, but there only of an average brightness. I examined it -carefully for position in the respective tubes; and on comparing them by -means of a pointer in the eyepiece, found it coincident with the ridge or -centre of the wedge-like bright-green broad band which is so conspicuous -in the air-tube spectrum. - -I think this edge-like centre has actually a line coincident with the -line I refer to; but if so, its intensity little exceeds that of the band -itself. - - -_Spark over Water._ - -[Sidenote: Spark over water: spectrum described.] - -To complete the set of air-experiments, I examined the same spark taken -from the surface of a small meniscus of water, placed in a glass cup upon -the lower platinum wire. In this case the air-spectrum was plainly, but -not brightly, seen at the violet end of the spectrum—the red, yellow, -green, and blue being filled with a continuous spectrum, through which -some of the air-lines faintly showed (see Plate XV. spectrum 7). - - -_Phosphoretted-Hydrogen Flame._ - -[Sidenote: Phosphoretted-hydrogen flame.] - -This was obtained from a hydrogen-bottle fitted with glass tubing, two or -three minute pieces of phosphorus being placed with the zinc. The flame -was of a bright yellow colour, with a cone of vivid green light in its -centre. - -[Sidenote: Spectrum described.] - -The spectrum was found to consist mainly of three bright bands in the -yellow, green, and green-blue respectively (see Plate XVI. spectrum 3). - -[Sidenote: Mons. Lecoq de Boisbaudran’s remarks on the spectrum -increasing in brilliancy when the flame is cooled.] - -The central band was very striking in its emerald-green colour, while -all the bands were remarkable as being very broad in proportion to the -slit (which, however, was not fine). The yellow band had a rich glow of -colour. My spectrum was mapped out at ordinary temperature, and I found -the bands sufficiently bright; but Mons. Lecoq de Boisbaudran, in his -‘Spectres Lumineux’ (texte, p. 188), has described how the brilliancy -of these bands is increased when the flame is artificially cooled -(_refroidie_). - -The idea of cooling the flame was due to M. Salet, who effected it either -by a jet of water or by an air-blast. - -The less refrangible bands seem the most susceptible to increase of -brilliancy. - -Mons. Boisbaudran also makes the important remark that the relative -intensities of the bands are in such case altered, adding:—“La -plus importante de ces modifications consiste en un renforcement -très-considérable de la bande rouge δ 97·03 (W.L. 5994) qui devient -vive de presque invisible qu’elle était en l’absence du refroidissement -artificiel de la flamme.” - -Full details of the changes are given by M. de Boisbaudran. - -The bearing of these observations as connected with the variable -character of the red line in the Aurora-spectrum seems to me in the -highest degree noteworthy. - - -_Iron-Spectrum._ - -[Sidenote: Iron-spectrum.] - -A comparison of this spectrum suggested itself, partly from the suspected -relations between the Aurora and solar corona, and partly from a -consideration of the views expressed by M. Gronemann and others in favour -of the Aurora having its origin in the fall of an incandescent meteoric -powder. - -[Sidenote: How obtained. Spectrum described. Mons. Lecoq de Boisbaudran’s -spectra also given.] - -The spectrum was obtained from a spark taken over a solution of -perchloride of iron in a small glass cup, and was remarkable for its -brightness in and about the green region. The lines varied considerably -in intensity, and with a fine slit the principal ones were sharp, -distinct, and clear. A group of three lines (α) stood out boldly in the -green as the most marked, and next to these a group of three others more -towards the violet end of the spectrum (see Plate XVI. spectrum 4). By -the side of my phosphoretted-hydrogen and iron spectra I have placed the -principal lines of Mons. Lecoq de Boisbaudran’s same spectra (reduced -to my scale), and with figures of wave-lengths for comparison with the -Aurora-spectrum (see Plate XVI. spectra 1 and 2). - -[Sidenote: Comparison of iron- and Aurora-spectrum.] - -A difficulty in comparing the iron-spectrum with that of the Aurora -arises from the large number of fine lines found in the former spectrum. -In a photograph (taken with the same prism as before described) of a -small piece of meteoric iron fused in an electric arc by the aid of 40 -Grove cells, about 154 lines are easily counted in the blue and violet -parts of the spectrum. Double this number at least would be seen with a -spectroscope of moderate dispersion in the region comprising the entire -set of auroral lines. - - -_Spectrum of Mercury._ - -[Sidenote: Mercury-spectrum. How obtained.] - -This spectrum is given as useful for comparison with the bright and -principal Aurora-line. It is easy to obtain with a small coil, the metal -being used as one electrode. The yellow lines are distinct and steady; -but the green, which is very bright, is apt to flicker as the spark moves -on the surface of the metal (see Plate XVI. spectrum 5). - - -_The following Table was compiled for the purpose of comparing the -foregoing results with the Aurora-spectrum._ - -[Sidenote: Table of coincidences.] - -TABLE showing comparative position of Aurora-lines with the principal -lines in the examined spectra. C. means coincident within the limits of -my instrument and scale, N. near, and VN. very near. - - -------------+--------+------+------+------+------+------+--------+------ - {| | | | | | | 4694 | - Aurora-lines{| 6297 | 5569 | 5390 | 5233 | 5180 | 5004 |to 4629 | 4350? - {| β. | α. | ζ. | δ. | δ. | γ. | ε. | ε. - -------------+--------+------+------+------+------+------+--------+------ - | | | | | C., | | Band | - Hydrogen-tube| | N. | | N. | same | |includes| - | | | | | W.L. | |2 lines.| - -------------+ +------+------+------+------+------+--------+ - | | | | | | | Band | - Coal-gas | | | | N. | VN. | |includes| - tube | | | | | | |1 line. | - -------------+ +------+------+------+------+------+--------+ - Oxygen-tube | | | | | VN. | | | - -------------+ +------+------+------+------+------+--------+ - | | Band | | | | Band | - Air, | | includes | | N. | |includes| - capillary | | | | | |2 lines.| - -------------+ +------+------+------+------+------+--------+ - | | | | |Band | | Band | - Air, violet- | * | | | | in- | C. |includes| ** - pole | | | | |cludes| |1 line. | - -------------+ +------+------+------+------+------+--------+ - Air, red-pole| | See Air, capillary. | - -------------+ +-------------------------------------------+ - Aurora-tube} | | See Air, capillary; and note bright line. | - and phos- } | | | - phorescent} | | | - tube } | | | - -------------+ +------+------+------+------+------+--------+ - | | | | | | | Band | - Air, spark | | N. | | | N. | C. |includes| - | | | | | | |2 lines.| - -------------+ +------+------+------+------+------+--------+ - Air, spark | | Continuous spectrum and faint | Band | - over water | | air-lines. |includes| - | | |2 lines.| - -------------+ +------+------+------+------+------+--------+ - Phosphoretted| | N. | Faint| Band | | | | - hydrogen | | | band.| in- | | | | - | | | |cludes| | | | - -------------+ +------+------+------+------+------+--------+ - Iron | | VN. | N. | VN. | VN. | | | - -------------+--------+------+------+------+------+------+--------+------ - - * No results in the examined spectra; but see Plate XIII. fig. 2. - - ** Too uncertain in position for comparison (see Plate XIII. fig. 1). - -Tested by coincidence, or close proximity of lines to those of the -Aurora, we arrange the spectra in the following order:—(1) iron, (2) -air-spark, (3) hydrogen, (4) air-tube, (5) phosphoretted hydrogen, (6) -carbon and oxygen. - -The air-tube spectrum might perhaps stand higher in the scale but for its -broad bands, which make comparison doubtful. Lines of oxygen possibly -escape detection in the Aurora from the faint character of its spectrum. - -The phosphorus and iron spectra are especially interesting in connexion -with Professor Nordenskiöld’s “metallic and magnetic cosmic dust in the -Polar regions” (see Phil. Mag. ser. 4, vol. xlviii. p. 546). - -[Sidenote: Additional Table of compared spectra.] - -As an addendum to the foregoing, on Plate IX. fig. 1 will be found a -Table I have prepared, in which a type Aurora and also Vogel’s and -Barker’s Auroræ are compared with eight other spectra, viz.:— - - S. Solar spectrum. - N. Nitrogen (air): Watts. - O. Oxygen (air): Watts. - C.H. Carburetted-hydrogen vacuum-tube: Watts. - C.I. Carburetted-hydrogen flame: Watts. - C.C. Blue base of candle-flame: Capron. - O.P. Oxygen vacuum-tube: Procter. - I. Iron: Watts. - -The divisions and vertical lines will guide the eye in making comparison -of the spectra. - - - - -CHAPTER XII. - -SOME NOTES ON PROFESSOR ÅNGSTRÖM’S THEORY OF THE AURORA-SPECTRUM. - - [The substance of these appeared in the ‘Philosophical - Magazine’ for April 1875, in conjunction with the “Comparison - of the Tube and other Spectra” (Chapter XI.), but they are now, - for the sake of convenience, made a separate article.] - - -[Sidenote: Professor Ångström’s propositions.] - -In a contribution by the late Professor Ångström to a solution of the -problem of the Aurora-spectrum (an abstract of which appeared in ‘Nature’ -of July 16, 1874), the Professor is stated, amongst other things, to have -laid down certain propositions in substance as follows:— - -[Sidenote: That the Aurora has two spectra.] - -1st. That the Aurora has two different spectra—the one comprising the one -bright line in the yellow-green only, and the other the remaining fainter -lines. - -[Sidenote: That bright line does not coincide with HC₂.] - -2ndly. That the bright line falls within a group of hydrocarbon lines, -but does not actually coincide with any prominent line of such group, and -that Dr. Vogel’s finding this line to coincide with a not well-marked -band in the air-spectrum must be regarded as a case of accidental -coincidence. - -[Sidenote: That moisture is _nil_ in Aurora region.] - -3rdly. That moisture in the region of the Aurora must be regarded as -_nil_, and that oxygen and hydrogen must alone there act as conductors of -electricity. - -[Sidenote: Ångström’s flask-experiment described.] - -Professor Ångström then details the examination of an exhausted dry -air-flask filled with a discharge analogous to the glow of the negative -pole of a vacuum air-tube. - -[Sidenote: Flask-spectrum compared with Aurora-spectrum.] - -The experiment is described as follows:—“Into a flask, the bottom of -which is covered with a layer of phosphoric anhydride, the platinum wires -are introduced, and the air is pumped out to a tension of only a few -millimetres. If the inductive current of a Ruhmkorff coil be sent through -the flask, the whole flask will be filled, as it were, with a violet -light, which otherwise only proceeds from the negative pole, and from -both electrodes a spectrum is obtained composed chiefly of shaded violet -bands.” The comparison of the spectrum of this violet glow with that of -the Aurora gives, according to Ångström, the following results:— - - Aurora-lines, wave-lengths 4286 4703 5226 - Violet light, wave-lengths 4272 4707 5227 - -Two weak light bands, found by Dr. Vogel at 4663 and 4629, are also -compared with other lines in the violet light 4654 and 4601; and the -Professor then concludes that it may be in general assumed that the -feeble bands of the Aurora-spectrum belong to the spectrum of the -negative pole, possibly changed more or less by additions from the banded -or the line air-spectrum. - -[Sidenote: Bright line is due to fluorescence or phosphorescence.] - -4thly. That the only probable explanation of the bright line is, that it -owes its origin to fluorescence or phosphorescence. The Professor remarks -on this point that “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 fluorescence.” He -notes also that oxygen and some of its compounds are fluorescent. - -[Sidenote: No need of Dr. Vogel’s theory of variability.] - -5thly. That there is no need, in order to account for the spectrum of the -Aurora, to have recourse to the “very great variability of gas-spectra -according to the varying circumstances of pressure and temperature” (Dr. -Vogel’s theory). Professor Ångström does not admit such variability, and -does not admit that the way a gas may be brought to glow or burn can -alter the nature of the spectrum. - -[Sidenote: Professor Ångström’s conclusions tested.] - -In order to test some of the Professor’s conclusions in an experimental -way, I examined some tube and other spectra not only for line-positions, -but also for general resemblance to an Aurora-spectrum. - -These experiments are detailed in the last Chapter, and the results are -comprised in Plates XIV., XV., and XVI., in which the spectra obtained -are represented in black for white. - -[Sidenote: Result of examination of the Professor’s propositions.] - -The result of the examination of Professor Ångström’s principal -propositions seems to be this:— - -1st. Two Auroral spectra. I agree in this, but question whether the -fainter lines may not possibly comprise more than one spectrum. - -2nd. I agree also that the bright yellow-green line falls, as Professor -Ångström describes, just behind the second line in the hydrocarbon yellow -group (see Plate V. fig. 7). And I find, in common with the Professor, no -well-marked or prominent line in the air-spectrum with which it accords. - -3rd. This may be conveniently divided into two parts, viz.:— - -A. The proposition that “moisture in the region of the Aurora must be -regarded as _nil_.” - -[Sidenote: Moisture probably not _nil_ in the Aurora region. Reasons for -this given. Aurora in vapour or mist. Frequently near to earth’s surface.] - -Here I see reason to differ, since (to quote a letter of Mr. Procter’s) -“the vapour-density of OH₂ is only 9 against 14 for N and 16 for O;” and -again, “electrical or heat-repulsion may carry water-dust up to enormous -heights.” There are, too, I think, circumstances connected with the -Aurora itself which make the assumption of moisture being _nil_ in the -Auroral regions untenable. The first of these is the fact that the white -arc, streamers, and floating patches of light, found in some Auroræ, -have frequently the peculiarly dense and solid look of vapour-clouds—a -circumstance with which I have been frequently struck. Mr. Procter and -others have also remarked that the Aurora is generally formed in a sort -of “mist or imperfect vapour.” The second, that Auroræ, or portions of -them, are frequently near to the earth’s surface. Instances of this are -given in the section on the Height of the Aurora, notably the experiences -of Sir W. Grove and Mr. W. Ladd. - -[Sidenote: Coincidence of Auroral lines with telluric solar lines.] - -On this point, too, note the peculiarities of the red line, which (and, -as I find, the green line also) are coincident with, or very close -to, telluric bands or groups of lines in the solar spectrum usually -attributed to moisture. (See Plate XIII. fig. 2.) - -[Sidenote: Continuous spectrum.] - -I think we may also claim the continuous spectrum in the Aurora in -further proof of water-vapour (see Plate XV. spectrum 7). The continuous -spectrum of the Aurora is also, to my observation, more local and dense -in the spectroscope than the glow generally seen between the lines or -bands in gas-spectra. - -[Sidenote: Violet-pole spectrum discussed. Most spectra have a general as -well as special character.] - -B. The question of the violet-pole spectrum. Here I make the remark that -in comparing other spectra with that of the Aurora, it is, I think, too -much the practice to trust to the coincidence (more or less perfect) of -one or perhaps two lines out of many; whereas we know by experience that -most spectra have so well-marked a general as well as special character -that, when once seen, they are recognized afterwards with the greatest -ease and without measurements. An experience and proof of this is found -in a set of “Photographed Spectra” which the Autotype Company have -reproduced for me. - -[Sidenote: Coincidence of one or two lines not sufficient to establish -identity.] - -Of course no two given spectra can be considered identical unless their -principal lines coincide; but, on the other hand, the coincidence -of one or two lines out of many, without other features, cannot be -satisfactorily or conclusively held to establish identity. - -[Sidenote: Ångström’s compared spectra.] - -In Professor Herschel’s letter (Phil. Mag. ser. 4, vol. xlix. p. 71), -Professor Ångström’s representation of the “spectrum of the glow -discharge round the negative pole of air-vacuum tubes” is given, in -comparison with the Aurora-lines and those of olefiant gas. This -illustration is here introduced. - -Ångström’s representation of the Spectrum of the glow discharge round the -negative pole of Air-vacuum tubes, and its comparison with the Spectrum -of the Aurora. - -[Illustration: Wave-lengths, in hundred-thousandths of a millimetre.] - -It is unfortunate that in this illustration and in Professor Herschel’s -paper the wave-lengths of the Aurora-lines are not given in figures, but -must be roughly calculated from the scale. Professor Herschel speaks of -Ångström’s drawing as representing a _normal_ spectrum, and as derived -from authentic sources, such as Vogel, Barker, and others; but beyond -this we are not certain as to its origin. - -In illustration of the difficulty of constructing any thing like a -general typical Aurora-spectrum I append a Table of eight Auroral spectra -taken at hazard:— - -[Sidenote: Table of compared Aurora.] - -Auroral lines and bands. - - ---------------------+----+---------+------+-----+-----------+----------- - Observers. |Red.| Yellow. |Green.|Blue.| Indigo. | Violet. - ---------------------+----+----+----+------+-----+-----+-----+------+---- - | | | | | | | |{4694}| - Vogel, April 9, 1871 |6297|5569|5390| 5233 | 5189| 5004| — |{ to }| — - | | | | | | | |{4629}| - | | | | | | | | | - Barker, Nov. 9, 1871 |6230|5620| — | — | 5170| 5020| 4820| — | — - | | | | | | | | | - | | | |{5330}| |{5050| 4930| 4740}| - Barker, Oct. 14, 1873|6300|5550| — |{ to }| — |{ to | to | to }|4310 - | | | |{5200}| |{4990| 4850| 4670}| - A. Clarke, junr., | | | | | | | | | - Oct. 24, 1870 | |5690| — | 5320 | — | — | 4850| — |4350 - | | | | | | | | | - Backhouse, 1873 |6060|5660| — | — | 5165| 5015| — | 4625 |4305 - | | | | | | | | | - Backhouse, Feb. 4, | | | | | | | | | - 1874 | * |5570| — | — | 5180| 4980| 4830| 4640 |4320 - | | | | | | | | | - H. R. Procter, 1870 | * | * | — | * | — | — | — | * | * - | | | | | | | | | - Lord Lindsay, 1870 | — | * | * | * | * | — | — | — | * - ---------------------+----+----+----+------+-----+-----+-----+------+---- - - * Mr. Procter’s and Lord Lindsay’s lines had no wave-lengths. - -[Sidenote: Ångström’s drawing discussed.] - -On examining Ångström’s diagram it certainly seems to me that, upon the -showing of the drawing itself, the coincidences are not very exact. -All three of the violet-pole bands appear to be less refrangible than -the Aurora-lines with which they are compared-the middle one (at 47) -considerably so, the one near E (at about 52·30) appreciably so, and the -third (at 43) slightly so. - -[Sidenote: Diagram of Vogel’s Aurora and violet-pole spectrum.] - -As it seemed desirable to adopt a specific Aurora-spectrum for -comparison, and to show such comparison on a somewhat larger scale than -Ångström’s drawing, I prepared the diagram shown on Plate XI. fig. 1. The -upper spectrum is Vogel’s, already described and figured on Plate XIII. -The lower spectrum is that of “Air, violet pole,” Plate XV. spectrum 2, -graphically shown. I can only find one absolute coincidence in the two -compared spectra in this diagram. - -It should, too, I think, be borne in mind that there is a great -difference in the character of the compared spectra, whether as shown in -Ångström’s drawing or mine—the bands of the violet-pole spectrum mostly -degrading towards the violet, while the lines or bands of the Aurora in -no way possess that character[14]. - -[Sidenote: Dr. Vogel’s violet-pole and Aurora-lines.] - -To assist in the foregoing violet-pole comparison I add the following -Table derived from Dr. Vogel’s memoir:— - - Violet-pole lines. Aurora-lines. - W.L. W.L. - 6100,} broad, moderately - 5945,} bright stripe 6297, very bright stripe. - - 5459,} broad, moderately {5569, brightest line of spectrum. - 5289,} bright stripe {5390, extremely faint line. - - 5224, very bright line 5233, moderately bright. - 5147, faint line 5189, moderately bright. - 5004, bright line 5004, very bright line. - 4912, fainter than last. - - 4808, very faint line. {6694,} - 4704, very intense line. {4663,} band less brilliant in - 4646, very faint line. {4629,} the middle. - - 4569, moderately bright. - 4486, moderately bright. - 4417, quite faint line. - 4346, moderately bright line. - 4275, very bright line. - -On examination of these figures it will be seen that 5224 and 5233 are -fairly close, and that 5004 is coincident. Beyond these there is little -to identify the spectra. - -[Sidenote: Conclusions arrived at adverse to the violet-pole theory.] - -As the general result of my observations and a comparison of the -foregoing spectra and tables, I see no reason for giving to the -violet-pole glow any special or distinguished place in a comparison with -the Aurora, and certainly not for assigning to it the nearly absolute -monopoly of the spectrum. It is true that the line γ in the violet-pole -glow (Plate XV. spectrum 2), which, by the way, degrades towards the -red, is in close coincidence with one of the Aurora-lines; but how -are the brighter bands α and β accounted for? These, as I have before -pointed out, alone survive when the tube is placed at a distance from -the slit. It is true they are thus reduced to shaded-off lines in lieu -of bands; but the difficulty still remains, that they are conspicuous -for their absence in the Aurora-spectrum. On the whole, I cannot but -conclude that Professor Ångström’s theory fails. At all events, if the -violet-pole glow-spectrum is to represent the Aurora-spectrum, it must -be under conditions different from those by which it obtains in dry-air -vacuum-tubes or flasks at ordinary temperature. - -[Sidenote: Phosphorescence or fluorescence of the yellow-green line.] - -4th. I feel more in accord with Professor Ångström’s memoir upon the -subject of the phosphorescence or fluorescence of the bright yellow-green -Aurora-line. - -[Sidenote: External features of Auroræ confirmatory of this.] - -I do not notice that the Professor touches upon the external features of -the Aurora in respect of this question. - -October 20, 1870.—I noted the grand Auroral display of that evening, -including “streamers of opaque-white _phosphorescent_ cloud very -different from the more common transparent Auroral diverging streams of -light.” - -February 4, 1872.—A fine display. The first signs 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 appearance in October 1870.” (Other instances of this effect -will be found in the section Aurora and Phosphorescence.) Day Auroræ, -too, we might suppose could hardly be seen without the presence of some -phosphorescent glow. - -[Sidenote: Other confirmatory circumstances. Conclusion in favour of the -theory.] - -Having regard to the near proximity of the phosphoretted-hydrogen band to -the bright Aurora-line, to the circumstance of this band brightening by -reduction of temperature (a phenomenon probably connected with ozone), -to the peculiar brightening of one line in the green in the “Aurora” and -“phosphorescent” tubes (the phosphorescent tubes probably containing -O), and to the observed circumstance that the electric discharge has -a phosphorescent or fluorescent after-glow (isolated, I believe, by -Faraday), I feel there is strong evidence in favour of such an origin to -the principal Aurora-line, if not to the red line as well. - -[Sidenote: Invariability of gas-spectra questioned.] - -5th. Professor Ångström opens a wide door to discussion in his -proposition of the invariability of gas-spectra, and I do not now attempt -to follow in detail this interesting part of the present subject. Suffice -it to say, that if the Professor lays down this proposition in its -strictest sense (I can hardly suppose he so meant it), there is, so far -as I am aware, no one spectrum that can at all claim comparison with the -Aurora-spectrum. Giving greater latitude to the Professor’s words, I -reply, upon competent authority, that lines vary in number and brilliancy -with temperature, and in breadth with pressure. Kirchhoff, too, in -speaking of vapour-films as increasing the intensity of lines, states -“it may happen that the spectrum appears to be totally changed when the -mass of vapour is altered.” We may, too, now add magnetism as capable of -effecting a change in certain spectra, not only as to brilliancy, but -even as to position of lines. (Chautard’s Researches, ‘Philosophical -Magazine,’ 4th series, vol. 1. p. 77, and experiments detailed in Chap. -III. of this work.) - - - - -CHAPTER XIII. - -THE OXYGEN-SPECTRUM IN RELATION TO THE AURORA (PROCTER AND SCHUSTER). - - -[Sidenote: Procter’s oxygen-spectrum.] - -In a communication to ‘Nature,’ Mr. H. R. Procter has pointed out an -apparent coincidence in position of several of the Auroral lines with -those of a spectrum occasionally obtained from air at low pressure with -a feeble discharge. It is, he says, sometimes exhibited in lumière -(phosphorescent?) tubes, and he believed it, in part at least, to be the -spectrum described by Wüllner (Philosophical Magazine, June 1869) as a -new spectrum of oxygen. - -[Sidenote: How obtained.] - -He had obtained it very vividly in pure electrolyzed oxygen with a feeble -discharge, but some perplexing observations made him doubtful of its -origin. - -Plate XI. fig. 4 gives a representation of this spectrum as shown by Mr. -Procter, except that my drawing is in black for white. - -[Sidenote: Compared spectra described.] - -The upper spectrum is that above mentioned, the centre one that of the -Aurora, the lower one the lines of Na and H for comparison. The Auroral -yellow-green line, in January 1870, was found by Mr. Procter coincident -with a bright line or band in the tube (with a spectroscope of a 60° -bisulphide prism, and magnifying-power about six). The third and fifth -lines in the Aurora seemed also to correspond with tube-lines. As to -these Mr. Procter says they were not bright enough to be compared with -the same accuracy as the yellow-green line, but that the positions could -not be far wrong. - -[Sidenote: Mr. Procter’s subsequent views. Yellow-green line traced to -some form of hydrocarbon.] - -Mr. Procter subsequently (‘Edinburgh Encyclopædia,’ art. “Aurora”) -considered he traced the yellow-green tube-line to some form of -hydrocarbon. On examination with instruments of greater dispersion, it -was found that, though more refrangible than the first band of citron -acetylene (candle-flame), it was less so than the Aurora-line. The -tube-band, too, was shaded towards the violet, which was not the case -with the Aurora-line. - -The question as between hydrocarbon and oxygen I did not then consider as -disposed of. With the lumière tubes the question might be open, but I did -not see how it could be in the case of the electrolyzed oxygen-spectrum. - -From a comparison of the tube-spectra, I have shown that although -the spectra of the carbon and oxygen tubes are proved to be, -photographically, as a whole, distinct, they have, as to position of -some of the principal lines in the central part of the spectrum, a very -close resemblance. - -[Sidenote: Probability that O may play a part in the Aurora-spectrum.] - -That oxygen may in some form play a part in the Aurora seems highly -probable; how far it is spectroscopically detected seems a different -question. - -[Sidenote: Difference between air-spark and tube-spectra.] - -Ångström and Herschel suggest its presence in the Aurora in connexion -with phosphorescence or fluorescence. With a spark-discharge in air at -ordinary pressure, a mixed spectrum of bright lines of N and O is found; -while in the case of Geissler vacuum-tubes (representing a glow-discharge -in a much more rarefied atmosphere) the N lines appear mainly to usurp -the spectrum. - -[Sidenote: H₂O tube referred to.] - -It must, however, be borne in mind that a Geissler tube, as to -temperature at least, in no way represents the conditions of the Aurora; -and when we remember the association of oxygen and ozone, and the way in -which the latter is affected by heat, it may well be that temperature -plays an important part in the matter. In proof of this conduct of -oxygen, it may be cited that, in the case of a H₂O tube, the H lines -come out sharp and brilliant in the spectrum, while what is seen of the -O lines is comparatively weak, misty, and ill-defined. Vogel, it will be -remembered, makes 5189 of the Aurora coincident with an O line. - -[Sidenote: Residual phosphorescence in Geissler tubes. Garland tube.] - -Professor Herschel has pointed out, and I have confirmed, that the -residual phosphorescence in Geissler tubes, after the spark has passed, -is probably associated with oxygen. He also alludes to the fact that when -one of the globes of a “Garland” tube was heated, it did not shine after -the spark had passed, apparently because of the destruction of the ozone -by heat. - -[Some experiments with a tube of this description will be found detailed -in Part III. Oxygen was not, I think, the gas it was filled with.] - -[Sidenote: Dr. Schuster’s tubes described.] - -Subsequently to my examination and comparison of the O and CO₂ spectra -before detailed, Dr. Arthur Schuster was good enough to send me three -vacuum-tubes of his own preparation, showing an oxygen-spectrum. - -One, with large disk-shaped brass electrodes, was unfortunately broken in -transit. Dr. Schuster informed me it showed the carbonic-oxide spectrum -as well as that of oxygen. The other two tubes had aluminium electrodes. -They were similar in shape to ordinary Geissler tubes, but had attached -to each a supplemental bulb containing dry oxide of manganese. -Illuminated by the larger coil, one of these tubes (which had a slight -crack in the manganese bulb) lighted up faintly; the other was fairly -bright, and the glow had a somewhat reddish tint. - -Plate XVIII. fig. 15 represents as the upper spectrum Vogel’s Aurora, -with W.L. numbers, as the middle spectrum the capillary part of Dr. -Schuster’s O tube, and as the lower spectrum the negative (violet) pole -of the same tube. - -[Sidenote: Spectra described.] - -The tube-spectra were mapped out with the aid of the diaphragm micrometer -before described. - -[Sidenote: Capillary.] - -The capillary spectrum was mainly distinguished by four bright sharp -lines—one in the red, between the red Aurora-line and D, two in -the green, but considerably more refrangible than the yellow-green -Aurora-line, while the fourth was found to be hydrogen F. The other lines -in the spectrum were considerably fainter, and misty and band-like. The -red line, though not brilliant, was fairly bright and sharp. - -The place of the less refrangible of the two bright bands in the -violet-pole spectrum was occupied in the capillary spectrum by a faint -glow only. - -[Sidenote: Violet-pole.] - -The violet-pole spectrum was recognized by two very bright broad -bands of light in the green, each including within its limits one of -the Aurora-lines. The bright red line in the capillary had a faint -representative in the violet-pole spectrum, as also had the two bright -lines in the green. Other fainter lines appeared in the blue, and three -fairly bright ones towards the violet. - -[Sidenote: Dr. Schuster’s remarks on the spectra.] - -Dr. Schuster remarks that one of these O bright bands is closely -coincident with a band in the CO spectrum, but that the CO band is bright -towards one edge and fades off gradually thence, while the O band is of -pretty uniform strength throughout. Dr. Schuster finds the wave-lengths -of the violet-pole O bands to be as follows:— - - 5205·0} - 5292·5} Brightest part 5255. - - 5552·8} - 5629·6} Brightest part 5586. - -[Sidenote: His tubes free from impurity.] - -He also gives as weak bands 5840-5900 and 5969-6010. Dr. Schuster comes -to the conclusion that the green line of the Aurora is not due to -oxygen, as, under considerable dispersion and with good definition, the -oxygen-bands can be broken up into a series of lines, when the brightest -part is found to lie at 5586, which is too much towards the red to -compare with the Aurora-line. He notices that the more refrangible of the -O bands corresponds with a line sometimes seen in the Aurora (Vogel’s -5233). The same remark will, however, apply to this last as to the other -coincidence, viz., that a broad band can hardly represent a line—at -least, the line can only be said to coincide in a loose and indefinite -way. It is evident that Dr. Schuster’s tubes were free from what must -now be considered an impurity in those examined by me and by Dr. Vogel, -and that Mr. Procter’s suspicions of carbon impurities in these, and the -ordinary oxygen-tubes, are thereby quite confirmed. - -[Sidenote: Experiments with an open Geissler tube.] - -In some experiments which we made (after receiving Dr. Schuster’s tubes) -with an open Geissler tube, so arranged as to connect with an air-pump -and gas-receiver, and thus from time to time to wash out the tube and -vary its contents, we found the same impure spectrum as in the case -of the sealed O tubes; and it seems to require a very large amount of -precaution to avoid these impurities. - -[Sidenote: Spectra of Dr. Schuster’s O tube examined.] - -Dr. Schuster was kind enough to examine the spectra I mapped out, and -which are shown in Plate XVIII. fig. 15, with the following results:—The -lines Oα, Oβ, Oγ are those he has referred to under that designation in -his communications to ‘Nature,’ and undoubtedly belong to oxygen. The -bands A, B, and C are the bands characteristic of the negative pole. -He finds A divided into two parts by a dark space. The spectrum of the -negative pole, under good exhaustion, stretches into the capillary part; -hence B appears in the capillary as a faint band. A similar thing happens -with nitrogen. I., II., III., and possibly 8 and 9, he thinks, are due -to the spark-spectrum of oxygen, obtained when the jar and a break are -interposed, the brighter lines of the line-spectrum being always present -at the negative pole. These last-mentioned lines I have already referred -to, as having been found by me in a tube showing phosphorescence after -the spark has passed. (Compare Plate XVIII. fig. 15, O violet pole, with -Plate XV. spectrum 5.) Nos. 1 and 2, he thinks, are due to some foreign -matter, as they are not in all his tubes. - -Dr. Schuster often finds that a spectrum due to the aluminium electrodes -is seen in tubes under great exhaustion; and this he considers is the -spectrum of aluminium oxide. A drawing of this spectrum is found in -Watts’s ‘Index of Spectra,’ plate iii., “Aluminium first Spectrum.” -To this, he thinks, are also due the bands, or sets of lines in my -aluminium-arc spectrum (‘Photographed Spectra,’ plate ii.), and he -believes lines 3, 4, 5, 6, and 7 in the mapped-out spectra are due to it. -It would thus appear that the lines due to O are few in number, and do -not well compare with the Aurora-spectrum. - - - - -PART III. - -MAGNETO-ELECTRIC EXPERIMENTS IN CONNEXION WITH THE AURORA. - - - - -INTRODUCTION. - - -[Sidenote: Object of experiments. Description of apparatus employed. -Electro-magnet. Battery.] - -The set of experiments detailed in Chapters XIV. to XIX. was mainly -conducted for the purpose of testing, in connexion with the Aurora, -the action of a magnet upon the electric glow _in vacuo_ and on the -spark at ordinary pressure. It also includes some observations on the -glow from the violet pole with and without the magnet, and on the -glow obtained from one wire only. The apparatus employed was a Ladd’s -electro-magnet, with poles 10¼ inches high by 2 inches across, each -pole being surrounded by a movable helix, composed of two sets of stout -copper wire wound together, so that they could be used either in one -length or as independent coils excited at the same time. The latter form -of arrangement was employed by us. In most of the experiments conical -armatures were employed for the purpose of bringing the action of the -poles to bear upon the subjects examined. A contact-maker was added to -the magnet, so that it could be put rapidly in or out of action without -disturbing the wires. The battery used to excite the magnet was of the -form known as that of Dr. Huggins, and consisted of four vulcanite -cells in a frame, each holding seven pints of bichromate solution, and -containing two carbon and one zinc plate, each 13½ by 6 inches. - -[Sidenote: Small coils. Larger coil. Magnetic curves obtained.] - -A winch and pulley enabled the whole set of plates to be lowered into -the liquid and withdrawn at pleasure, and the large quantity of solution -gave the battery a considerable amount of constancy. We found it could be -used for two evenings’ work, of four hours each, without any material -dropping in power. For obtaining the glow in the Geissler and other small -tubes, a Ruhmkorff coil, giving a ½-inch spark, excited by one plate of a -½-gallon bichromate (bottle form), was used. For the glow in the larger -tubes and the spark in air a larger coil, giving a two-to three-inch -spark, and worked by two ½-gallon double-plate bichromates, was employed. -Notes were taken of the experiments, and drawings of the effects at the -time; and these are reproduced almost literally in the text and Plates -comprised in this Part. To ascertain the direction and extent of the -magnetic curves, we covered large sheets of cardboard, placed over the -poles, with iron filings; excited the magnet so as to obtain the curves, -and then obtained permanent prints from the filings by spraying the -cardboard with tannin solution. The magnetic effects were thus found to -extend to a radius of at least ten inches (see diagram, Plate XVII. fig. -1, showing magnet-poles and curves on a ¼ scale). - -[Sidenote: Chautard’s investigations kept in view. Evidence obtained of -change in colour, form, &c. of Aurora. Ångström’s flask-experiment tried.] - -In the vacuum-tube experiments we held Mons. J. Chautard’s investigations -(on the action of magnets on rarefied gases in capillary tubes rendered -luminous by the induced current, Phil. Mag. 4th series, vol. 1. p. 77) -in view. We obtained in our experiments plenty of evidence of a change -of colour and form in the discharge under the magnetic influence; -and both simple and compound spectra were found to be much varied by -the exaltation or suppression of some parts of the spectrum, so that -apparently new lines sprang up; but we failed to trace actual change of -position or wave-length in any given line, though we carefully looked for -it. A portion of our researches was directed to the subject of Ångström’s -experiment of filling a dry flask with a violet glow, analogous to -that from the negative pole. We entirely failed in obtaining the same -result while two wires and an uninterrupted circuit were employed. When, -however, we attached a negative wire only (the other wire being left -free) to an exhausted globular receiver, we obtained an effect very -similar to that referred to in Prof. Ångström’s memoir. - -[Sidenote: General results of experiments. Bulb effects noticed as a mode -of analysis of gases.] - -The general result of the experiments was to prove, assuming the Aurora -to be an electric discharge, the great influence the magnetic forces may -exercise on the colours, form, motions, and probably the spectrum also -of that phenomenon. It is easy to conceive that the variation in number, -and intensity of the lines which has been remarked in Auroral spectra -may have its origin in such a cause. The influence of the magnet on the -capillary stream was mainly in colour and intensity; but in the bulbs the -effects were still more marked and striking, and, in a greater or less -degree, different in the case of each gas which we examined. A careful -and extended study of these effects, conjointly with the changes in the -spectrum, might possibly form a new and valuable mode of analysis of -compound gases. This is well illustrated in the case of the iodine and -sulphur tubes which we examined. - -[Illustration: Plate XVII.] - - - - -CHAPTER XIV. - -EXAMINATION OF GEISSLER TUBES UNDER ACTION OF THE MAGNET. - - -_Nitrogen-tubes._ - -[Sidenote: Nitrogen-tube No. 1. Discharge described. Spectrum described. -Capillary stream. Positive bulb. Violet-pole glow.] - -(1) A small Geissler tube (No. 1) was lighted up by the small coil. -The capillary part showed a very bright, slightly rosy-tinted stream. -Negative bulb was filled with rosy-purple light, the violet-pole glow -being confined to the extent of the electrode. Positive bulb of the -same rosy-purple colour, but stream slightly contracted in volume. Glow -throughout quiescent, and no stratification in the tube. A compound-prism -spectroscope, taking in the whole of the spectrum, showed in the -capillary stream, from yellow to red, a fairly bright wedge, having a -dark band in the centre, and six bright columns, with dark lines at -intervals, shading off on either side. On the more refrangible side of -the yellow, the spectrum was composed of a set of bright bands and lines -in the green, blue, and purple, one line only (in the green) standing -out very bright. In the yellow and red no bright line stood out alone. -The positive bulb gave a fainter spectrum of the same character, mainly -confined to the centre, the violet, yellow, and red not being well seen. -When the violet-pole glow was examined, the general character of the -spectrum was quite changed: a brilliant broad band in the violet, a -bright narrower one in the blue, and two bright lines in the green, with -intermediate fainter lines throughout, were the main features. The yellow -and red part of the spectrum was also changed. The yellow was fairly -and evenly distinct up to the dark band; then came a somewhat brighter -orange band, and after that the red, but rather obscure and cut off. No -absolutely bright line could be traced in the red. - -[Sidenote: Nitrogen-tube No. 2. Glow described. Difference of spectra of -capillary stream and violet-pole glow. Junction of the violet-pole glow -and capillary stream.] - -(2) To compare the capillary stream and the violet-glow, a second -nitrogen-tube (No. 2) was used. This tube was larger in bulk and bore -than No. 1. The glow in the bulbs was considerably fainter and more -salmon-coloured; and there was much stratification in both, extending -to the capillary bore. (This stratification was considered due to H, -as the three principal lines of that gas came out very brightly in the -spectrum.) The difference of the spectra of the capillary stream and of -the violet-pole glow was extremely well marked—the former consisting of -a set of bright lines and bands of fairly uniform intensity, while the -latter was split up into a few bright bands with fainter lines between. -The yellow and red of the violet-glow were very weak as compared with -the same region of the capillary spectrum. No bright line appeared in -the red. The tube being properly adjusted for the purpose, the junction -of the violet-pole glow and the capillary red-glow was easily observed. -The bright bands of the violet-pole were seen to run into the capillary -line-spectrum, and then, gradually getting finer and more pointed, to -fade out. - -[Sidenote: Tube No. 1 between the poles of the magnet. Change of colour -in capillary stream. “Tailing-over” of capillary stream.] - -(3) The capillary part of tube No. 1 was arranged between the poles of -Ladd’s electro-magnet, the conical ends of the armatures almost touching -the tube (Plate XVII. fig. 1). With the magnet not excited, the capillary -stream was bright and of a slightly rosy-yellow tinge. It varied a little -in apparent diameter with the current. As soon as the magnet was excited -the capillary stream, as also (in a less degree) that in the bulbs, -were seen to contract, and to change from a _rosy_ tint to a distinctly -_blue-violet_. The polished armatures, acting as reflectors, showed this -change of tint in a most marked manner each time the magnet was excited. -At the same time the capillary stream was seen to run into the negative -bulb, as if overflowing, and with an effect resembling the “tailing-over” -of a gas-flame. This effect took place each time the magnet was excited, -and was not found at the positive-bulb end. - -[Sidenote: Spectrum examined.] - -Occasionally, when the magnet was excited, flashes of light were -discharged in the negative bulb from the capillary towards the -violet-pole. The spectrum was then carefully examined. No change was -seen in the actual position of any of the lines or bands when the tube -was influenced by the magnet, but those towards the violet end of the -spectrum were conspicuously brightened. - -[Sidenote: Negative bulb between poles of the magnet. Positive bulb -within action of the magnet.] - -(4) The extremity of the negative bulb was now placed between the poles -of the magnet. A bright violet-coloured arc, following the magnetic -curve, was at once formed, as in the case of the large Plücker tubes; and -at the same time a straight stream of not very bright light ran along the -bulb. The positive bulb was next placed within the action of the magnet; -and immediately a brilliant spiral of flickering light appeared in the -bulb, lighting it up, and reminding one in shape of the spiral which -water forms on being poured from a lipped jug (see Plate XVII. fig. 9). - -[Sidenote: Spiral formed.] - -This was repeated each time the magnet was excited. The spiral, though -flickering in character, was permanent in form, and inclined to the side -of the tube which was in contact with the N pole of the magnet. - - -_Oxygen-tubes._ - -[Sidenote: O tube No. 1; spectrum described.] - -A tube (No. 1) was lighted up and examined with the spectroscope, and -found to give the spectrum shown on Plate XIV. spectrum 3, but with a -strong set of H lines in addition. - -[Sidenote: O tube No. 2; spectrum described.] - -A second tube (No. 2) was then lighted up. The spectrum was a bright one, -similar to the foregoing, the principal H lines being present, but not -strong. - -[Sidenote: Tube-glow described. Effect upon glow when magnet excited. -Bulbs between poles of the magnet. Effect of magnet on spectrum.] - -The red region was indistinct, and showed no prominently bright line. -The bulbs were mainly of a slightly blue-grey tint, with a steady glow. -Capillary stream quite pale white, with a very slight tinge of red. -Violet-glow small and confined to the electrode. Upon the magnet being -excited, the capillary stream became intensely brighter, and the glow -in both bulbs contracted into a single bright stream, which curved -towards the sides of the bulbs at right angles to the magnetic poles, and -changed from side to side with the current. This effect was very marked, -and was more apparent in the positive than the negative pole. A faint -stratification was seen in both bulbs. Upon either bulb being placed -between the armatures, the glow left the electrode point and condensed -into one bright stream, running along the side of the tube and curving at -each end (Plate XVII. fig. 10). No trace whatever of tendency to form a -spiral was seen. The spectrum with the magnet on was very conspicuously -brightened up throughout. A set of fluted bands with a bright line -among them appeared in the red, and several lines or bands appeared in -the violet which could not be seen before. The bright red line, upon -measurement, proved to be the hydrogen-line C. It thus seemed brighter -in proportion than the F line, although, with the magnet off, the latter -was well seen, while the C was not. No actual change in position of the -spectrum-lines could be detected. - -[It is to be noticed that the O tubes employed were those used by me -in former experiments, and had the bright lines now attributed to -hydrocarbon impurity. Their bulb-effects differed, however, entirely from -those of the CO₂ tube. (Compare figs. 10 and 11, Plate XVII.)] - - -_Hydrogen-tubes._ - -[Sidenote: H tube, No. 1; glow described.] - -A small H Geissler tube (No. 1) was selected, and lighted up by the small -coil. The capillary was a bright white-pink stream, with a tendency to -redden at times. The bulbs were both of a faint blue-grey tint, with -coarse lenticular stratification. The violet-pole glow was pale and -white as compared with that of N. - -[Sidenote: When the magnet was excited, whole character of tube changed. -Unexcited spectrum described. Effect when magnet was excited.] - -When the magnet was excited, the whole character of the tube changed. -The capillary stream diminished in brightness and in apparent volume, -and changed to a deep amber-yellow. The bulbs lost some of their light, -and their coarse stratification; being, in lieu, filled with a vertical -condensed stream of moderate light, in which a fine stratification only -was seen. The stream in the positive bulb had a tendency to the spiral -form. The capillary, each time the magnet was excited, “tailed over” -into the negative bulb, as in the case of N, looking as if it were -squeezed out of the capillary bore. The unexcited spectrum was found to -consist of the usual principal lines of H on a continuous glow, with the -intermediate bands and finer lines, which are usually suspected to be due -to impurity. The sodium-line was also seen. When the magnet was excited, -the spectrum grew much fainter—the continuous glow in the red and blue, -and the red and blue lines, nearly disappearing, and the line in the -green alone shining out conspicuously. No change of place in the lines -could be noticed. - -[Sidenote: No. 2 H tube; effects described.] - -A longer H tube (No. 2) was then tried, with similar effects, except that -the diminution in brightness was not so conspicuous. When the negative -bulb of the tube No. 1 was placed between the poles of the magnet, a -stream of light was formed, and the stratification became finer. The same -effect took place with the positive bulb, with a tendency to the spiral -form. - - -_Water-Gas (H₂O) tube._ - -[Sidenote: Water-gas (H₂O) tube; effects produced described.] - -A faint purple glow was seen in each bulb, the tube not lighting-up -brightly. The capillary showed a slightly rosy-tinted, grey stream of -brighter light. With the magnet on, the glow in the bulbs was condensed -into a single bright stream. The capillary brightened up, and assumed a -yellow tint—this effect being principally confined to that portion which -was between the conical ends of the armatures, and gradually diminishing -as the distance increased from these. Without the magnet, the principal -H lines showed brightly in the spectrum, the O lines being misty and -indistinct. With the magnet on, the O lines and spectrum generally -brightened up. - - -_Ammonia-tube._ - -[Sidenote: Ammonia-tube; lighting-up described. Spectrum described.] - -This tube was difficult to light up. Hardly any light was seen in -the bulbs, except a very faint purple glow at the electrodes. In the -capillary part a fairly bright stream of purple-white light appeared. The -spectrum was a faintly shown one of N and H. The effect of the magnet -was to reduce the brightness of the glow in the capillary, but with -little marked action on the bulbs, except to condense the faint glow into -a slightly bright stream running along the side of the tube. - -On a subsequent examination the tube and spectrum both brightened up -under the influence of the magnet. The N lines, which were faint without -the magnet, shone out under its influence distinctly—the red and yellow -parts of the spectrum specially showing this effect. The H lines also -brightened up, but hardly so much in proportion as the N. - - -_Carbonic-Acid tube._ - -[Sidenote: Tube marked C A; lighting-up described.] - -A Geissler tube marked C A was examined. Capillary stream a brilliant -bluish white; bulbs grey-blue, with a slight tint of green; slight -stratification in positive bulb; stream diffuse, not quite filling -the bulbs, and changing in volume as the coil-break was touched; glow -round the violet-pole considerable, but markedly white in tint, rather -than violet; stratification strong in capillary. With magnet excited, -the capillary stream diminished in volume, but greatly increased in -brightness. It “tailed over” into the negative bulb, and the stream -through both bulbs curved towards the sides. A slight pattering noise -was heard in the tube. In the positive bulb bright, imperfectly formed, -saddle-shaped rings of light, with a tendency to spiral formation, were -seen, somewhat similar to the effects in the Plücker tube after described -(see Plate XVII. fig. 11). - -[Sidenote: Effects when magnet was excited.] - -The whole spectrum, under influence of the magnet, became much brightened -up. Faint bands in the red came out bright, as also did some in the -violet. The violet-glow was examined (without the magnet), and the light -was found condensed into four prominent shaded bands, one red, one -yellow-green, one green, and one blue, with fainter bands seen between. - - -_Chlorine-tubes._ - -[Sidenote: Chlorine-tube No. 1 lighted-up. Action of magnet upon the tube -and spectrum.] - -A chlorine-tube (No. 1) was lighted-up with the small-coil. Capillary -stream of a pale green tint. Bulbs with very little glow in them; -spectrum pale, and not very distinct. Under action of the magnet this -tube brightened up throughout, and the glow became more condensed, and -ran to the sides of the tube. The spectrum also brightened, the faint -lines becoming stronger, but the general character was preserved. - -[Sidenote: Chlorine-tube No. 2 lighted-up. Effect on glow when magnet was -put on.] - -A second chlorine-tube (No. 2) was then tried. Both bulbs were -completely filled with a dense white (very slightly rosy-tinted) opaque -light, and capillary the same, but brighter. A very slight violet tinge -was seen at the negative pole. When the magnet was put on, both bulbs -were at once filled with flickering bright streams of light, running -towards the side of the tube, according to the direction of the current. - -The capillary stream at the same time changed from white to an intense -bright green. The spectrum without the magnet consisted of sets of -lines, with two well-marked absorption-spaces between, all seen somewhat -faintly, as if through a mist. - -[Sidenote: Changes in spectrum when magnet was excited.] - -When the magnet was put on, the marked character of the absorption spaces -was lost. The sets of lines in the yellow-green and green started up -intensely bright, while those in the blue only slightly brightened. - -The misty appearance was altogether lost, and the bright lines all shone -up upon a perfectly dark background, with a strikingly metallic look; we -could not, however, trace change of position or actually new lines. It -seemed as if lines which had been faint in the yellow-green and green -region suddenly increased in intensity, the other parts of the spectrum -not being similarly influenced. They quite flashed up when sudden contact -was made with the magnet commutator. - - -_Iodine-tubes._ - -[Sidenote: Iodine-tube No. 1.] - -This tube (No. 1) had been used for photographic purposes, and the bulbs -were partly obscured by a white deposit. - -[Sidenote: Lighting-up described. Effect of touching one wire with the -finger.] - -On lighting it up, both bulbs were filled with a violet-grey diffused -light, with much coarse well-marked lenticular stratification. This -stratification was mainly lost on changing the direction of the current, -but made its reappearance when one conducting-wire was touched with a -finger. This effect was still more marked when one finger of each hand -was applied to the wire. The capillary stream was of a pale lemon-yellow. -On putting on the magnet the light in the whole tube was nearly -extinguished, a faint thin stream of condensed light running through the -centre of the tube alone remaining. - -[Sidenote: Effect of the magnet.] - -On placing the bulbs between the magnet-poles, effects were produced -similar to those in the case of the tube, after described (p. 144, and -marked Si Fl₆), but in a less marked degree. - -[Sidenote: Tube again tested.] - -The iodine-tube was subsequently again tested, and it lighted-up better -than on the last occasion, showing nearly the same effects in bulbs and -capillary, the former having somewhat of a rosy tint and the latter an -amber. - -[Sidenote: Magnet effects. The spectrum described. Change when magnet was -excited.] - -On exciting the magnet, the capillary part of the tube changed from -amber to a decided light green. The spectrum, without the magnet, -gave one very bright line, and several less bright ones near, in -the blue-green. The rest of the spectrum, with the exception of the -absorption-spaces, was misty and continuous, with lines showing faintly -through. The red and yellow portions of the spectrum were quite bright. -When the magnet was excited, the spectrum entirely changed. The red -and yellow portions of the spectrum, and the misty continuous light, -all quite disappeared; while a set of sharp lines on the yellow-green -and green flashed up bright and clear, and stood out alone upon a dark -background, in which the absorption-spaces were lost. The effect was very -strongly marked, and gave a totally different character to the appearance -of the spectrum. The change seemed to arise from the suppression of one -part of the spectrum, and the increase in intensity of the lines in the -other part. - -[Sidenote: No change in line-position.] - -The principal lines could not be traced to change in actual position. - -This tube differing somewhat from a second one we examined (No. 2) in -tint of glow and spectrum, it suggested itself to us that there might be -a partial mixture of N or H (or both) with the iodine vapour, giving rise -to some of the brighter parts of the spectrum which were extinguished -under the action of the magnet. - -[Sidenote: Comparison of iodine-tubes No. 1 and No. 2. Comparison of the -spectra.] - -We therefore compared these two tubes, viz. the old one (No. 1) and the -new one (No. 2), and also their spectra, by means of a comparison-prism -on the slit of the spectroscope. To the eye, the tubes differed much -in appearance. No. 1 had a distinct transparent rosy tint throughout, -with considerable coarse flickering stratification; and this contrasted -strongly with the dense whitish light of tube No. 2, which showed neither -movement nor stratification. The spectra were also found different in -general look. That of tube No. 1 was strongly tinged in the red and -yellow, and showed a bright continuous spectrum, crossed by many sharp -lines, with little trace of absorption-spaces. The spectrum of No. 2 -was much whiter in tint, showed very little of the red and yellow, and -the absorption-spaces were very dark. A few bright lines, mainly in the -yellow-green and green, were faintly seen. - -[Sidenote: The two tubes examined in detail. No. 2.] - -The two tubes were then examined separately in detail. No. 2, excited -by the magnet, showed curious effects. The glow was rendered weak and -intermittent, and the rosy tint almost disappeared. The capillary changed -to a decided green colour, and the positive electrode was surrounded -by a yellow glow. The changes in the spectrum were no less decided. -Without the magnet, the spectrum was found to be a bright continuous one -of H (with a full set of principal and intermediate lines) and N—the N -spectrum being rather faint and misty, with very slight, if any, traces -of the iodine-spectrum. On the magnet being excited, the spectrum changed -as if by magic; the H and N spectra disappeared (except hydrogen F, which -still faintly remained), and the iodine lines, mostly in the yellow-green -and green, shone out wonderfully sharp and bright on quite a dark ground. -No. 1, upon examination, showed between the magnet-poles only the same -changes as on last occasion. The spectrum seemed to be one of iodine, -with the addition of slight traces of the H spectrum. - -[Sidenote: Effects discussed.] - -On excitation of the magnet, the misty continuous part of the spectrum -nearly disappeared, and the bright lines shone up sharply upon the dark -background as before. The effects in the case of both tubes were strongly -marked. The impression as to tube No. 2 was that, without the magnet, -the slight iodine-spectrum was overpowered and masked by the N and H -spectra; while under the influence of the magnet the N and H spectra -were almost altogether suppressed, the iodine-spectrum being at the same -time intensified. The disappearance of the continuous spectrum under the -action of the magnet in No. 1 (with the supposition it was mainly H) -would be accounted for in the same way. - - -_Bromine-tubes._ - -[Sidenote: Bromine-tube No. 1. Lighting-up described. Effect of the -magnet. Bromine-tube No. 2. Effect of magnet.] - -This tube (No. 1) had been previously worked for photographic purposes. -Excited by the small coil, the whole tube was filled with a faint -flickering light. The positive bulb contained a faint purple glow, with -a yellow-green tinge at the electrode, a curious flickering stream of -light flashing from the electrode to the side of the tube. The negative -pole showed pretty much the same effect as the positive. The capillary -stream expanded at the opening into the positive bulb, but ran in a -condensed stream into the negative bulb. In colour it was of a rather -bright lilac. Upon putting the magnet on, the light-glow in the tube was -at once and permanently extinguished, the coil still working as if the -current passed. The same effect happened repeatedly; but now and then the -tube lighted-up for a second, showing spiral arrangement in the bulb. -We tried another bromine-tube (No. 2): it lighted-up easily; both bulbs -were filled with a purple stream of light; capillary stream bright grey. -The glass of the tube was strongly fluorescent and of a yellow tinge. -When the magnet was excited the stream of light was somewhat condensed in -the bulbs, and flew to the side of the tube; while the capillary stream -at the same time brightened. The spectrum without the magnet was fairly -bright; it increased in brightness under the influence of the magnet, -and additional lines appeared; but we considered them to be only faint -existing ones brightened up. No change in the position of the principal -lines was traced. - - -_Silicic-Fluoride tubes._ - -[Sidenote: Si Fl₆ tube. Lighting-up described. Effect of magnet.] - -(1) A tube marked Si Fl₆ had been worked for photographic purposes; it -lighted-up easily. Both bulbs were filled with a brown-pink diffused -light, inclined to condense into a stream in the positive bulb. The -violet glow was very bright, and nearly filled the space round the -electrode. The capillary stream was of a bright violet tint. The effect -of the magnet was to decrease the intensity of the light throughout the -whole tube. - -In the positive bulb the stream broke up into a number of vibrating -streamlets, with little bright threads of light intermixed, which flew -towards the side of the tube at right angles to the magnetic poles. There -was an inclination to spiral arrangement in the streamlets. This stream -changed from side to side of the tube coincidently with change in the -magnetic poles. At the negative pole the violet glow formed an arc in the -direction of the magnetic curves, while a spiral of fainter (positive?) -light was formed in the upper part of the bulb. A slight ringing sound -was heard in the tube. - -[Sidenote: Comparison of Si F₄ and Si Fl₆ tubes.] - -(2) We compared two tubes (Si F₄ and the one marked Si Fl₆). The Si Fl₆ -tube in general effect, and in its spectrum, when lighted-up, resembled -Si F₄. We compared the one tube under the influence of the magnet with -the other not so, by means of a comparison-prism on the slit. As the -spectroscope and second tube were necessarily removed some distance from -the magnet, the spectrum of the tube between the poles was not bright. -We could not trace a change of position in any of the principal lines. -The tube between the poles was brightened up when the magnet was in -action[15]. - - -_Sulphuric-Acid (SO₃) tubes._ - -[Sidenote: SO₃ tube No. 1; lighting-up described. Effect of the magnet. -Changes in the spectrum.] - -(1) Excited by the small coil, both bulbs of this tube (No. 1) lighted-up -brightly, with a misty light-blue tinted stream of opaque light, a -yellow glow appearing at the negative pole. The capillary stream -partook of the same blue tint, but was whiter and brighter. Under the -magnet’s influence, the glow in the bulbs flew to the side of the tube -in flickering streams of light, the capillary at the same time changing -to a distinctly green tint. The spectrum without the magnet consisted -of four fairly bright bands of light in the yellow, green, blue, and -violet, connected by a faint misty continuous spectrum (O or possibly the -hydrocarbon spectrum found in O tubes by way of impurity). - -When the magnet was excited, this spectrum entirely disappeared; -and a set of bright metallic-looking lines upon a dark background -(line-spectrum of S) took its place. This effect was produced whenever -the magnet was excited, and we tried it several times, to make sure of -the complete change. After a time, when the magnet, battery, and the -coil-power were all weaker, with the magnet on, we obtained a compound of -both spectra, the bright lines being seen upon the continuous spectrum in -which the bands appeared. When the magnet was taken off, the bright lines -disappeared, and the O spectrum alone remained. - -[Sidenote: SO₃ tube No. 2 examined.] - -(2) We also tried another SO₃ tube (No. 2) which had been worked for -photographic purposes, and was suspected of a carbon impurity. Without -the magnet, the spectrum was very like that of the first tube; but when -the magnet was excited, the spectrum only brightened, and no bright -metallic-looking lines appeared. - - -_Sulphur-tube._ - -[Sidenote: Sulphur-tube. Lighting-up (without heating) described.] - -(1) A small bent vacuum-tube containing some solid sulphur, excited by -the smaller coil, and without being heated, gave a narrow stream of -bright blue-green light running straightly through it. With the magnet -on, this stream was deflected in the bulbs, and the capillary changed -from a blue-green to a distinct rosy tint. - -[Sidenote: Effect of magnet. Changes in the spectrum.] - -Without the magnet, the spectrum consisted of four bright bands, with a -continuous spectrum between, resembling that of SO₃ tube No. 1. With the -magnet on, the spectrum brightened, especially in the yellow and red, -which were dull before; and a set of lines appeared upon it (a line or -band in the yellow especially showing) which were not seen before. The -lines were distinct, but not very bright. The action on the capillary was -noticed to be strongest just between the conical points of the armatures; -and, in accordance with this, the central part of the spectrum-band in -the red and yellow showed an increased brightness. - -[Sidenote: Effects when one of the bulbs of the tube was heated. Changes -in the spectrum under influence of magnet.] - -(2) One of the bulbs of the tube was then gradually heated with a -small gas-flame. The single stream in the heated bulb became somewhat -deflected and broken up into a number of smaller streams; and these, -when placed under the magnetic influence, had small spark-like threads -of light running among them. The capillary, as the tube was heated, -and the sulphur rose in it, changed somewhat in tint, and, under the -magnetic influence, became of yellow-rose hue. As the heat was applied -to the bulb the bands of sulphur gradually appeared in the field of -the spectroscope, until at last the band-spectrum of sulphur entirely -took the place of the spectrum seen in the cool tube. The magnet being -excited, the spectrum changed at once, a set of bright sharp lines -(line-spectrum of S) appearing upon a faint and dull image of the -band-spectrum. - -This effect was constantly repeated upon the magnet being excited. The -magnet being taken off, the band-spectrum alone was to be seen. - - - - -CHAPTER XV. - -EFFECT OF MAGNET ON A CAPILLARY GLASS TUBE. - - -[Sidenote: Capillary portion of a Geissler tube tested in three ways.] - -The capillary portion of a Geissler tube was cut away from the bulbs, -cleaned, and connected by a small vulcanite tube with the gas-pipe in -the room conveying coal-gas at ordinary pressure. The flame was small -and oval in shape, 8 millims. high, by 4 millims. wide, and burnt quite -steadily. (Plate XVII. fig. 13.) - -[Sidenote: No effect on flame.] - -(1) The capillary tube was placed between the poles of the excited -magnet, almost, but not quite, touching them; no effect at all was -produced on the flame. - -(2) The tube was placed so that the conical ends of the armatures were -allowed to compress the centre of it between them; still no effect was -produced on the flame. - -(3) The tube was placed so that the straight sides of the armatures -compressed it between them; still no effect took place on the flame. - -[Sidenote: Flame between poles of magnet.] - -(4) The flame itself was placed between the poles of the magnet. It was -slightly drawn towards one pole with an inclination to form the magnetic -curve. - -[Sidenote: Quill glass tubing tested. No effect on the flame.] - -(5) A piece of quill glass tubing was selected, 5 millims. in diameter -and 1 millim. thick, and drawn out to a point, the end of which was -snapped off and the tubing connected as before. The flame was 20 millims. -high, and 5 millims. across, and somewhat lambent. On being placed (1) -between the conical ends and (2) between the flat ends of the armatures, -no effect could be seen on the flame. - -[Sidenote: Effect on taper and spirit-lamp flames.] - -(6) A small taper-flame was placed between the poles of the magnet: no -effect was produced, except that the flame gave a slight “jump” each time -the magnet was excited. A spirit-lamp flame was tried with a similar -result. - - -_Action of Magnet on a bar of heavy glass._ - -[Sidenote: Heavy glass bar and mounting described.] - -A piece of heavy yellow-tinted glass was selected, being a bar 10 -centimetres in length, and 8 millimetres square. This was mounted in a -frame with a Nicol prism at one end, and a double-image prism (next the -eye) at the other. - -[Sidenote: Placed along poles of magnet. Effect of magnet on -candle-images.] - -(1) The glass bar and mounting were placed upon and along the poles -of the magnet (in the direction of the magnetic curves), and the -double-image prism and Nicol were so adjusted that two images of a candle -were seen—the one below bright and normal, the one above, by rotation of -the prism, as nearly as possible extinguished (Plate XVII. fig. 4). On -exciting the magnet the faint image at once conspicuously brightened, -at the same time assuming a slightly green tinge. To get full effect of -brightening, it seemed necessary to have good pressure-contact between -the battery-wires and the binding-screws. - -[Sidenote: Effect on using a tourmaline as analyzer.] - -(2) Using a tourmaline as analyzer in lieu of the double-image prism, -the candle-flame was seen alternately brightened and darkened, as the -tourmaline was rotated; and when the image was obscured by rotation, -excitation of the magnet caused it to brighten strongly. This effect was -accompanied by the apparent removal of a dusky red patch or spot, which -occupied the centre of the field when the flame was obscured. - -[Sidenote: Bar placed at right angles to the poles: no effect produced.] - -(3) The bar of glass and double-image prism being placed between the -conical ends of the armatures, but at right angles to, instead of along, -the poles, upon excitation of the magnet no effect at all was produced. - -(4) The bar and prism being placed in the same position between the flat -ends of the armatures, no effect at all was produced. - -[Sidenote: Slight effect on second experiment.] - -(4_a_) Experiment No. 4 was repeated. It was thought that on excitation -of the magnet the secondary image slightly brightened; but there was a -doubt about it, and the effect (if any) was slight. - -[Sidenote: Effects produced when a biquartz was introduced.] - -(5) The apparatus was now changed for one of the following -arrangement:—1, a rotating Nicol prism next the eye; 2, the glass bar; -3, a biquartz with the halves horizontal; 4, another Nicol prism. The -neutral-passage tint of the biquartz was found to be rather green (from -mixture with the yellow of the glass). - -[Sidenote: Change in colour of the halves.] - -(i.) Placed _along_ the poles of the magnet and the magnet excited, a -change of tint was seen in both halves of the biquartz, the slightly -purple-reddish tint of the upper half passing into a full purple. Effect -not so marked as with the double-image prism. - -(ii.) Placed _across_ flat ends of the armatures (as in experiment No. 4) -no effect was seen. - - - - -CHAPTER XVI. - -EFFECT OF MAGNET ON WIDE AIR (AURORA) TUBE. - - -[Sidenote: Wide air-tube described.] - -A large, wide air-tube was tried; it was 14½ inches long by 1 inch -in diameter, of the same bore throughout, and with straight platinum -electrodes. - -[Sidenote: Magnet effect when tube placed vertically between conical -armatures.] - -(1) To excite it the larger coil was used. The tube was filled with -bright, steady, rosy light, and beautiful stratification, which, as -it flickered, seemed to incline to a continuous spiral (Plate X. fig. -8). This stratification was very close and fine, and extended nearly -throughout the tube. On excitation of the magnet (the tube having -been placed _vertically_ between the conical armatures), the glow was -condensed into a bright solid line or stream of light at the point -which lay directly between the poles. This line or stream expanded into -an elongated funnel-shape as it retreated from this centre towards -the extremities of the tube, the stratification showing itself more -distinctly as the glow of light became less dense (Plate XVIII. fig. 3). -The stream of light was driven away at right angles to the poles, and -changed from side to side of the tube with the direction of the current. - -[With the small coil this tube showed only a flickering stream of light, -with very slight indications of stratification.] - -[Sidenote: Effect when tube placed horizontally between the armatures.] - -(2) The tube was placed _horizontally_ between the conical ends of -the armatures. The condensed stratified stream of light flew upwards -and downwards (according to direction of current) instead of to the -respective sides of the tube. - -[Sidenote: Tube placed along the poles of the magnet.] - -(3) The tube was placed along the poles of the magnet. In the interval -between these the stream was driven upwards, but at either end sideways, -right or left according to whether the pole was N. or S. (Plate XVIII. -fig. 4). The result gave a complete spiral of stratified condensed light -within the tube. - - -_Note on Stratification._ - -[Sidenote: Stratification in small tubes arranged in series.] - -The current from the large coil was sent through a set of five small -French vacuum-tubes, of equal calibre, containing salts of strontium and -calcium, and showing phosphorescent effects. These tubes were arranged in -single series; and, from the colour of the glow-discharge, were presumed -to contain rarefied air in contact with the salts. - -A strong coarse stratification was seen in the central (No. 3) tube. -Tubes Nos. 2 and 4 also showed stratification, but in a less degree; -while the outside tubes, Nos. 1 and 5, showed no stratification at all. -The current was steady, and these effects did not fluctuate. - - -_Effect of Magnet on Plücker (Air-) Tube._ - -[Sidenote: Plücker air-tube. Lighting-up described. Effect of magnet on -the positive-pole stream.] - -(1) A Plücker air-tube was selected of the form shown on Plate V. fig. -1, and was excited by the small coil. The ring was used for the positive -pole, the straight electrode for the negative. When lighted up, the -tube glowed with a perfectly steady and quiescent light. The negative -electrode was surrounded by the usual bright violet glow, extending -itself and being gradually lost at a short distance from the wire, while -the ring let fall a faint, tubular, salmon-coloured, diffused stream of -light, which met the violet glow as it approached the negative pole. The -tube was then placed vertically between the poles of the electro-magnet, -the armatures being almost in contact with the sides of the tube around -the negative pole. On excitation of the magnet, an instantaneous change -took place. The stream of light from the positive pole contracted itself, -so that it became of a long funnel-shape (the ring forming the mouth of -the funnel), while it tapered almost to a point where it met the violet -glow. - -[Sidenote: Effect on negative violet glow.] - -The stream also became very brilliant (the sides of the tube being left -proportionately free from light), and crossing it were a set of bands, -or striæ, having a waving or vibratory motion. The whole of the negative -violet glow was simultaneously gathered into a brilliant narrow arc, -which stretched across between the poles of the magnet. These effects are -shown on Plate V. fig. 1. The edges of the arc were remarkably sharp and -well defined, and with no surrounding aura or shading off. - -[Sidenote: Arc of light followed the magnetic curves.] - -By moving the tube between the armatures it was seen that the arc of -light followed the magnetic curves. If the tube was moved upwards, the -arc curved towards the zenith, if downwards, contrariwise; and a middle -position could be selected, in which the edges of the arc were nearly -parallel. Moving the tube a short distance from the pole had the effect -of rendering the arc more diffuse, but not of otherwise altering its -character. - -[Sidenote: Direction of the current changed. Effects on glow described.] - -(2) The direction of the current in the tube was then changed; and, -without the magnet, the ring electrode was surrounded by a diffused -violet glow; while the straight wire gave forth a faint salmon-coloured -stream of light, spreading up to the ring. - -[Sidenote: Magnet effects described. On negative pole. Rings from -positive pole described. Effects on rings of making and breaking contact -with magnet. Shape of rings described.] - -On excitation by the magnet (the positive pole being now placed between -the armatures), the violet glow of the negative pole contracted into a -compact mass round the ring electrode. At the same time from the positive -pole sprang a set of bright saddle-shaped rings, which increased in size -as they advanced; and spreading upwards with a rapid but smooth motion -towards the negative pole, closely approached to, but never actually -came in contact with, the violet glow. The positive end of the tube -was otherwise but slightly lighted, and the sudden appearance of this -brilliant stream of rings of light was very striking. A single bright ray -was also seen running from the positive wire, in a somewhat transverse -course, along one side of the tube. When wire-contact with the magnet -ceased, so that it was not excited, the rings ran back in succession to -the positive pole and disappeared, and by making and breaking contact -they were caused to advance and retire at will. They were accompanied by -a waving or vibratory motion, and were evidently of the same character -as the smaller striæ or bands mentioned as seen when the ring formed -the positive pole. The general appearance was that of a hollow cone of -light (the base towards the negative pole), composed of brilliant rings -with dark spaces between, which appeared and expanded under the magnetic -influence, and contracted and disappeared on its removal. The rings did -not appear to be flat disks, but were somewhat curved or saddle-shaped. -They reminded one much of the diatom _Campylodiscus spiralis_; that is -to say, they were apparently flat if looked at from above, but like a -figure of 8 when viewed sideways, the peak of the saddle forming a kind -of brilliant point or apex. - -All this is difficult to describe; but an illustration from a sketch made -of the tube is given on Plate XVII. fig. 2. - -[Sidenote: Negative pole placed vertically on the magnet.] - -(3) The negative pole (straight electrode) was then placed vertically on -one of the poles of the electro-magnet. On excitation, the violet glow -was contracted into a small upright brush or column of bright light, with -a slight inclination to curvature. - -[Sidenote: Tube laid horizontally across poles of magnet.] - -(4) The same Plücker tube was laid horizontally across the poles of the -electro-magnet (without armatures), the respective electrodes being above -each pole. - -[Sidenote: Effects produced.] - -From the negative (straight electrode) pole sprang a dense and compact -arc of violet light, in the direction of the magnetic curves, which -terminated at the upper circumference of the tube, but which, if -prolonged, would have followed the curves to the opposite pole. The -stream from the positive pole was very considerably brightened, as in the -other experiments, but did not appear in the form of rings or waves. It -assumed that of a bright steady continuous glow, which formed round the -tube a not perfectly continuous, but distinct and well-marked, spiral. -This form of discharge seems connected with the peculiar contour of -the rings mentioned in experiment 2. One might, indeed, conjecture the -spiral-shaped glow to be a ring of light extended or drawn out towards -the negative pole. - -[Sidenote: Effects like those obtained by Gassiot.] - -Experiment No. 2 seems in result very like that of Gassiot’s with his -grand battery and the Royal Institution magnet, the effects (though of -course upon a smaller scale) being similar to those obtained by him. - - -_Effect of Magnet on Plücker Tube (Tin Chloride)._ - -[Sidenote: Plücker tube (tin chloride). Lighting-up described.] - -A large Plücker tube was examined, which had a bulb attached at each end, -communicating with the central portion by a narrow neck or constriction. -On connexion with the small coil, a narrow stream of pale diffused -cobalt-blue light ran along the whole tube, from point to point of the -electrodes, the positive wire at the same time glowing with an aura of -amber-yellow light. (See Plate XVII. fig. 3, where the narrow stream of -light is shown by dotted lines.) At the two necks or constrictions the -stream of light was perceptibly brightened. - -[Sidenote: Effects of magnet upon the stream.] - -When the magnet was connected, the stream in the positive bulb was not -much changed, but only slightly bent. In the central partition of the -tube and in the negative bulb, the stream of light was broken and split -into a number of smaller streams, and at the same time bent or forced -against the sides of the tube. (See Plate XVII. fig. 3.) - -[Sidenote: Peculiar noise within the tube.] - -In the central partition, the blue streamlets were accompanied by a -number of spark-like threads of golden light, which shone out among them -as the whole vibrated against the side of the tube; at the same time a -peculiar pattering, as of a miniature hail-storm within the tube, made it -ring with a slightly metallic tinkle. - -The direction of the bending or deflection of the stream was at right -angles to the axis of the poles of the magnet, and changed from side to -side of the tube as the direction of the current from the coil was varied. - -[Sidenote: Spectrum described. Without magnet. With the magnet excited.] - -In the positive bulb the stream, instead of joining the point of the -electrode, left this and ran along one side of the whole length of the -wire. (See effect, Plate XVII. fig. 3.) The spectroscope was applied to -the neck of one of the bulbs where the stream was bright. Without the -magnet a faint continuous spectrum, mainly of the blue and green, with -very slight traces of the yellow and red, was seen. Upon this, five or -six faint but sharp and metallic-looking lines were seen. On the magnet -being excited, the continuous spectrum was not changed; but the sharp -lines shone out brighter and clearer, one in the blue being especially -conspicuous. These lines were measured with a micrometer; and their -places being compared with Lecoq de Boisbaudran’s “Spectres lumineux,” -they were easily recognized to be those of tin. On each excitation of the -magnet the same brightening of the lines took place. - - -_Effect of Magnet on Tin-Chloride Geissler Tube._ - -[Sidenote: Geissler tube, Sn Cl₄, examined. Glow described. Effect of the -magnet.] - -We then examined a Geissler tube, marked Sn Cl₄. When first excited -by the small coil, the spark passed freely. The glow in the bulbs was -of a diffused, light purple tint; the positive electrode had a bright -yellow glow around it. The capillary stream was of a sharp green-yellow, -at times brightening up to a metallic-looking green. When the magnet -was first employed, the tube distinctly and permanently brightened up -throughout. - -[Sidenote: Spiral formed in positive bulb. Glow in tube extinguished.] - -The negative bulb was not much changed in appearance; but in the positive -bulb a curious permanent and steady cloud-like spiral, of a purple -colour, made its appearance, and lasted while the tube was under the -magnetic influence. (See Plate XVII. fig. 12.) After a short time the -tube seemed to lose a great deal of its conducting-power, and to light -up in a feeble and intermittent manner, brightening only when the coil -was made to work its best. While in this condition, the magnet (which had -been previously disconnected) was excited, and at once what moderate glow -was still shining in the tube was totally extinguished. At first it was -thought some accident might have happened to the conducting-coil wires; -but repeated trials satisfied us that the effect was due to the magnetic -influence alone. Efforts were made, by looking to the coil and battery, -to brighten up the tube as at first, but they quite failed; and it was -evident some change had taken place in its conducting qualities. This -tube was accidentally broken, so that we had no opportunity to renew the -experiments. - -[Sidenote: Another tin-chloride tube tried. Glow described.] - -We subsequently tried another tin-chloride tube, purchased of Mr. -Browning. This lighted up like the former tube, but brighter. There was -an amber glow at the junction of the negative bulb which adjoined the -capillary part. This was lost on putting on the magnet. At the same -time a perceptible pattering ringing noise was heard in the tube, and -metallic-looking threads of light ran through the bulbs. - -[Sidenote: Spectrum described.] - -Without the magnet, the spectrum was a continuous faint misty one, -with bright lines of tin occasionally flashing up. With the magnet, -the tin lines at once shone out bright, strong, and clear upon a black -background, the change in effect being very marked. - - - - -CHAPTER XVII. - -EFFECT OF MAGNET ON BULBED PHOSPHORESCENT TUBE. - - -[Sidenote: Large phosphorescent bulbed tube. Lighting-up described. -Spectrum described. Glow when discharge stopped, described.] - -Mr. John Browning kindly lent me a large phosphorescent tube with five -bulbs, said to be filled with anhydrous sulphurous-acid gas (SO₂). (See -Plate XVIII. fig. 1.) This tube lighted up beautifully with the large -coil. The connecting tubular parts of it were filled with a bright, -beaded, transparent, rosy light; while the bulbs glowed with a more -opaque blue-tinted effect. The spectrum of the tubular part was found to -agree exactly with the principal bright band seen in a SO₃ Geissler tube. -The spectrum of the bulb-glow was a faint green-blue continuous one, with -bright bands or lines faintly flashing up at times. When the discharge -was stopped, the tube still glowed with a moderately bright, opaque, -grey-green light. This glow gradually faded out, always commencing with -the bulb forming the negative or violet pole, and so dying out, bulb by -bulb, towards the positive pole. The negative-pole bulb at times was, on -suddenly stopping the current, hardly lighted at all, the other bulbs -being luminous. - -[Sidenote: Comparison with SO₃ Geissler tube.] - -(1) We compared the large tube with a SO₃ Geissler tube, by means of -a comparison-prism on the slit, with the result before detailed. The -Geissler tube, however, showed no after-glow. - -[Sidenote: Effects in bulbs on lighting-up the tube described. Effects of -reversal of the current. After-glow restored by passing of current.] - -(2) We lighted up the Browning tube with the large coil. The negative -bulb was always the least filled with the blue opaque vapour, and the -other bulbs increased in vapour-density in the order they approached -towards the positive bulb. When the current was reversed, so that the -negative and positive glow changed places, the negative bulb still -remained transparent, although the positive opaque glow had (presumably) -been thrown into it. When the after-glow had quite disappeared in the -bulbs, it was again strongly restored, by the passing of the current for -a few seconds only through the tube. - -[Sidenote: Effect of reversal of current on positive-pole glow.] - -(3) The tube was well excited, and the four bulbs (other than the -negative one), upon stopping the current, glowed strongly. The current -was then sent through reversed, so as to throw the negative glow for a -few seconds into the positive bulb. The after-glow in the positive bulb -was at once extinguished. On once more reversing the current, it was only -restored after a certain amount of continuance of the positive stream. - -[Illustration: Plate XVIII.] - -[Sidenote: Effect of change of current on the three central bulbs.] - -The time during which the negative glow was thrown into the positive bulb -did not appear sufficient to have heated it. After rapidly changing the -direction of the current several times and then stopping it, the three -central bulbs alone had an after-glow, the two extreme ones having none, -being both equally transparent. - -[Sidenote: Effect of heat on the bulbs. Effect of cooling by ether-spray.] - -(4) A moderate heat from a spirit-lamp was applied to the centre bulb -(_a_) while the current was on; and also (_b_) when this was stopped, -and the bulb glowed. In the first case the bulb was found to get more -transparent; and in the second case the after-glow disappeared in a -proportionately shorter time in the heated bulb than in the others. To -test the result of cooling the bulbs, the negative-pole bulb and also the -central one were each subjected to the action of ether-spray, and also -of ether and water-spray mixed. This was done, (_a_) when the current -was passing, and (_b_) when it was stopped and the glow only was in the -bulb. The bulbs were cooled until a marked cold effect to the touch was -produced. We did not notice any difference in the behaviour of the bulbs -so treated as compared with the others, either when the current was -passing or in the case of the after-glow. - -[Sidenote: Negative-pole bulb between the armatures of magnet. Effects on -negative and positive glow.] - -(5) We placed the negative-pole bulb between the conical points of the -armatures, and excited the magnet. The negative glow contracted itself -into a condensed violet-tinted crescent, in accord with the magnetic -curves. The positive glow of the same bulb lost its beaded (stratified) -character, and was condensed into a bright stream of light, which latter -protruded from the small inner tube and formed a spreading spiral set of -cloud-rings within the bulb (see Plate XVIII. fig. 2). The action of the -magnet seemed to be exercised in subduing the stratification, condensing -the glow into a bright stream of light, and forcing the latter to “tail -over” at each extremity of the tubular joints into the bulbs—this effect -extending even so far as the second bulb. - -When the positive bulb was placed between the poles of the magnet, the -glow was simply condensed into a bright stratified stream, which flew to -either side of the bulb. - -[Sidenote: Effect of magnet on glow in bulb No. 4.] - -(6) _a._ Bulb No. 4 (see Plate XVIII. fig. 1) was placed between the -poles of the excited magnet, and the current was passed and then stopped. -The glow in that bulb faded away out of its order, and earlier than in -ordinary cases (nearly as soon as No. 2). - -[Sidenote: Other bulbs tested in similar manner.] - -_b._ The same and other bulbs were tested in a similar manner. In all -cases the bulb influenced by the magnet, when the current was stopped, -was found perceptibly fainter in after-glow. - -[Sidenote: Effect of magnet upon the after-glow itself.] - -_c._ The tube was arranged with one of the bulbs between the poles -of the unexcited magnet; the current was passed and stopped, and -the after-glow obtained. The magnet being then quickly excited, the -after-glow in the bulb, under its influence, faded out; and the bulb -became transparent, perceptibly sooner than under ordinary circumstances. -We tried this several times, with the same result in each case. - -[Sidenote: Mr. Thompson’s experiments on action of magnets upon liquid -rings.] - -_Note._—In relation to these experiments, it may be mentioned that Mr. -S. P. Thompson, of Bristol, is reported to have studied the action of -magnetism upon rings of coloured liquid projected through water, and -to have observed their retardation and partial destruction in passing -through a powerful magnetic field. - -[Sidenote: Mr. Ladd’s explanation of some of the phenomena observed.] - -Mr. Ladd has suggested to me that some of the phenomena produced -indicate a driving of the gas in the direction from the negative to the -positive pole—a theory which is supported by the action of the magnet -on the bulbs, if this be considered a repulsive one as regards the gas -influenced. - - -_Effect of Magnet on small Phosphorescent (powder) Tubes._ - -[Sidenote: Tubes containing phosphorescent powders described.] - -We examined six vacuum-tubes containing phosphorescent powders, which, -upon exposure to sunlight and removal to the dark, or after passing of -the electric current over them, continued to glow in the tubes after the -exciting cause had ceased. They were of thin glass, and of equal calibre -throughout. - -One was 6½ inches long and ⅝ inch in diameter, and had no label; the -other five were 7½ inches long and ½ inch in diameter, and were labelled -respectively:— - - Strontium vert, - ” jaune, - Calcium violet, - ” orange, - ” vert-bleuâtre. - -[Sidenote: Lighting-up of the tubes described. Effect of magnet on -⅝-diameter tube. Spectrum without magnet.] - -The powders in tubes of this description are said to contain either -sulphide of strontium, or calcium, or sulphate of quinine. The -first-mentioned tube shone with a white and bright light, and probably -contained the latter substance. The general effect of the current on the -tubes was similar in all cases. Under a sufficiently strong current, -they lighted up with a brilliant, slightly green-white glow; in which, -however, by looking sideways, it was possible to detect a delicate rosy -tint. Any colours beyond these in the tubes seemed to depend on the -powders enclosed in them. When the current was stopped, the powders -alone glowed in accordance with the colours mentioned on the labels, the -rarefied gas or air in the tubes not giving any after-glow, as in the -case of the sulphurous-acid tube. When the ⅝-diameter tube was excited -by the small coil, the effect of the magnet was to entirely suppress and -extinguish the glow. When this and the other tubes were worked with the -larger coil, the spectrum, without the magnet, was bright and continuous, -either showing no lines or else very faint traces of them, and, extending -through the whole range of colours was brightest in and about the green. - -[Sidenote: Magnet effect on glow. Same on spectrum.] - -With the magnet excited, a bright line of pink light was condensed -against the upper side of the tube; while the glow in the tube generally -became very decidedly fainter, except at the electrodes, which still -preserved a certain amount of brilliancy. The spectrum also was much -changed. The bright continuous glow became much fainter, and many sharp -and fairly bright lines were seen upon it. These lines were, as to -character, not easy to recognize. Hydrogen (F) was, however, plainly -distinguished; and other lines, which we considered to be N, were common -to all the tubes. Some lines were also remarked as being, without the -magnet, not so constant. - -[Sidenote: Tubes examined and compared for spectra.] - -Calcium orange and calcium violet, compared for spectra, were identical; -the two strontium tubes hardly so, but with strontium vert a bright -continuous spectrum mainly hid the lines. - -Strontium jaune and calcium orange were not alike; strontium vert and -calcium violet differed. Calcium orange and calcium vert-bleuâtre were -considered alike; but the comparison was not easy, as the calcium vert -was bright, and the lines were only seen faintly upon the continuous -spectrum. - -In order not to shift the powders, the tubes were laid horizontally, and -two spectra simultaneously examined across the tubes. - - -_Lighting-up Tubes with One Wire only (Marquis of Salisbury’s -Observations)._ - -[Sidenote: One wire only connected with an electrode.] - -The vacuum-tubes employed were examined in the usual way, but one wire -only was connected with an electrode. The other wire was attached to the -end of a glass rod, and circuit was from time to time completed while the -tube was before the spectroscope. - -The large coil was used. In all cases, with the one wire, the glow was -very faint as compared with that of the closed circuit. - -[Sidenote: Ether vapour.] - -(1) _Ether Vapour._—With both wires, in company with the usual bright -bands of the carbon spectrum, shading-off towards the violet, the H lines -were very sharp and brilliant. With the one wire only, the carbon bands -were left faintly shining, with both sides nebulous alike, and with no -shading-off towards the violet. (We were not quite sure whether this -was not the effect of the reduction of the light.) The H lines, though -originally stronger than the carbon bands, quite disappeared from the -spectrum. - -[Sidenote: Coal-gas.] - -(2) _Coal-gas._—The same effects were produced; but we thought we could -detect very faint traces of the H lines. - -[Sidenote: Nitrogen.] - -(3) _Nitrogen._—The N lines, as well as those of H (also seen in the -tube), were much fainter with one wire, but the H lines more so in -proportion. - -[Sidenote: Hydrogen.] - -(4) _Hydrogen._—Only a marked reduction in brilliancy of the whole -spectrum. - -[Sidenote: Oxygen, N and H.] - -(5) _Oxygen._—An impure tube, showing O (some of the lines hydrocarbon?), -N, and H spectra simultaneously. With one wire the O lines still remained -fairly bright, the N and H being only faintly seen. - -[Sidenote: Water-gas.] - -(6) _Water-gas._—Same effect. - -[Sidenote: Turpentine vapour.] - -(7) _Turpentine Vapour._—Same effect as ether, but the H lines could be -faintly seen. - - - - -CHAPTER XVIII. - -ACTION OF THE MAGNET ON THE ELECTRIC SPARK. - - -[Sidenote: Apparatus employed.] - -The magnet was excited with two plates of the large battery, and the -larger coil with the other two plates, the action in both cases being -strong. - -1. A spark from the coil was passed between two platinum wire electrodes, -about three centimetres apart. - -[Sidenote: Spark and aura described.] - -It consisted centrally of a thin stream of bluish-white light, vividly -bright, around which was seen a narrow, uniform, diffuse, yellow-tinted -aura, which accompanied the spark in all its movements. The spark always -struck across from the extreme points of the electrodes (see Plate XVII. -fig. 5). - -[Sidenote: Effect of magnet upon the aura.] - -2. On being placed between the conical poles of the excited magnet -the bright thread of the spark did not change; but instead of the -inconsiderable yellow-tinted aura which accompanied the unmagnetized -spark, there now struck out, at right angles to the magnet-poles, a -thin rosy-tinted half-disk of aura-like flame. This extended aura ran -considerably along each electrode, though the spark proper still struck -from the points. - -[Sidenote: Extended aura described.] - -The aura was somewhat larger in extent upon one electrode than on the -other. In the first case, it sprang from a considerable number of minute -illuminated points; on the other electrode, these illuminated points -were fewer in number, and the flame was more purple in tint. Reversing -the current these effects were reversed. The aura was uniformly thin and -disk-like, and the curved edge remarkably true in shape (see Plate XVII. -fig. 6). - -The lateral direction of the aura was changed when the current was -reversed. - -[Sidenote: Aura not proportionate to length of spark.] - -3. The aura was found not proportionate to the length of the spark. When -the electrodes were approached, so as to very much shorten the spark, the -aura still sprang out to a distance and extent quite out of proportion -to the length of the spark. Even when the electrodes were approached so -close that the spark was very short indeed, still, under the magnetic -influence, a very considerable aura made its appearance. - -[Sidenote: Effect of working coil-break upon the aura.] - -4. Upon working the coil-break, it was found that in proportion as the -contact screw was drawn apart from the break, so the aura gradually -diminished in extent, until at last, by continuing to increase the -distance between the screw and the break, a point was reached when thin -bright sparks, without any aura, passed. Upon the screw being worked up -closer, thicker sparks passed, and the aura again made its appearance. As -the aura diminished in size it gradually changed in tint from yellowish -rose-pink to purple. - -[Sidenote: Spark taken in glass bulb.] - -5. The spark was taken in a glass bulb, the tube in which it was blown -being open at both ends, with the same effect as in the open air. - -6. A plate of glass was laid on the poles of the magnet, and the spark -was passed _along_ the poles (in the same direction as the heavy glass -was laid in the Faraday experiment). No aura was formed. The points were -then moved round, so as to carry the spark at right angles to the poles, -and the aura was formed as before. - -[Sidenote: Aura could be blown away from the spark.] - -7. The aura, it was found, could be blown away at right angles to the -spark. When strongly urged, it assumed the shape of a flickering tongued -curtain of flame, flying away in the contrary direction to that from -which the current of air proceeded, and again returning to its original -shape as the impulse was removed. The spark proper was not influenced -(see Plate XVII. fig. 8). - -[Sidenote: Effect of withdrawing spark from central position between the -poles.] - -8. As the spark was withdrawn from its central position between the poles -of the magnet, the convex edge of the aura became gradually less perfect, -and assumed a ragged and broken-up appearance, the inequality at times -amounting almost to jets or flickering sprays of light. The spark was -also slightly curved away from the electrodes (see Plate XVII. fig. 7). - -[Sidenote: Magnet had no effect upon condensed spark.] - -9. A condenser of four coated plates was introduced into the circuit, -causing a sharp brilliant blue-white spark, apparently divided into -streams and with no aura. The magnet had no effect whatever upon this -form of spark. - - - - -CHAPTER XIX. - -THE DISCHARGE IN VACUO IN LARGER VESSELS, AND MAGNETIC EFFECTS THEREON. - - -A Tate’s air-pump was used, and the spark from the larger coil. The -exhaustion could not be carried very far. - -[Sidenote: Globular receiver described. Discharge described.] - -(1) A globular receiver was used, having brass caps for exhaustion, -and platinum wires passing through the opposite sides for electrodes -(see Plate XVIII. fig. 6). With partial exhaustion, from the positive -electrode proceeded long, sharp, bright, rosy sparks, striking in zigzags -across the receiver. From the negative terminal sprang a larger number of -bluer and more diffuse streams of light, like spiders’ webs; and these -were enveloped, for a short distance from the terminal, in a slight misty -aura. Both sets played round the sides of the glass as well as across. - -[Sidenote: Bell-shaped receiver described. Discharge described.] - -(2) A bell-shaped receiver, with terminals inserted at the sides and one -also at the top, was next used (see Plate XVIII. fig. 9). When the side -terminals were employed, the effect was much the same as in the last -case. When the top terminal was used for one wire (the other wire being -connected with the pump-plate) a single stream of bright rosy light ran -from the upper terminal to the plate. First striking the central part of -the plate, the stream then glided towards one of the lateral terminals, -and so to the edge of the receiver. After partly discharging itself by -contact with the terminal, the stream as rapidly retreated to the centre -of the plate again—this effect being from time to time repeated while the -current was passing. The current being reversed, a number of bright, but -weaker and more diffused, streams of light had the appearance of shooting -from the upper electrode, and of striking upon the plate below; with a -tendency to fly off from where they struck, in a similar manner to the -single stream before described. Where each stream touched the plate a -brilliant point of light appeared, and a strong pattering noise was heard -in the receiver. - -[Sidenote: Bell-shaped receiver without electrodes. Induction discharge -described.] - -(3) Another bell-shaped receiver of similar shape was used. This had no -electrodes forming a direct communication with the interior; but, in -lieu of these, two wafers of thin sheet brass were cemented, one inside -and one outside the glass, opposite to one another. On connexion being -made with the outside wafer, the effects produced by induction were -similar to, and very nearly as strong as, those in the cases where direct -communication with the interior of the receiver was made. - -[Sidenote: Long large tube exhausted and illuminated. Spiral form of -discharge.] - -(4) A large tube, 24 inches long and 2 inches in diameter, with ball and -point electrodes respectively, was exhausted, and the current passed -through it. The effects were similar in most respects to those produced -in the globular and bell receivers, but the streams of light assumed a -distinctly spiral form in their passage (see Plate XVIII. fig. 5). This -tube when placed between the poles of the magnet showed no effect, except -a slight condensation of the streams of light towards the sides of the -glass. - -[Sidenote: Globular receiver again used.] - -(5) The globular receiver first described was again used (the Tate pump -having been cleaned and working easier). - -[Sidenote: Phosphorescent after-glow succeeding the spark.] - -(_a_) When exhaustion was as good as it could be got, the spark struck -across in a single, slightly expanded, stream of rosy light, having a -tendency to curve upwards (see Plate XVIII. fig. 6). The electrodes had -but little glow round them, only just enough to distinguish the poles -apart. When the flow of the stream was interrupted by breaking contact -with one terminal, so that sparks passed in succession, we thought we -detected a faint blue phosphorescent after-glow succeeding each spark. - -[Sidenote: Positive wire only attached.] - -(_b_) The positive wire only was attached to one electrode, the negative -being unconnected. A set of faint whity-blue cobweb-looking streams of -light spread from the electrode all over the receiver, having a vibratory -motion. The spaces between these were dark, and there was no aura—the -effect being similar, but not quite so bright and pronounced, as when -both wires were attached (see Plate XVIII. fig. 7). - -[Sidenote: Negative wire only attached.] - -(_c_) The negative wire only was attached. The cobweb streams were -absent, or only shot out very occasionally. The main effect was a -straight nebulous stream of violet light, which commenced at the -electrode and spread out in a fan-shape towards the lower brass cap of -the receiver; while, at the same time, an aura or glow of similar light, -but fainter in quality, spread from the electrode over at least one half -of the receiver. This aura would no doubt have filled a small flask (see -Plate XVIII. fig. 8). - -[Sidenote: Effect of gradual exhaustion on the discharge.] - -(_d_) When exhaustion was first commenced, both electrodes of the -receiver (both wires being connected) threw out spider-web-like streams, -as in Experiment 1, pale blue from the one pole and somewhat rosy from -the other. - -As the exhaustion progressed the pale-blue streams disappeared, while the -rosy flickering ones diminished in quantity and extent until ultimately -a single rosy stream of light crossed the receiver as in Experiment -5_a_. Upon admitting the air, these effects took place in an inverse -order—the single stream being gradually broken up, and the spider-webs -taking its place. - -[Sidenote: Globular receiver placed on poles of the magnet. Magnet -effect.] - -(_e_) The exhausted globular receiver was placed upon the poles of the -excited magnet, with the stream at right angles to them. Looking across -the S. pole of the magnet, the negative electrode was on the left hand, -and the positive on the right. The effect of the magnet on the stream -was apparently to split it up into several; but this appearance must -have been due to vibration only, as a revolving mirror showed the stream -as single. When the current was reversed, the stream which, without the -magnet, was somewhat flickering and vibrating, slightly straightened at -the positive pole, and the whole stream became steadier. - -[Sidenote: Single wires attached.] - -(_f_) Single wires were successively attached to the negative and -positive poles, and the cobweb streamers and glow before described -obtained. The magnet was found to have no decided effect on either of -these. - -[Sidenote: Plücker tube placed between poles of magnet with negative wire -only attached.] - -(6) The Plücker air-tube (Plate XVII. fig. 2) was placed between the -poles of the magnet, and the negative wire only was connected with the -straight electrode. A pale violet glow was seen round this electrode, and -another, but rather fainter, glow of a similar description at the ring -electrode, the intermediate space being filled with a salmon-coloured -light. This violet glow was condensed into an arc by the action of the -magnet. Reversing the current, the violet glow still remained at each -electrode, and that between the poles of the magnet was still influenced -into an arc. - -[Sidenote: Geissler tube substituted, with similar results.] - -(7) An air Geissler tube was substituted for the Plücker tube, with very -much the same result. Whichever wire was attached, a violet glow appeared -at the connected electrode, and a fainter one of the same character at -the other; and the magnet influenced both. The connecting salmon-coloured -glow was faint. - -[Sidenote: Globular receiver treated with phosphoric anhydride.] - -(8_a_) The globular receiver had some phosphoric anhydride shaken into -it; and it was then exhausted. The cobweb streamers and violet glow each -appeared according to which wire was connected. There was no marked -difference between the receiver with the anhydride and without; except -that in the former case the streamers and glow were reduced in extent and -strength, and were comparatively faint. - -[Sidenote: Discharge in water-vapour described.] - -(_b_) The anhydride having been washed out, first with plain and -afterwards with distilled water, some drops of the latter were allowed -to remain in the receiver. On exhaustion a vapour-cloud was formed, and -the discharge passed (both terminals being connected with the coil) -through this. The rosy stream of light was formed as usual, but was -more flickering and unsteady. As the exhaustion was lessened, the rosy -stream disappeared, and the cobweb streams began to fill the receiver. -These were, however, not so bright and sharp as in a dry receiver, but -were faint and broad; while some diffused and nebulous streams of light, -running (slightly bent) from pole to pole, and from ¼ to ⅜ of an inch -broad, were intermixed with them. When one wire only was connected, the -glow and streamers from the electrode were very faint. - -[Sidenote: Large bell-receiver and plate described. Receiver exhausted -and stream of light formed.] - -(9) A large bell-shaped receiver, 11 × 8 inches, was next used. It was -open at the bottom, which was ground as usual; and had a small opening -at the top, also carrying a ground edge. A solid brass plate was -prepared, ground only round the edge (in order to take the receiver), -and in the centre of this brass plate were inserted two disks of soft -iron, corresponding in position and size with the poles of the Ladd -electro-magnet (see Plate XVIII. fig. 11). When this plate was placed on -the magnet, the poles and disks were in contact; and the disks became -N. and S. poles within the receiver. A small brass plate carrying a tap -and exhaust-tube, and a binding-screw for attaching an electrode within, -closed the receiver at top. The receiver and plate being placed on the -magnet-poles, the former was exhausted until the discharge became a rosy -slightly-diffused stream of light; with a small unilluminated space -between it and the negative pole, where the usual violet glow appeared -round the wire. - -This stream of light was used for the experiments after detailed. In some -cases the conical armatures were placed within the receiver, in others -the disks alone were used as the magnetic poles. - -[Sidenote: Effect on same when magnet excited.] - -(_a_) With the apparatus arranged as shown on Plate XVIII. fig. 10, and -the magnet excited, a violet glow appeared round the end of the wire -which was negative. A small unilluminated space then intervening, the -stream ran in a curve between the wire and the armature, which latter was -positive. The stream was not steady and had a tendency to rotate; but as -this was better observed with the disks only, it is described further on. - -[Sidenote: Experiments with the conical armature removed. Vibrating -stream.] - -(_b_) The conical armature within the receiver was removed, and the -stream allowed to connect with the centre of the pump-plate. When the -magnet was excited, the stream was violently projected at right angles -to the poles, with a vibrating movement to either side according to the -direction of the current. When the wire was positive the movement was -towards the left, with a slight inclination towards the N. pole. When -the wire was negative the movement was to the right, but in a rather -strong curve towards the N. pole. The vibrating motion was very distinct, -and gave the appearance of six or seven streams running off at regular -intervals (see Plate XVIII. fig. 12). - -[Sidenote: Rotating wire over S. pole.] - -(_c_) The wire was next placed over the centre of the disk forming the -S. pole. With the wire negative and the pole positive, rotation of the -stream was decidedly, but not very strongly, from right to left from the -centre of the plate (as the hands of a watch). With the wire positive -and the pole negative, rotation was strongly left to right, with a -disposition to spiral twist in the stream (see Plate XVIII. fig. 13). - -[Sidenote: Same over N. pole.] - -(_d_) The wire was placed over the disk forming the N. pole. With the -wire negative and the pole positive, rotation of the stream was left to -right. With the wire positive and the pole negative, rotation was right -to left (see Plate XVIII. fig. 14). - -[Sidenote: Stream thrown across the receiver above the magnet-poles.] - -(_e_) The stream was thrown across the receiver from the lateral -binding-screws above, and at right angles to, the disks, and afterwards -in the opposite direction, _i. e._ along them. In neither case was there -any marked change when the magnet was excited. - -(_f_) The conical armatures were placed with the pointed ends upon the -disks in the receiver, and the stream thrown above and along them. It -diverged—one part running straight across between the electrodes, whilst -another stream and some cobwebs ran from each electrode to its nearest -pole. The streams and cobwebs flickered a good deal. There was no marked -change when the magnet was excited. - - -_Some of Baron Reichenbach’s Magnetic Researches tested._ - -[Sidenote: Baron Reichenbach’s researches.] - -In 1846 Dr. W. Gregory published an abstract of Baron Reichenbach’s -‘Researches on Magnetism and on certain allied subjects, including a -supposed new Imponderable.’ - -[Sidenote: Auroræ considered to be magnetic lights. Flames seen by -“sensitive” persons.] - -From a paragraph in this work, it would seem that the Baron considered -his observations as tending to an explanation of the Aurora Borealis; -and, since it was generally admitted that these phenomena occur within -our atmosphere, that there appeared a great probability of Auroræ being -visible magnetic lights. The Baron, in the original work, fully describes -the Aurora Borealis; and concludes it must be similar in its nature -to the flames of light seen streaming from the magnet-poles by Mdlle. -Reichel and other sensitive patients of the Baron’s. It is unfortunate -that these flames were only seen by certain “sensitive” persons. The -drawings given of them, too, show no analogy to the magnetic curves. - -[Sidenote: Magnet tested for such flames.] - -Having the opportunity of a powerful magnet in that used during our -tube-experiments, we made an attempt to detect the Baron’s magnetic -flames, on or around the poles of our magnet, in a perfectly dark room. -Arrangements were made to silently connect and disconnect the battery -with the magnet, without the knowledge of any one except the operator. -The experiment proved a complete failure; no flames or discharges of -light of any kind were to be seen. The observers were five in number, two -gentlemen and three ladies, but not one of the party proved “sensitive.” - -[Sidenote: Mr. Brooks’s experiments on action of the magnet on a -sensitive photographic plate.] - -Some experiments made by Mr. W. Brooks, and detailed in a paper read by -him before the South London Photographic Society, seem to corroborate -(to a certain extent) the statements made by the Baron in regard to the -influence of the magnet on a sensitive photographic plate. - -Remembering, however, how it has been demonstrated that light may be -“bottled up” as an actinic source for a considerable period of time, it -seems a question whether the images obtained were not due to some such -source rather than to any magnetic aura. - - -SUMMARY OF THE FOREGOING EXPERIMENTS AND THEIR RESULTS. - -[Sidenote: Summary of the experiments.] - -Chapter XIV. Action of magnet on glow and spectrum of Geissler gas -vacuum-tubes demonstrated. - -Chapter XV. Action of magnet on glass capillary tube negatived. Faraday’s -experiment with heavy-glass bar repeated. - -Chapter XVI. Action of magnet on glow in wide air-tube demonstrated. Note -on stratification. In Plücker tube, action of magnet on negative pole -(arc formed) and positive pole (Gassiot’s rings produced) demonstrated. -Effects of magnet upon glow and spectrum of tin-chloride vacuum-tubes -demonstrated. - -Chapter XVII. Effect of magnet upon after-glow in a bulbed phosphorescent -tube demonstrated. Effect of magnet upon glow in small phosphorescent -(powder) tubes examined. Marquis of Salisbury’s experiments (lighting-up -with one wire only) tested, and confirmatory results arrived at. - -Chapter XVIII. Action of magnet on aura of electric spark demonstrated. - -Chapter XIX. Effects of magnet on discharges _in vacuo_ in larger vessels -demonstrated. Ångström’s flask experiment tested; same results not -obtained unless one wire only was connected. Experiments demonstrating -the action of a magnet on an electric stream, viz. vibration between, and -rotation round, poles. Baron Reichenbach’s magnetic flames tested without -result. - - - - -CHAPTER XX. - -SOME CONCLUDING REMARKS. - - -It is usual, in concluding a work on a special subject, to sum up its -contents, and to examine the general results arrived at. This, however, -it is not easy to do in the present case. The contents of our volume -comprise a short history of the Aurora, its qualities and spectrum; and -a statement has been given of the several conclusions at which various -observers have arrived as to its character and causes. In the present -state of our knowledge of the subject, to add an opinion to these might -seem to savour of presumption; and the questions involved may perhaps -be better treated as still _sub judice_, and as requiring further and -fuller evidence before arriving at a verdict. The following observations -must therefore be taken rather as further notes and memoranda, than as -conclusions. Apart from the spectroscopic questions involved, the oldest -and most received theory of the Aurora—that of its being some form of -electric discharge in the more rarefied regions of the atmosphere,—seems -to hold its own: and if, as is probable, some form of phosphorescence is -involved in the discharge, M. Lecoq de Boisbaudran’s observations on the -brightening of the red line under the influence of cold, and the falling -of the yellow-green line within a band of phosphoretted hydrogen, come -into play; and a connexion, though slight and imperfect, may be in this -respect traced between the discharge and its spectrum. The experiments -detailed in Part II. seem to have an important bearing, as showing the -very marked effect of the magnet on the rarefied glow, as well as on the -spark in air at ordinary pressure. The well-defined arc formed by the -aura of the spark, the flickering jets which replace the even edge of -the arc when partially withdrawn from the magnetic influence, and the -streamers formed when the aura is blown away from the spark (Plate XVII. -figs. 6, 7, and 8), are certainly highly suggestive of frequent forms -of Auroral discharge; and, but for trial and failure, might lead one to -expect results from a comparison of the line air-spectrum with that of -the Aurora. The experiments with a wire attached to one electrode only, -show how the glow may be affected and varied in colour and character when -the discharge is interrupted and incomplete. Differences in electric -tension may also considerably vary the character of the discharge. - -The influence of the magnet in exciting and brightening the glow and -spectrum of one gas, while it depresses and extinguishes the glow and -spectrum of another gas in the same tube, suggests an explanation of -the observed variation in intensity, and difference in number, of the -Aurora-lines. Intensity of lines depending on temperature, and this -again on resistance, and it appearing that resistance is influenced by -the magnetic action, the same effects of brightening or depressing of -the spectrum are probably produced in the Aurora, as in the vacuum-tubes -placed between the magnet-poles. - -In the Marquis of Salisbury’s observations, paraffin-vapour gave C -and H lines when connected with both poles of the battery, but C -lines only when connected with one pole; and in that case the lines -were equally sharp on both sides. These observations (repeated in our -experiments) may afford an explanation why the hydrogen-lines are not -seen in the Aurora-spectrum; although there can be hardly any doubt -that the phenomenon usually takes place in air more or less moist. -Professor Ångström’s researches on the violet-pole glow are not entirely -corroborated by our experiments; and it seems doubtful whether his -results in the exhausted flask were not obtained from the negative pole -only. One great difficulty in the comparison of the Aurora-spectrum -with the violet pole of air-tubes and some other spectra (including -oxygen), arises from the presence in the latter of broad bands; and it -is difficult to understand how these bands can be aptly compared with -the definite, though faint, lines observed by Dr. Vogel and others in -the Aurora-spectrum. It must, too, be borne in mind that the conditions -under which we may consider the Aurora to obtain, are such as can be -only very imperfectly imitated in the laboratory. Auroræ also no doubt -differ in density and thickness of layer; and Kirchhoff’s observation -must be remembered:—“That if thickness of a film of vapour be increased, -the lines are increased in intensity, the bright lines more slowly than -the fainter; and it may happen that the spectrum appears to be totally -changed when the mass of the vapour is altered.” Were it possible to test -with the spectroscope a cloud or film of gaseous vapour corresponding -in some degree in density and thickness with an Auroral discharge, we -might perhaps get nearer the truth. Mr. Procter also remarks (as we -proved in our magnet experiments):—“That frequently very small traces -appropriate to themselves the whole of electrical discharges at low -pressures, and completely mask the spectra of any other gases present.” -The oxygen-spectrum, with its possible variation by the conversion of -that gas into the allotropic condition termed ozone, seemed at first -to afford a prospect of close relation to the Aurora-spectrum; which, -however, disappeared on closer examination. If nitrogen could be modified -in some such way as oxygen is converted into ozone, it might perhaps -afford another opportunity for investigation; but we have no evidence -at present of such a change. The spectrum of nitrogen is usually found -singularly distinct and persistent; and, except as varied from band to -line by intensity of the discharge, not liable to alteration[16]. - -Colours of lines are functions of wave-length, subject, however, to the -observation that in a weak spectrum the colours lose their intensity. The -red line in the Aurora has sometimes been found brighter than the green. -It has been suggested that the red and green may be independent spectra; -but the variations of tint observed in the capillary of hydrogen and -other tubes according to resistance of the current, demonstrate that the -varying colours of the Aurora may be connected with the lighting-up of -particular parts of the spectrum, and do not necessarily indicate that -different gases and spectra are excited. - -Absorption may also play an important part in the nature of the -Aurora-spectrum (Zöllner’s theory that the lines are really spaces -between absorption bands). Most gases will give a continuous spectrum -under certain circumstances, even at a low pressure. - -The question of cosmic dust is inviting, but the facts collated hardly -warrant at present its probable connexion with the Aurora. - -If Auroræ were composed of incandescent glowing meteors, it would be -reasonable to expect to find in the spectrum the lines of iron, a metal -constituting so prominently the composition of meteorites. No connexion -between the iron and the Aurora-spectra is, however, proved; though it -may be suspected. The iron-spectrum, as remarked elsewhere, contains so -many lines that some may, as a mere accidental circumstance, closely -agree with the Aurora-lines. - -The iron-lines are, it may be remarked, as a rule, sharper and finer than -the Auroral lines, though it is possible that these characteristics might -vary if the spectrum were obtained in a rarefied medium. Tubes with iron -terminals are said to evolve a compound gas of H and Fe. I have not had -an opportunity to verify this. - -It may be added that the comparative faintness of the more refrangible -lines of the Aurora-spectrum suggests a feeble resistance to the exciting -current, and a low temperature inconsistent with a meteoric theory; and -this is not contradicted by the brightness of the red and green lines, -if these are due to a phosphorescent origin. Expansion of a line is -recognized to be dependent on pressure, and consequently the breadth -of the green or red lines might indicate the height of the Aurora; -while their brightness or otherwise might also give some idea as to its -density. No observations in this direction have, as far as I am aware, -been recorded. - -As the general result of spectrum work on the Aurora up to the present -time, we seem to have quite failed in finding any spectrum which, as to -position, intensity, and general character of lines, well coincides with -that of the Aurora. Indeed, we may say we do not find any spectrum so -nearly allied to portions even of the Aurora-spectrum, as to lead us to -conclude that we have discovered the true nature of one spectrum of the -Aurora (supposing it to comprise, as some consider, two or more). The -whole subject may be characterized as still a scientific mystery—which, -however, we may hope some future observers, armed with spectroscopes -of large aperture and low dispersion, but with sufficient means of -measurement of line positions, and possibly aided by photography, may -help to solve. The singular absence of Auroræ has, for some time past, -given no opportunity in that direction. May some of my readers be more -fortunate in obtaining opportunities of viewing the glorious sky-fires, -and assist to unravel so interesting a paradox! - - - - -APPENDICES. - - - - -APPENDIX A. - -REFERENCES TO SOME WORKS AND ESSAYS ON THE AURORA. - -(Most of these are cited in the ‘Edinburgh Encyclopædia’ and the -‘Encyclopædia Britannica.’) - - -Musschenbroek, Instit. Phys. c. 41. - -‘Trai. Phys. et Hist. de l’Aurore Boréale,’ par M. de Mairan. Paris, 1754. - -Beccaria, ‘Dell’Elettricismo Artif. e Nat.’ p. 221. - -Smith’s ‘Optics,’ p. 69. - -D’Alembert’s ‘Opuscules Mathématiques,’ vol. vi. p. 334. - -‘Philosophical Transactions’ as under:— - - Vol. Pages - - 1716 406 - 1717 584, 586 - 1719 1099, 1101, 1104, 1107 - 1720 21 - 1721 180, 186 - 1723 300 - 1724 175 - 1726 128, 132, 150 - 1727 245, 301 - 1728 453 - 1729 137 - 1730 279 - 1731 53-55 - 1734 243, 291 - 1736 241 - 1740 368 - 1741 744, 839, 840, 843 - 1750 319, 345, 346, 499 - 1751 39, 126 - 1752 169 - 1753 85 - 1762 474, 479 - 1764 326, 332 - 1767 108 - 1769 86, 307 - 1770 532 - 1774 128 - 1781 228 - 1790 32, 47, 101 - -‘Miscell. Berolinens.’ 1710, vol. i. p. 131. - -‘Comment. Petrop.’ tom. i. p. 351, tom. iv. p. 121. - -‘Acta Petrop.’ 1780, vol. iv. p. 1. - -‘Mem. Acad. Paris,’ 1747, pp. 363, 423; 1731; 1751. - -‘Mem. Acad. Berl.’ 1710, vol. i. p. 131; 1747, p. 117. - -Schwed. ‘Abhandlungen,’ 1752, p. 169; 1753, p. 85; 1764, pp. 200, 251. - -Bergman, ‘Opusc.’ vol. v. p. 272. - -‘Americ. Trans.’ vol. i. p. 404. - -‘Mém. de Mathémat. et Phys.’ tom. viii. p. 180. - -Rozier, vol. xiii. p. 409; vol. xv. p. 128; vol. xxxiii. p. 153. - -Franklin’s Works, vol. ii. - -Weidler, ‘De Aurora Boreale.’ 4to. - -Nocetus, ‘De Iride et Aurora Boreale, cum Notis Boscovisch.’ Rome, 1747. - -Chiminello, ‘Mem. Soc. Ital.’ vol. vii. p. 153. - -Gilbert’s ‘Journal,’ vol. xv. p. 206; and (particularly) Dr. T. Young’s -‘Nat. Phil.’ vol. i. pp. 687, 716, and vol. ii. p. 488. - -Wiedeburg, ‘Ueber die Nordlichter.’ Jena, 1771. - -Hüpsch, ‘Untersuchung des Nordlichts.’ Cologne, 1778. - -Van Swinden, ‘Recueil de Mémoires.’ Hague, 1784. - -Wilke, ‘Von den neuesten Erklärungen des Nordlichts,’ Schwed. Mus. -Wismar, 1783. - -Dalton’s ‘Meteor. Observ.’ 1793, pp. 54, 153. - -Loomis, ‘Sill. Journal,’ 2nd series, xxxii. p. 324; xxxiv. p. 34. The -same, 3rd series, v. p. 245; B. V. Marsh, 3rd series, xxxi. p. 311. - -Oettingen and Vogel, Pogg. Ann. cxlvi. pp. 284, 569. - -Galle and Sirks, _ibid._ cxlvi. p. 133; cxlix. p. 112. - -Silbermann, ‘Comptes Rendus,’ lxviii. pp. 1049, 1120, 1140, 1164. - -Prof. Fritz, “Geog. Distrib.,” Petermann’s Mitth., Oct. 1874. - -Zehfuss, ‘Physikalische Theorie.’ Adelman, Frankfort. - -‘Nature,’ iii. pp. 6, 7, 28, 104, 126, 346, 348, 510; iv. pp. 209, 213, -345, 497, 505; x. 211 (Ångström). - -‘Edinburgh Astronomical Observations,’ vol. xiv. 1870-1877. - -‘English Mechanic,’ No. 461 (January 23, 1874), pp. 445-447; and No. 462, -pp. 475, 476. - - - - -APPENDIX B. - -EXTRACTS FROM THE MANUAL AND INSTRUCTIONS FOR THE (ENGLISH) ARCTIC -EXPEDITION, 1875. - - -_Note on Auroral Observations. By Prof. STOKES, Sec. R.S._ - -The frequency of the Aurora in Arctic regions affords peculiar facilities -for the study of the general features of the phenomenon, as in case the -observer thinks he has perceived any law he will probably soon, and -repeatedly, have opportunities of confronting it with observation. The -following points are worthy of attention:— - -_Streamers._—It is well known that, at least as a rule, the streamers -are parallel to the dipping-needle, as is inferred from the observation -that they form arcs of great circles passing through the magnetic zenith. -It has been stated, however, that they have sometimes been seen curved. -Should any thing of this kind be noticed, the observer ought to note the -circumstances most carefully. He should notice particularly whether it -is one and the same streamer that is curved, or whether the curvature is -apparent only, and arises from the circumstance that a number of short, -straight streamers start from bases so arranged that the luminosity as a -whole presents the form of a curved band. - -Have the streamers any lateral motion? and if so, is it from right -to left or left to right, or sometimes one and sometimes the other, -according to the quarter of the heavens in which the streamer is seen, or -other circumstances? Again, if there be lateral motion, is it that the -individual streamers move sideways, or that fresh streamers arise to one -side of the former, or partly the one and partly the other? Do streamers, -or does some portion of a system of streamers, appear to have any uniform -relation to clouds, as if they sprang from them? Can stars be seen -immediately under the base of streamers? Do streamers appear to have any -definite relation to mountains? Are they ever seen between the observer -and a mountain, so as to appear to be projected on it? This or any other -indication of a low origin ought to be most carefully described. - -When streamers form a corona, the character of it should be described. - -_Auroral Arches._—Are arches always perpendicular to the magnetic -meridian? If incomplete, do they grow laterally? and if so, in what -manner, and towards which side? Do they always move from north (magnetic) -to south? and if so, is it by a southerly motion of the individual -streamers, or by new streamers springing up to the south of the old ones? -What (by estimation, or by reference to known stars) may be the breadth -of the arch in different positions in its progress? Do arches appear -to be nothing but congeries of streamers, or to have an independent -existence? What relations, if any, have they to clouds? and if related, -to what kind of clouds are they related? - -_Pulsations._—Do pulsations travel in any invariable direction? What -time do they take to get from one part of the heavens to another? Are -they running sheets of continuous light, or fixed patches which become -luminous, or more luminous, in rapid succession? and if patches, do these -appear to be foreshortened streamers? Are the same patches luminous in -successive pulsations? - -_Sounds_ (?).—As some have suspected the Aurora to be accompanied by -sound, the observer’s attention should be directed to this question when -an Aurora is seen during a calm. If sound be suspected, the observer -should endeavour, by changing his position, brushing off spicules of -ice from the neighbourhood of the ears, his whiskers, &c., to ascertain -whether it can be referred to the action of such wind as there is on -some part of his dress or person. If it should clearly appear that it -is not referable to the wind, then the circumstance of its occurrence, -its character, its relation (if any) to bursts of light, should be most -carefully noted. - -These questions are prepared merely to lead the observer to direct -his attention to various features of the phenomenon. Answers are not -demanded, except in such cases as definite answers can be given; and -the observer should keep his attention alive to observe and regard any -other features which may appear to be of interest. It is desirable that -drawings should be made of remarkable displays. - -Observations with Sir William Thomson’s electrometer would be very -interesting in connexion with the Aurora, especially a comparison of the -readings before, during, and after a passage of the Aurora across the -zenith. - - -_Spectroscopic Observations. By Prof. G. G. STOKES, Sec. R.S._ - -_Spectrum of the Aurora._ - -The spectrum of the Aurora contains a well-known conspicuous bright line -in the yellowish green, which has been accurately observed. There are -also other bright lines of greater refrangibility, the determination of -the positions of which is more difficult on account of their faintness, -and there are also one or more lines in the red, in red auroras. - -Advantage should be taken of an unusually bright display to determine the -positions of the fainter lines. That of the brightest lines, though well -known, should be measured at the same time to control the observations. -The character of the lines (_i. e._ whether they are strictly lines, -showing images of the apparent breadth of the slit, or narrow bands, -sharply defined or shaded-off) should also be stated. - -Sometimes a faint gleam of light is seen at night in the sky, the -origin of which (supposed from the presence of clouds) is doubtful. A -spectroscope of the roughest description may in such cases be usefully -employed to determine whether the light is auroral or not, as in the -former case the auroral origin is detected by the chief bright line. -The observer may thus be led to be on the look-out for a display which -otherwise might have been missed. - -It has been said, however, that the auroral light does not in all cases -exhibit bright lines, but sometimes, at least in the eastern and western -arch of the Aurora, shows a continuous spectrum. This statement should be -confronted with observation, special care being taken that the auroral -light be not confounded with light which, though seen in the same -direction, is of a different origin, such, for example, as light from a -bank of haze illuminated by the moon. - -Sir Edward Sabine once observed an auroral arch to one side (say north) -of the ship, which was in darkness. Presently the arch could no longer -be seen, but there was a general diffuse light, so that a man at the -mast-head could be seen. Later still, the ship was again in darkness, and -an auroral arch was seen to the south. - -Should any thing of the kind be observed, the whole of the circumstances -ought to be carefully noted, and the spectroscope applied to the diffuse -light. - - -_Polarization of Light. By W. SPOTTISWOODE, M.A., LL.D., Treas. R.S._ - -It has been suggested that the Aurora, inasmuch as it presents a -structural character, may afford traces of polarization. Having reference -to the fact that the striæ of the electric discharge in vacuum-tubes -present no such feature, the probability of the suggestion may be -doubted. But it will still be worth while to put the question to an -experimental test. - -If traces of polarization be detected, it must not at once be concluded -that the light of the Aurora is polarized; for the Aurora may be seen on -the background of a sky illuminated by the moon, or by the sun, if not -too far below the horizon, and the light from either of these sources is, -in general, more or less polarized; therefore, if the light of the Aurora -is suspected to be polarized, the polariscope should be directed to an -adjacent portion of clear sky, free from Aurora, but illuminated by the -moon or sun as nearly as possible similar, and similarly situated to the -former portion; and the observer must then judge whether the polarization -first observed be merely due to the illumination of the sky. - -The presence of polarization is to be determined:— - -(1) With a Nicol’s prism, by observing the light through it by turning -the prism round on its axis, and by examining whether the light appears -brightest in some positions and least bright in others. If such be the -case, the positions will be found to be at right angles to one another. -The direction of “the plane of polarization” will be determined by -that of the Nicol at either of these critical positions. The plane of -polarization of the light transmitted by a Nicol, is parallel to the -longer diagonal of the face; and, accordingly, the plane of polarization, -or partial polarization, of the observed light is parallel to the longer -diameter of the Nicol when the transmitted light is at its greatest -intensity, or to the shorter when it is at its least. - -(2) The observation with a double-image prism is similar to that with a -Nicol. This instrument, as its name implies, gives the images which would -be seen through the Nicol in two rectangular positions, both at once, -so that they can be directly compared; and when in observing polarized -light the instrument is turned so that one image is at a maximum, the -other is simultaneously at a minimum. Both these methods of observation, -(1) and (2), are especially suitable for faint light; because in such a -case the eye is better able to appreciate differences of intensity than -differences of colour. - -(3) The observation with a biquartz differs from (1) only by holding a -biquartz (a right-handed and a left-handed quartz cemented side by side) -at a convenient distance beyond the Nicol, and by observing whether -colour is or is not produced. If the Nicol be so turned that the two -parts of the biquartz give the same colour (choose the neutral tint, -_teint de passage_, rather than the yellow), we can detect a change in -the position of the plane of polarization by a change in colour, one -half verging towards red, the other towards blue. This observation is -obviously applicable to a change in the plane, either at different parts -of the phenomenon at the same time, or at the same parts at different -times. - -(4) We may use a Savart’s polariscope, which shows a series of coloured -bands in the field of view. For two positions at right angles to one -another corresponding to the two critical positions of a Nicol, these -bands are most strongly developed; for two positions midway between the -former the bands vanish. In the instruments here furnished, the plane of -polarization of the observed light will be parallel to the bands when the -central one is light, perpendicular to them when the central band is dark. - - -_Instructions in the use of the Spectroscopes supplied to the Arctic -Expedition._ - -_By J. NORMAN LOCKYER, F.R.S._ - -_Spectroscopic Work._ - -Scales prepared on Mr. Capron’s plan, together with forms for recording -positions, also accompany the instrument. - -_A._ In using these, carefully insert the principal solar lines in their -places on the forms, as taken from a fine slit, and keep copies of this -scale for use. If the slit opens _only on one_ side, note on scale in -which direction the lines widen out, whether towards red or violet. Also -fill up some of these forms with gas and other spectra, as taken at -leisure _with the same instrument_ and scale. - -When observing, close the slit (after first wide opening it) as much as -light will permit, and then with pen or pencil record the lines as seen -upon the micrometer-scale on the corresponding part of the form, and note -_at once relative intensities_ with Greek letters, α, β, &c. (or numbers). - -Reduce at leisure line-places on scale to wave-lengths, and note as to -each line the _probable limits of instrumental error_. _B._ - -In case the auroral spectrum is so faint that the needle-point or -micrometer-scale is invisible, half of the field of view may be covered -with tinfoil, with a perfectly straight smooth edge running along the -diameter of the field, in perfect focus, and parallel to the lines of the -spectra. The reading-screw being set to 10, the bending-screw should then -be adjusted so that the green line of the Aurora is just eclipsed behind -the blackened edge of the tinfoil. A similar eclipse of other lines will -give their positions. - -In this instrument the reference-prism is brought into action by turning -the slipping piece to which is fixed the two terminals. Care should be -taken that the prism itself is adjusted before commencing observations, -as it may be shaken out of position on the voyage. The tubes provided -for the reference-spectra may be either fastened to the terminals or -arranged in some other manner. The air-spectrum may also be used as a -reference-spectrum. To get this, two wires should be screwed into the -insulators, their ends being at such a distance apart and in such a -position that the spectrum is well seen. - - -_General Observations regarding the Spectrum of the Aurora[17]._ - -_C._ Note appearance, colour, &c. of _arc_, _streamers_, _corona_, and -_patches_ of light. - -Get compass positions of principal features, and _note any change of -magnetic intensity_. If corona forms, take its position and apparent -height. - -Look out for _phosphorescence_ of Aurora and adjacent clouds. Listen -for reported sounds. Note any peculiarity of cloud scenery, prior to or -pending the Aurora. - -Sketch principal features of the display, and indicate on this sketch the -parts spectroscopically examined. - -Examine line in _red_ specially in reference to its assumed connexion -with _telluric_ lines (little _a_ group), and note _as to its brightening -in sympathy with any of the other lines_. - -Examine line in yellow-green (Ångström’s) as to _brightness_, _width_, -and _sharpness_ (_or nebulosity_) at the edges. Notice as to a peculiar -_flickering_ in this line sometimes seen; note also whether this line is -_brighter_ (or the reverse) _with a fall of temperature_. Note _ozone_ -papers at the time of Aurora. - -Note whether the Auroræ can by their spectra be classed into distinct -types or forms, and examine for _different spectra_ as under:— - - α. The auroral _glow_, pure and simple. - - β. The white arc. - - γ. The streamers and corona. - - δ. Any phosphorescent or other patches of light, or light cloud - in or near the Auroræ. _D._ - -The information collected together in the ‘Manual’ should be carefully -consulted, and the line of observations suggested by Ångström’s later -work followed out. To do this, not only record the positions of any -features you may observe in the spectrum, but endeavour to determine, -if any, and if so which, of the features vary together. Compare, for -instance, the two spectra of nitrogen in the Geissler tube supplied, by -observing first the narrow and then the wider parts of the tube. It will -be seen that the difference in colour and spectrum results simply from an -addition to the spectrum in the shape of a series of channelled spaces -in the more refrangible end in the case of the spectrum of the narrow -portion. - -Try to determine whether the difference between red and green Auroras may -arise from such a cause as this, and which class has the simpler spectrum. - -See whether indications of great auroral activity are associated with the -widening or increased brilliancy of any of the auroral lines. - -Remember that if auroral displays are due to gaseous particles thrown -into vibration of electric disturbance, increased electric tension -may either (1) dissociate those particles and thus give rise to a new -spectrum, the one previously observed becoming dimmer; or (2) throw the -particles into more intense vibration without dissociation, and thus give -rise to new lines, those previously observed becoming brighter. - -Careful records of auroral phenomena from both ships may enable the -height of some, observed from both, to be determined. It will be very -important that those the heights of which are determined by such means -should be carefully observed by the spectroscope, in order to observe -whether certain characteristics of the spectrum can be associated with -the height of the Aurora. - - - - -APPENDIX C. - -EXTRACTS FROM PARLIAMENTARY BLUE BOOK, CONTAINING THE “RESULTS DERIVED -FROM THE ARCTIC EXPEDITION 1875-76.” (Eyre and Spottiswoode, 1878.) - - -_Auroras observed 1875-1876, at Floebery Beach and Discovery Bay._ - -_By Lieutenant A. C. PARR, R.N._ - -Though the auroral glow was often present, and served in some degree to -lighten the darkness of the sky during the long winter, when the moon was -absent, the actual appearances of the Aurora itself were few, and the -nimbus worthy of any particular remark extremely small. Those which were -stationary assumed the form of low arches, with streamers flashing up to -them from the horizon, and usually to the eastward. But the more common -form was for an arch to appear low down in some part of the sky where the -glow was brightest; at first it was very faint and narrow, but as it rose -gradually in the heavens it would increase both in size and intensity, -till on arriving near the zenith, with its ends extending nearly to the -horizon, it would be about the breadth of three or four rainbows, and its -colour that of white fleecy clouds lit up by the rays of the full moon. -On reaching this point, however, its course was nearly run; for after -appearing to remain stationary, as little white gaps would suddenly rend -the arch asunder, the portions thus detached seemed to roll together and -concentrate all their brightness in the smaller space, and then gradually -fade away and become extinct. Sometimes a very pale green would show -itself in the more luminous patches, and once or twice there was a slight -suspicion of red; but never was the whole sky illuminated by streams -running in all directions, and forming coronæ, while these colours varied -every moment. - -When instead of the arch rising up from the horizon a streamer appeared, -its origin was in the north. From the northern horizon it would stretch -out towards the zenith, passing nearly overhead, and reaching to within -a few degrees of the land to the south. In appearance they would be the -same as the arches, but sometimes a second would grow out of the first, -and on one occasion three were visible at the same time. They had lateral -motion either from east to west, or west to east, but there was no -flashing to brighten them, and they gradually faded away. - -The time at which Auroras usually occurred was between 9 P.M. and -midnight, the last display being on February 19th, commencing at 11 -P.M. It was a beautifully clear night, without mist or haze of any -description, and small stars visible close down to the horizon. At -the above-named hour two arches made their appearance, and remained -stationary; the lower one was the brighter, being of a pale green colour, -its centre bearing E.S.E. (true), and having an altitude of about 5°, -with a breadth of about twice that of a rainbow. The second arch was -concentric with the first, and about 7° above it, but rather broader and -fainter. These arches maintained their altitude, the upper one at about -the same intensity, but that of the lower one varied considerably. It -would gradually lighten up, then send flashes to the upper one, then -break up and fade away; before, however, it had quite disappeared, -flashes would come up to it from the horizon which seemed to endue it -with new life, for the arch would be reformed, brighten up, and the same -performance would be again repeated. This occurred three or four times in -the course of three quarters of an hour; but the flashes from the horizon -never extended beyond the lower arch, and those from the lower never went -beyond the upper. During this display the citron-line was obtained very -clearly with the spectroscope, but no other lines were visible. - -On six or seven occasions Auroras were visible at the same time on board -both the ‘Alert’ and ‘Discovery;’ but the absence of characteristic -features makes it impossible to determine whether they were the same -display, or merely two distinct ones which happened to occur at the same -time. But as by far the larger number of those recorded in the one ship -were not visible at the other, it was certainly only under exceptional -conditions that they could be simultaneously observed at both stations, -if, indeed, they ever were. Auroras seemed to appear indifferently both -when there was wind and when it was calm, with either a high or low -barometer, and seemed quite unconnected with the temperature, although on -an occasion the thermometer was observed to fall 3° during the display, -and to rise 2° almost immediately afterwards. But it was never seen -illuminating the edges of clouds, as we saw it on the passage home, nor -playing about the outline of the land, and never was there the slightest -suspicion of sound being produced by it. - -The opportunities for observing the spectrum of the Aurora in this -position have been most unsatisfactory, as the displays were small in -number and deficient in brilliancy. - -The form they generally assumed was to rise like an arch from a portion -of the horizon where there was a luminous glow, at first very faint, but -gradually increasing in brilliancy till near the zenith, where it would -remain stationary for a short time and then break up and disappear. -Sometimes they would rise up as streamers, but only occasionally was more -than one visible at a time, and they lasted for such a short time, that -even if they had been bright it would have been very difficult to make -satisfactory observations. - -Very few showed any signs of colour, and those only the slightest tinge. -Nearly all that were observed gave the citron-line with the small pocket -spectroscope with more or less distinctness, though no signs of any other -lines were ever seen; but on only two occasions was it bright enough to -get the line with Nury’s spectroscope, and then only for such a short -time that a satisfactory measure could not be obtained. - - * * * * * - -Then follows a descriptive list of the Auroræ seen, from which I have -selected three of the finest, viz. January 2nd, February 14th, and -February 19th, 1876. - -January 2nd, 1876. Lieut. Parr. _Floeberg Beach._—9 P.M. Streams of -Aurora. Stars shining brightly. - -Register. _Discovery Bay._—9 P.M. Observed an Aurora like a pale band of -light in the form of an arch whose centre was on the true meridian and -15° from the zenith. It shortly afterwards broke up into feathered edges, -their direction being a little to the eastward of the zenith. The arch -grew fainter, and shifted to the eastward of the meridian four points; -the left extremity of the arch faded away, and the right assumed the -shape of the folds of a curtain doubled over. The weather was clear and -calm. The display lasted upwards of 30 minutes. - -A spectroscope, one of Browning’s 8-in. direct-vision, was directed -towards the Aurora, but the light was not sufficient to give any spectrum. - -The temperature was -39°. Barometer 29·56 inches. No wind. Clouds stratus -2. Eight meteors were observed during the time the Aurora was visible. - -February 14th. Register. _Discovery Bay._—At 2 A.M. a faint Aurora -passing across the heavens from S.E. to S.W. was observed, like an arch -of a pale colour. It lasted only a short time, and was very indistinct. -Temperature -47°. Barometer 30·44 inches. No wind or clouds. - -Lieut. Aldrich. _Floeberg Beach._—2 A.M. A faint Aurora towards the S.W. -Weather calm. Cumulus-stratus clouds 3. Temperature -46°. 8 P.M. Faint -flashes of Aurora in the E. and S.W. - -Lieut. Aldrich and Lieut. Parr. _Floeberg Beach._—11.50 P.M. A moderately -bright arch of Aurora extended from due N. to about S.S.W., where it -terminated close down to the horizon in a crook turned to the eastward. -In a few moments a streamer flashed from the end of the crook parallel -to the first and right across the heavens, its edges being quite sharp -and parallel to each other. A third streamer shot up a minute afterwards, -but did not extend more than 80° upwards. The streamers were visible for -a very short time, the first remaining longest. The second-named arch -gradually faded away till within a few degrees of the S.S.W. horizon, and -(still being a continuation of the crook) bent round to the eastward, -and towards the horizon, going on to what was left of the stump of the -third arc. A lateral motion to the eastward now began, the whole body -gradually turning round until it disappeared about due south. Stars were -visible through it at its brightest, but not very distinctly. This is -the most intense and variegated Aurora we have experienced, but scarcely -any colours were to be seen. Temperature -51°. Barometer 30·43 inches, -stationary. Calm weather. Clouds cumulus 1. Preceded and followed by calm -weather. - -Meteorological Register. _Discovery Bay._—9.15 P.M. An Aurora was -observed to the southward, spreading out like a fan in separate ways. -It was faint. A few cirro-stratus clouds were visible, apparently -between the observer and the Aurora. It lasted about 40 minutes, and -then gradually faded away. Temperature -47°. Barometer 30·51 inches, -stationary. No wind. Clouds cirro-stratus 4. - -February 19th. Meteorological Report. _Discovery Bay._—9.45 P.M. An -Aurora like a fluted arch, with rays flashing towards the Pole, was -observed spanning the hills from the south to the east. The direction of -the lines of light from all parts of the arch was towards the zenith. -Above the arch a pale band of colour appeared, like a secondary arch -above the other. It appeared very much as if it was caused by the -reflected light of the Aurora. The Aurora was bright for a few seconds, -and then gradually died away. It lasted altogether about 30 minutes. -The centre of the arch bore S.E., having an altitude of about 30°. The -secondary arch was about 15° above the former. Both arches were of a pale -light colour, the upper one very faint. Temperature -34°. Barometer -29·87 inches, rising rapidly. Weather calm. Misty. No clouds. - -Lieut. Parr. _Floeberg Beach._—An Aurora appeared shortly after 11 -P.M., consisting of bright arch, whose centre bore about E.S.E., and -had an altitude of about 5°, with a second broader and fainter arch -about 7° above the first. These arches maintained their altitudes, the -upper one at about the same intensity, but that of the lower one varied -considerably. It would gradually brighten up, then send streamers up to -the second, then break up into light patches, and gradually fade away. -This happened three or four times during the 40 minutes that the display -lasted. At times streamers would come up from the horizon to the lower -arch, for it was a splendidly clear night, and seemed to brighten it up, -but none of them extended beyond it. Neither did the streamers from the -lower arch extend beyond the upper one. It was slightly green in colour -when brightest, and the citron-line was well defined, but no others were -visible. Temperature -46°. Barometer 29·95 inches, steady. Weather calm. -Cumulus clouds 4. Misty. - - -TABLE of DATES when AURORAS were observed by the ARCTIC EXPEDITION, -1875-76. - - ----------------+--------------------------+----------------------- - | H.M.S. ‘Alert,’ | H.M.S. ‘Discovery,’ - Date. | Floeberg Beach. | Discovery Bay. - ----------------+--------------------------+----------------------- - 1875, October 25| 11.45 P.M. Faint. | Cloudy. - ” ” 26| 10 P.M. Very faint. | 10 P.M. - ” ” 30| Sky obscured. Faint. | Ditto. - ” November 1| Ditto. Faint, but well | - | marked. | Ditto. - ” ” 2| 9 to 10 P.M. Arches and | - | streamers. | A few clouds. - ” ” 21| Ditto. Bright | 9 to 10 P.M. - | streamer. | and 10 to 11 P.M. - ” ” 22| 2 P.M. and 8 P.M. Slight,| - | red. | Clear sky. - ” ” 25| 9.30 A.M. Character not | - | recorded. | Ditto. - ” ” 26| 10 A.M. Stream of light.}| A few clouds. - ” ” 26| Cloudy to 10 P.M., }| - | bright afterwards. }| 10 P.M. - ” ” 27| Midnight. Slight. | 11.40 P.M. - ” ” 28| 1 A.M. Bright streak. | Clear sky. - ” ” 29| Cloudy, brighter at 11 | - | A.M. Faint glow. | 9.30 A.M. - ” ” 30| A few clouds. Very faint.| 4.30 A.M. - ” ” 30| 5 P.M., 8 P.M., and 10 | - | P.M. Flashes. | 5 P.M. - ” December 2| Evening. Streamers. | Clear sky. - ” ” 3| 1 A.M. Flashes. | Ditto. - ” ” 3| Bright sky. Faint Aurora.| 2.30 P.M. - ” ” 16| 10 P.M. Slight; showed | - | citron-line. | 11 P.M. - ” ” 19| 3 P.M. to 5 P.M., faint; | - | and 9 to 10 P.M., | Very clear sky. - | moderately bright arc. | - ” ” 22| 10 P.M. Slight. | Ditto. - ” ” 23| 6 P.M. Ditto. | Ditto. - ” ” 24| Misty, a few stars | - | visible. Arch. | 9 A.M. - ” ” 26| Very bright sky. Faint. | 6 P.M. - ” ” 29| Ditto. Very faint. | 6.15 P.M. - ” ” 31| 4 P.M. Same. | Sky obscured. - 1876, January 1| 5 P.M. and 11 P.M. | - | Slight. | A few clouds. - ” ” 2| 9 P.M. Described and | - | figured. | 9 P.M. - ” ” 17| Very bright sky. Very | - | faint streamers. | 9.25 A.M. - ” ” 18| 9.45 P.M. and 10.5 P.M. | - | Character not recorded. | 10.15 P.M. - ” ” 19| Very bright sky. Faint. | 9.45 P.M. - ” ” 20| 2 A.M. Slight. | 2.30 A.M. - ” ” 23| 7.55 A.M. and 2 P.M. | - | Slight. | 8.45 P.M. - ” ” 24| Bright sky. Slight flash.| 2 A.M. - ” ” 24| 5 P.M. and 11.15 P.M. | - | Faint Aurora. | Very clear sky. - ” ” 27| 2 A.M. to 3.45 A.M. | - | Faint. | 1 A.M. to 4 A.M. - ” ” 27| Very bright sky. Faint | - | double arch. | 8.30 P.M. - ” ” 28| 6 P.M. and 7 to 9 P.M. | - | Faint flashes. | 7.20 P.M. - ” ” 30| 8 P.M. Streak. | 7.50 to 9 P.M. - ” ” 31| 8.30 A.M. and 7.30 | - | P.M. Very faint. | 8.25 A.M., 5.30 P.M. - ” February 3| 10 P.M. Slight flash. | Very clear sky. - ” ” 11| Sky obscured. Very faint.| 11 P.M. - ” ” 13| 11 P.M. Flashes. | Clear sky. - ” ” 14| 2 A.M., 9.15 to 10 P.M. | - | Described and figured. | 2 A.M. and 11.50 P.M. - ” ” 19| 9.45 P.M. | 11 P.M. - ” ” 20| 2 A.M. Very faint. | 2.30 A.M. - ” ” 22| 2 A.M. Character not | - | recorded. | Very clear sky. - ” ” 24| Bright sky. Very faint. | Midnight. - ” ” 26| 10 P.M. and 11 P.M. | - | Faint flashes. | Sky obscured. - ----------------+--------------------------+----------------------- - -I have added to the above Table the character of the Aurora in each -instance as taken from the fuller descriptions given.—J. R. C. - - -_Auroras and Magnetic Disturbances._ - -The appearances of Auroras and the synchronous movements of the -declinometer-magnet were subjects of special observation during the stay -of the ‘Alert’ and ‘Discovery’ at their winter-quarters. The Table on -page 187 gives the dates and hours when Auroras were visible. On all -occasions they were observed to be faint, with none of those brilliant -manifestations which are described by our own officers as seen at Point -Barrow, and by the Austro-Hungarian Expedition at Franz-Josef Land, where -the magnetical instruments were so sensibly disturbed. - -These phenomena were not observed either in the ‘Alert’ or the -‘Discovery,’ especially no connexion between magnetical disturbances and -the appearances of Auroras could be traced. - -This is quite in accordance with the remarks of previous observers within -the region comprehended between the meridians of 60° and 90° west, and -north of the parallel of 73° north. For example:— - -In the Phil. Trans. 1826, Part IV. p. 76, Capt. Parry and Lieut. Foster -remark, in the discussion of their magnetical observations at Port -Bowen:—“As far, however, as our own observations extended, we have -reason to believe that on no occasion were the needles in the slightest -degree affected by Aurora, meteors, or any other perceptible atmospheric -phenomenon.” - -Again, in the Smithsonian Contributions, vol. x., 1858, Mr. A. Schott, -in his discussion of Dr. Kane’s observations at Van Rensselaer Harbour, -in 1854, remarks—“In conformity with the supposed periodicity of this -phenomenon as recognized by Professor Olmstead, no brilliant and complete -Auroras have been seen; with an exception of very few, they may all be -placed in his fourth class, to which the most simple forms of appearances -have been referred.” The following statement is given in the same page -as a footnote:—“The processes have no apparent connexion with the -magnetic dip, and in _no_ case did the needle of our unifilar indicate -disturbance.” - - * * * * * - -The following description of the Aurora observed on 21st November, 1875, -is given by Commander Markham and Lieut. Giffard, in their abstract of -observations at Floeberg Beach:— - -“Between 10 and 11 P.M. bright broad streamers of the Aurora appeared -10° or 15° above the north horizon, stretching through the zenith, and -terminating in an irregular curve about 25° above the south horizon, -bearing S.S.W. During the Aurora’s greatest brilliancy the magnet was -observed during five minutes to be undisturbed.” - - [_Note._—I applied for a loan of the lithographic stones - to enable me to give copies of the three diagrams of - Auroræ referred to in the Arctic “Results;” but the Lords - Commissioners of H.M. Treasury refused this, except on the - terms of my paying one third of the original cost of production - of such diagrams. I did not think it worth while to accept - these conditions. Only one of the drawings has any special - interest; and this is a “curtain” Aurora, similar to that - figured on Plate II. of this work.—J. R. C.] - - - - -APPENDIX D. - -THE AURORA AND OZONE. - - -[Sidenote: Aurora and ozone. Dr. Allnatt’s notes and conclusions deduced -therefrom.] - -While Part I. was in the press, Dr. Allnatt, formerly of Frant, and for -many years the well-known meteorological contributor to ‘The Times’ -newspaper, kindly placed at my disposal his large series of notes. Upon -an examination of these we came to the following conclusions:— - -1. That Auroral periods are also periods of comparative abundance of -ozone. - -2. That instances are by no means wanting in which an abnormal -development of ozone appears to be coincident with the manifestation of -an Aurora. - -[Sidenote: Year 1870 remarkable for sun-spots, auroræ, and ozone.] - -In reference to the first point, it is found, as the result of an -examination of Dr. Allnatt’s notes, that particular years and months are -notable at once for Auroræ and for ozone in abundance. 1870 was one of -these years, and was specially recorded by Dr. Allnatt, in his ‘Summary -for the Year,’ as remarkable for sun-spots, Auroræ, and ozone. - -[Sidenote: Particulars of some of the monthly records.] - -The month of February in that year was marked by intense cold and -brilliant Auroræ. Atmospheric electricity was feeble, but ozone was, -throughout the month, well developed; and there was no tangible period of -antozone. - -In the month of April of the same year, eight days consecutively (19th to -26th) were marked for ozone 10, the maximum of Dr. Allnatt’s scale. - -In May of the same year there were magnificent Auroræ, and atmospheric -electricity was intense. Ozone was scanty; but this was accounted for by -the wind being generally E.N.E., ozone being mostly developed with a W. -or S.W. wind, and a moist state of the atmosphere. - -In August 1870 the unusually large number of 22 days were recorded for a -maximum of ozone. - -September 1870 was hardly less remarkable, with 19 days of maximum. It -was recorded that there were splendid Auroræ during this month, and the -solar spots were very large. - -October 1870 had 20 days of maximum ozone, and November had several -fine Auroræ and maxima of ozone noted. In fact, nearly every month in -that year was referred to by Dr. Allnatt for displays of Aurora (of -both Arctic and Antarctic forms) and for a development of ozone very -considerably above the average. - -[Sidenote: Year 1871.] - -The year 1871 had more or less of the same character. In the month -of October of that year, fine Auroræ were prevalent, and ozone was -registered as at its maximum during 22 days. - -There seems reason to conclude that if a systematic comparison of annual -or other periods of Aurora and ozone development were made, it would -result in disclosing a connexion (probably an intimate one) between the -two phenomena. - -[Sidenote: Instances showing a connexion between a specific Aurora and an -ozone maximum.] - -With reference to the second point, the following (among other) instances -may be quoted, for the purpose of showing a connexion between a specific -Aurora and an ozone maximum. - -The Aurora of 24th September, 1870, was splendid and universal, being -seen in Europe, Asia, Africa, America, and Australia. Ozone reached, on -the morning of the 24th, 8 of the scale (the scale running from 1 to 10), -and, on the morning of the 25th, 10, the maximum. - -In October 1870 there were grand displays on the 14th, 20th, 22nd, 24th, -and 25th, and ozone was correspondingly abundant, as is seen by the -following Table:— - - +-------------------+----------+------+------------------------+ - | Date. | Aurora. |Ozone.| | - +-------------------+----------+------+------------------------+ - |1870, October 14th.|Aurora. | 8 |The display of the 24th | - | ” ” 20th.|Aurora. | 10 | was accompanied by | - | ” ” 21st.|None seen.| 5 | the formation of a | - | ” ” 22nd.|Aurora. | 10 | corona, and that of | - | ” ” 23rd.|None seen.| 8 | the 25th was splendidly| - | ” ” 24th.|Aurora. | 10 | seen in Edinburgh. | - | ” ” 25th.|Aurora. | 8 | | - +-------------------+----------+------+------------------------+ - -The foregoing figures somewhat point to the conclusion that ozone -quantity rises and falls coincidently with the Aurora displays. - -The following seems, however, a case still more strongly in point. - - +-------------------+-------+---------------------+------+ - | Date. | Wind. | Aurora. |Ozone.| - +-------------------+-------+---------------------+------+ - |1871, January 25th.| E.S.E.| None seen. | 0 | - | ” ” 26th.| N.N.W.| None seen. | 2 | - | ” ” 27th.| E.S.E.| Aurora at night in | | - | | | N. and S. horizons.| 10 | - | ” ” 28th.| E. | None seen. | 8 | - | ” ” 29th.| S.E. | None seen. | 2 | - +-------------------+-------+---------------------+------+ - -It is curious, in examining the above Table, to note how the ozone rose, -notwithstanding an east wind, from 0 on the 25th, and 2 on the 26th, to -10 on the 27th, when the Aurora appeared, and 8 on the 28th, when it -might have lingered; and how it again descended to 2 on the 29th. - -The case of the Aurora of 6th of October, 1869, when a broad belt of -Aurora was in the north, is also an illustrative one, as will be seen by -the following data:— - - +------------------+------+------+-------+ - | Date. |Wind. |Ozone.|Aurora.| - +------------------+------+------+-------+ - |1869, October 5th.|S.S.W.| 1 | — | - | ” ” 6th.|S.S.E.| 5 |Aurora.| - | ” ” 7th.|S.S.W.| 10 | — | - | ” ” 8th.| S. | 10 | — | - | ” ” 9th.| S.E. | 5 | — | - +------------------+------+------+-------+ - -The Aurora of the night of the 6th was here represented by the -ozone-paper of the morning of the 7th with a maximum of 10, which lasted -till the 8th. - -[It should be borne in mind, in examining these Tables, that the Aurora -is of the night of the given date, while the ozone-papers are taken and -recorded in the morning of the date quoted.] - -[Sidenote: Other instances.] - -We will now take instances where the ozone has not reached its maximum; -but even in these cases a certain amount of rise and fall of the ozone -development towards and from the Aurora is traceable. - - +-------------------+--------+------+-------------------------+ - | Date. | Wind. |Ozone.| Aurora. | - +-------------------+--------+------+-------------------------+ - |1871, April 8th| S.S.E. | 5 |Aurora on 9th, but | - | ” ” 9th| S.S.E. | 8 | wind E. and unfavourable| - | ” ” 10th| S.E. | 5 | to ozone. | - +-------------------+--------+------+-------------------------+ - |1871, November 9th| N. | 5 |Aurora on all three | - | ” ” 10th| N.W. | 8 | nights. | - | ” ” 11th| N. | 5 | | - +-------------------+--------+------+-------------------------+ - |1872, February 3rd| S.W. | 4 |Aurora on night of the | - | ” ” 4th| S.S.W. | 5 | 4th represented by | - | ” ” 5th| S.W. | 8 | ozone-paper of morning | - | ” ” 6th| S.W. | 5 | of the 5th. | - +-------------------+--------+------+-------------------------+ - -Other cases are, we are bound to say, found, in which ozone was either -not remarkable for quantity, or positively fell during the Aurora, as, -for instance, this:— - - +-----------------+------+------+------------------------------+ - | Date. | Wind.|Ozone.| Aurora. | - +-----------------+------+------+------------------------------+ - |1874, March 16th |W.N.W.| 6 |Aurora on the 18th represented| - | ” ” 17th | S.W. | 6 | by test-paper of | - | ” ” 18th | W. | 5 | the 19th with only two | - | ” ” 19th |S.S.W.| 2 | degrees of discoloration. | - +-----------------+------+------+------------------------------+ - -It is, however, possible that such instances may be accounted for, either -by some reaction on the test-papers after they have been coloured, or -by some accidental antagonistic circumstance affecting the tests. The -following is a case well illustrating this:— - - +------------------+------+------+---------------------------------+ - | Date. | Wind.|Ozone.| Aurora. | - +------------------+------+------+---------------------------------+ - |1874, January 31st|N.N.W.| 6 |There was an Aurora on | - | ” February 1st| N.W. | 8 | the night of the 2nd represented| - | ” ” 2nd| N.W. | 2 | by the ozone-paper | - | ” ” 3rd|N.N.W.| 4 | (4 only) on the | - | ” ” 4th|E.N.E.| 8 | morning of the 3rd. | - +------------------+------+------+---------------------------------+ - -This instance would seem strongly opposed to the theory of a connexion -between Aurora and ozone but for the fact that on the 2nd, when the -Aurora was seen at night, and on other days in the same month, Dr. -Allnatt has recorded a strong wave of antozone to have swept over the -whole of England, and blanched the ozone-papers, however deep their -coloration might have previously been. Indeed, it is easy to understand -that some antozonic influence may, at times, disturb the evidence of the -test-papers, even in so elevated and apparently pure an atmosphere as -that of Frant. - -It may not be considered that the foregoing instances are enough to -establish a case of ozone=Aurora; but there seems, at least, sufficient -to base a requisition for further inquiry upon. - -It would, too, be interesting to investigate whether Auroræ and ozone -development are respectively localized. Mr. Ingall’s fine Aurora, seen -at Champion Hill, S.E., July 18th, 1874 (_antè_, pp. 22 and 23), was not -observed at Frant, and the ozonoscopes there were described as blanched -by antozone. - - - - -APPENDIX E. - -INQUIRIES INTO THE SPECTRUM OF THE AURORA. - -BY H. C. VOGEL.[18] - - -The frequent appearance of the Aurora in the past winter, as well as -this spring, has given me opportunity to institute exact inquiries into -the spectrum of the Aurora. It is known that the nature of Auroræ is as -yet but little explored. It has been considered necessary to abandon -the former view—that they are discharges of the electricity collected -at the poles—because it has been hitherto found impossible to bring the -chief lines of the Aurora-spectrum into coincidence with the spectra of -the atmospheric gases. Theoretical considerations, based on the great -alterations to which the spectrum of the same gas is subject under -varying conditions of temperature and density, have very recently led -Zöllner to the opinion that probably the spectrum of the _Aurora does not -coincide with any known spectrum_ of the atmospheric gases, only because -it is a spectrum of another form of our atmosphere hitherto incapable of -artificial demonstration[19]. - -The following article will show how far I have succeeded, in conjunction -with Dr. Lohse, in supporting this view by exact observations of the -Aurora-spectrum itself, as well as by comparison with the spectra of the -gases constituting the air. - -The star-spectrum apparatus belonging to the 11-inch equatorial of the -Bothkamp Observatory was used for these observations. It consists of -a set of prisms _à vision directe_, five prisms with refracting angle -90°, slit, collimator, and observing telescope. The lowest eyepiece -(magnifying four times) of this telescope was employed. The telescope is -capable of being moved in such a way, by the aid of a micrometer-screw, -that different portions of the spectrum can be brought into the centre -of the field of vision. As fractions of the rotation of this screw are -marked, the distances of the spectral lines can be readily found. - -Repeated measurements of 100 lines of the solar spectrum have enabled -me, upon the basis of Ångström’s Atlas (‘Spectre normal de Soleil’), to -express the indications of the screw directly in wave-lengths. - -In place of the cross wires originally introduced into the focus of the -observing telescope, I have inserted a tiny polished steel cone, the very -fine point of which reaches to the centre of the field of vision. The -axis of this cone stands perpendicular to the length of the spectrum, -therefore parallel with the spectral lines, and the setting of the point -of the cone on the latter is accomplished with great sharpness. If the -spectrum is very faint, or consists only of bright lines, the cone is -lighted by a small lamp. For this purpose, opposite to the point of the -cone, there is an opening in the telescope, through which, regulated by -a blind, light can be thrown on the point. As the latter is polished, a -fine line of light thus appears, which extends to the centre of the field -of vision, and the brilliancy of which can be altered by withdrawing -the lamp to a greater distance or lowering the blind, so that even the -faintest lines of a spectrum can be brought with facility and certainty -into coincidence with this line of light. - -The head of the micrometer-screw is divided into 100 parts, and each -part, in the neighbourhood of the Fraunhofer line F, answers to about -·00016 wave-length. The probable error of position on one of the -well-marked lines in the sun’s spectrum amounts to about 0·008 of a turn -of the screw with the lowest eyepiece of the telescope. I have subjected -the screw itself to a thorough examination with reference to such range, -as well as to periodical inequalities in the single worms of the screw, -but could discover no error exceeding 0·01 of a turn of the screw. I -have to mention, further, that after each observation in the position in -which the instrument was used, readings followed on the sodium-lines, -or on some of the hydrogen-lines, in order to eliminate errors which -might arise in the unavoidable disturbance of any particular part of the -spectral apparatus. - - -1. _Observations of the Aurora._ - -1870, Oct. 25th.—A very bright Aurora. In the brightest parts, besides a -very bright line between D and E, several other fainter lines were to be -discerned, situated further towards the blue end of the spectrum. They -appeared on a dimly-lighted ground, and stretched out over the Fraunhofer -lines E and _b_ to about midway between _b_ and F. Towards the red end -the spectrum was terminated by the bright line first mentioned. No -measurements could be taken, as the apparatus had not yet undergone the -above-mentioned alterations, and even the brightest line of the spectrum -did not diffuse sufficient light to be able to perceive the fine cross -wires. The red rays of the Aurora were not examined. - -1871, Feb. 11th.—Towards ten o’clock appeared in the north-west a very -bright light-bow of greenish colour as the edge of a dark segment. -Even with a very narrow slit, the line between D and E could be well -recognized and measured. The average of six readings gave 7·11 turns, -equal to 5572 wave-length. In a small spectroscope of low dispersion -which is arranged on Browning’s plan, a few more lines placed further -towards the blue could be recognized (as in October). Towards the red end -of the spectrum no lines were observable. The greatest development of the -Aurora was about midnight. Magnificent rays rose to about 60° elevation; -they had the same greenish colouring as the bow of light, and the -appearance of the spectrum also was exactly the same. I again obtained -two sets of measurements: the average of six readings in the first set -gave 7·10 turns, 5572 wave-length; in another part of the heavens at the -same time 7·10 was the result of four readings. - -On Feb. 12, towards eight o’clock, the intensity of the Aurora was -already great enough to allow measurements of the brightest line. The -average of six readings gave 7·09 turns, or 5576 wave-length. Dr. Lohse -took observations later, with the same apparatus, and found from six -readings 7·12 turns, or 5569 wave-length. - -Yet the appearance of the spectrum in the spectroscope of low dispersion -was essentially distinct from that of February 11th. The green continuous -spectrum was present; it extended from the bright Aurora-line to the -lines _b_ of the solar spectrum, and was traversed by some bright lines. -Between band _b_ and F, was another line standing alone, out beyond F, in -the blue part of the spectrum, a clear bright stripe; and just before G a -very faint broad band of light was perceived. - -Amongst the rays which, later on, shot upwards, and were coloured red -at their ends, another very intense red line appeared in the spectrum -between C and D, yet placed nearer to C[20]. - -April 9th.—An exceedingly brilliant Aurora, of which the greater -development took place in the early morning hours. Magnificent red -sheaths rose up to the zenith. The spectrum was like that observed on -February 12th, only much more intense, so that the lines could be seen -and measured with the larger spectral apparatus. In the brightest part of -the Aurora was the dark segment; the spectrum consisted of five lines in -the green, and a somewhat indistinct broad line or band in the blue. - -The red rays, on the other hand, allowed us to recognize seven lines, -whilst the bright line again appeared in the red part of the spectrum. -I could not again perceive the faint stripe observed on February 12th, -in the vicinity of line G. The mean measurements of four readings on an -average, for each line, gave:— - - ------+-------+-------+-------+------------------------------------------ - Turns | Prob- | | Prob- | - of | able | Wave- | able | - screw.|errors.|length.|errors.| Remarks. - ------+-------+-------+-------+------------------------------------------ - 4·62 | ·0037 | 6297 |·00014 |Very bright stripe. } On - 7·12 | 9 | 5569 | 2 |Brightest line of the spectrum; } a - | | | | becomes noticeably fainter at } faintly - | | | | appearance of the red line. } lighted - 7·92 | — | 5390 | — |Extremely faint line; unreliable } ground. - | | | | observation. } - 8·71 | 21 | 5233 | 4 |Moderately bright. - 8·95 | 49 | 5189 | 9 |This line is very bright when the red line - | | | | appears at the same time, otherwise equal - | | | | in brilliancy with the preceding one. - 10·06 | 20 | 5004 | 3 |Very bright line. - 12·33 | — | 4694 | — |}Broad band of light, somewhat less - 12·59 | 22 | 4663 | — |} brilliant in the middle. - 12·88 | — | 4629 | 3 |Very faint in those parts of the Aurora - | | | | in which the red line appears. - ------+-------+-------+-------+------------------------------------------ - -April 14th.—Faint Aurora; only the bright line in the green could be -recognized in its spectrum. The mean of two readings gave 7·12 turns, or -5569 wave-length. - -I append a table of the wave-lengths of the brightest line, as exactly -measured on four evenings:— - - 1871, February 11 5573 - ” 12 5573 - April 9 5569 - ” 14 5569 - -Therefore the average result (if only half-weight is allowed to the -last observation, because it only depends upon two readings) gives for -the wave-length of the brightest line 5571·3, with a probable error of -·000·92. According to Ångström[21], the wave-length of this line is 5567; -according to Winlock[22], on the other hand, 5570. - - -2. _On the Spectra of some Gases in Geissler’s Tubes, as well as on the -Spectrum of the Atmospheric Air._ - -Numerous experiments have been made in order to find out some admitted -connexion between the spectrum of the Aurora and the spectra of the -principal gases composing the atmosphere. I limit myself to noticing some -of the often-repeated observations in Plücker’s tubes, which contained -oxygen, hydrogen, and nitrogen, as well as the observations of the -spectrum of the air under different conditions. The experiments were -made with a small inductive apparatus, in which the length of the spark -between platinum points in ordinary air was 15 millims. at the most. As -Zöllner (in the pamphlet mentioned) comes to the conclusion, that if -the development of the light in the Aurora, according to the analogy of -gases brought to glow in rarefied spaces, is of an electric nature, it -must belong to very low temperature—in order to bring the gases enclosed -in the tubes to glow at the lowest possible temperature, I have always -employed such weak currents that the gas was only just steadily alight. - -The following observations have been repeated often and at various times. -The figures are averages of the indications of the micrometer-screw, so -that the uncertainty of the figures will, in the rarest cases, amount -to no more than 0·015 turn of the screw, and must be reckoned somewhat -more highly only in the case of completely faint misty lines. The -spectrum apparatus was that described above, and the slit was nearly -the same in every experiment, and so narrow that the sodium-lines could -be seen separated. The measurements, for the most part, extend only to -the Fraunhofer line G, as I feared lest, through further turning the -telescope by means of the micrometer-screw, too great a pressure might be -exercised on the worms of the latter. - - -I. _Oxygen._ - -_a._ In the narrow part of the Plücker tube. - - +------+------------+--------------------------------------------------+ - |Screw.|Wave-length.| Remarks. | - +------+------------+--------------------------------------------------+ - | 3·97 | 6562 |Moderately bright. | - | 5·04 | 6146 |Very bright. | - | 6·98 | 5603 |Very bright, misty towards the violet. | - | 8·19 | 5332 |Faint. | - | 8·95 | 5189 |Moderately bright. | - |10·97 | 4870 | ” ” | - |11·02 | 4863 |Faint. | - |11·26 | 4829 |Bright; misty towards the red end of the spectrum.| - |13·30 | 4583 |Very faint. | - |14·05 | 4506 |Moderately bright. | - |15·55 | 4372 | ” ” | - +------+------------+--------------------------------------------------+ - -_b._ In the wide part of the Plücker tube. - - +------+------------+------------------+ - |Screw.|Wave-length.| Remarks. | - +------+------------+------------------+ - | 6·98 | 5603 |Very faint. | - | 8·95 | 5189 |Very bright. | - |11·26 | 4829 |Moderately bright.| - +------+------------+------------------+ - -The lines near 3·97 and 11·02 belong to hydrogen. Probably traces of -aqueous vapour were present in the tube, which were decomposed by -the galvanic current. These two lines are not to be found in a lower -temperature in the broad part of the tube. It is striking that the red -nitrogen-line near 5·04 is also missing there. In the narrow part of the -tube the lines stand out in the green on a very dimly-lighted ground, -whilst in the wider part they appear on a perfectly dark ground. - - -II. _Hydrogen._ - -_a._ In the narrow part of the tube. - - +------+------------+--------------------------------------------------+ - |Screw.|Wave-length.| Remarks. | - +------+------------+--------------------------------------------------+ - | 3·98 | 6558 |Very bright. | - | 6·16 | 5813 |Moderately bright, on both sides very faint lines.| - | 7·01 | 5596 |Moderately bright.} On a dimly lighted ground, | - | 7·18 | 5555 |Moderately bright.} which becomes fainter towards| - | 7·77 | 5422 |Faint. } the violet. | - | | | | - | 8·95 | 5189 |Moderately bright.} | - |10·03 | 5008 |Faint. } On a faint steadily bright | - |10·55 | 4929 |Moderately bright.} ground. | - | | | | - |11·04 | 4861 |Very bright. } From 11·5 to 12·9 a bright | - |12·86 | 4632 |Moderately bright.} ground, which towards the | - | | | } violet becomes very bright. | - | | | | - |13·32 | 4581 |Very faint. } | - |14·05 | 4506 | ” } On a dull ground. | - |15·90 | 4342 |Very bright. } | - +------+------------+--------------------------------------------------+ - -_b._ In the broad part of the tube. - - +-------+------------+------------------+ - | Screw.|Wave-length.| Remarks. | - +-------+------------+------------------+ - | 5·30 | 6063 |Faint. | - | 7·00 | 5598 |Bright. | - | 8·96 | 5187 |Very bright. | - | 11·28 | 4828 | ” | - | 14·04 | 4507 |Moderately bright.| - +-------+------------+------------------+ - -The lines appeared on a perfectly dark ground. - -The tube shows in the narrow part the hydrogen-spectrum of the first -order; the lines in the green do not coincide with the lines of the -nitrogen, though some lines belonging to nitrogen are found. Here, too, -most probably small particles of aqueous vapour have been enclosed in -the tube and are decomposed. Very striking is the spectrum in the broad -part of the tube; nothing is to be seen of the bright shining lines Hα -3·98, Hβ 11·04, Hγ 15·90; on the other hand, four very bright lines and -one quite faint one are in the red end of the spectrum, which appear, -in opposition to the spectrum of the narrow part, not on a partially -lighted, but on an entirely dark ground. The appearance is very striking -if we bring the tube in front of the slit; and so, by degrees, at first -the light in the narrow part, then the light at the connecting-point of -the narrow and wide parts, and, finally, the light in the latter fall -upon the slit. At the connecting-point of the wide ends of the tube the -three well-known hydrogen-lines decrease in intensity, the continuous -ground of some parts of the spectrum disappears, and a new line appears -near 11·28, which has about the same brilliancy as Hβ. - -A comparison with the spectrum of oxygen shows the bright lines which are -in the spectrum in the wide end of the tube as belonging to that element. -The heat evolved by the current appears insufficient to bring the -hydrogen to glow, whilst by it the oxygen, which is of a more rarefied -character, becomes incandescent. An alteration of the direction of the -current has no influence on the appearance. - - -III. _Nitrogen._ - -_a._ In the narrow part of the tube. - - +------+------------+--------------------------------------------------+ - |Screw.|Wave-length.| Remarks. | - +------+------------+--------------------------------------------------+ - | 3·84 | 6620 |}Several faint, broad, close lines, increasing in | - | 4·85 | 6213 |} brilliancy as they approach the violet end. | - | | | | - | 5·30 | 6063 |}Broad bright lines, | - | 5·51 | 6000 |} so close together | - | 5·69 | 5948 |} that the intervening spaces appear | - | 5·87 | 5896 |} like fine dark lines. This part of | - | 6·04 | 5846 |} the spectrum is very bright, but | - | 6·20 | 5802 |} not uniform, being brighter towards | - | 6·43 | 5741 |} the violet end. | - | | | | - | 6·96 | 5607 |}Group of faint but at the same time | - | 7·13 | 5567 |} very broad lines. The | - | 7·28 | 5532 |} last is the brightest. | - | | | | - | 7·55 | 5470 |}The dark intervening spaces are somewhat broader,| - | 7·74 | 5428 |} the bright lines somewhat more intense than | - | 7·92 | 5389 |} in the preceding group, and all of almost equal | - | 8·09 | 5353 |} brilliancy. | - | | | | - | 8·32 | 5306 |} | - | 8·50 | 5272 |}Very faint fine lines. | - | 8·69 | 5237 |} | - | | | | - | 9·01 | 5178 |Very bright broad misty line. | - | | | | - | 9·67 | 5066 |Very bright line. } | - |10·25 | 4975 |” ” } | - |10·66 | 4913 |” ” } The bright lines | - |11·03 | 4862 |Very faint line. } are sharply | - |11·41 | 4811 |Bright line. } defined towards | - |12·11 | 4721 |” ” } the red end of | - |12·57 | 4666 |Faint line. } the spectrum, | - |12·57 | 4644 |Bright, broad, misty line. } fading away towards | - |13·42 | 4570 |Very bright line. } the other end of | - |14·24 | 4487 |” ” } the spectrum. | - |15·02 | 4417 |Bright line. } | - | | | | - |15·66 | 4363 |Bright lines. } Bright lines sharply defined | - |15·72 | 4357 | ” } towards red end, indistinct | - |15·87 | 4345 |Bright line. } towards other end of | - |16·72 | 4273 | ” } spectrum. | - +------+------------+--------------------------------------------------+ - Here follow several lines. - -_b._ In the wide part of the tube. - - +-------+------------+------------------------------+ - | Screw.|Wave-length.| Remarks. | - +-------+------------+------------------------------+ - | 6·20 | 5802 |Faint, indistinct, broad line.| - | 7·72 | 5433 |Dull stripe. | - | 8·20 | 5330 |Faint line. | - | 8·94 | 5191 |Very faint line. | - | 9·03 | 5175 |Broad band of light. | - | 9·90 | 5029 |Dull band of light. | - | 10·68 | 4911 |Moderately bright line. | - | 11·42 | 4809 |Faint line. | - | 12·59 | 4663 |Bright line. | - | 13·43 | 4569 | ” | - | 14·07 | 4504 |Moderately bright line. | - | 14·25 | 4486 |Very bright line. | - | 15·85 | 4347 | ” ” | - | 16·76 | 4273 |Moderately bright line. | - +-------+------------+------------------------------+ - -_c._ At the aura of the negative pole. - - +-------+------------+------------------------------------------------+ - | Screw.|Wave-length.| Remarks. | - +-------+------------+------------------------------------------------+ - | 5·18 | 6100 |} Broad, moderately bright stripe, indistinct | - | 5·70 | 5945 |} at the edges. | - | | | | - | 7·60 | 5159 |} Broad, moderately bright stripe. | - | 8·41 | 5289 |} | - | | | | - | 8·76 | 5224 |{ Very bright line, somewhat indistinct towards | - | | |{ the violet. | - | | | | - | 9·19 | 5147 |Faint line. | - | 10·00 | 5004 |Bright line, indistinct towards the red. | - | | | | - | 10·67 | 4912 |{ Somewhat fainter than the last, indistinct | - | | |{ towards the red. | - | | | | - | 11·43 | 4808 |Very faint line. | - | 12·25 | 4704 |Very intense, broad, indistinct towards | - | | | the violet. | - | 12·73 | 4646 |Very faint line. | - | 13·43 | 4569 |Moderately bright, indistinct towards the | - | | | violet. | - | 14·25 | 4486 |Like the last. | - | 15·03 | 4417 |Quite a faint line. | - | 15·86 | 4346 |Moderately bright line. | - | 16·76 | 4275 |Very bright line. | - +-------+------------+------------------------------------------------+ - -Here follow several other lines. - -The observations in the different parts of the tube show plainly the -dependence of the spectrum on the temperature. The aura of the negative -pole gives the line near 10·07 so characteristic of the air-spectrum. -This is the same line which is met with in the spectra of most of the -nebulæ. The very striking groups of lines in the red and yellow in the -spectrum of the narrow part of the tube disappear entirely in the wide -part. If we compare the spectra with those above quoted, of oxygen and -hydrogen, we find line Hβ very faint in the spectrum of the narrow part -of the tube near 11·03; on the other hand, oxygen-lines appear in the -broad part near 8·20, 8·94, and 14·07. Thence I would conjecture that -the tube was not filled with pure nitrogen, the appearance of which is -precise, but with dry rarefied air, since Wüllner’s researches have -proved that dry air yields the same spectrum as nitrogen gas. Perhaps -the air in the tube examined by me had not been thoroughly dried, and -thus the appearance of some lines of the elements before named is to be -explained. - -I must further mention that the electrodes of the tubes consisted of -aluminium; yet a comparison of the spectra observed and the aluminium -spectrum has shown no connexion between them. - - -IV. _Atmospheric Air._ - - +-----------+------------+------------------------------------------+ - | Screw. |Wave-length.| Remarks. | - +-----------+------------+------------------------------------------+ - | 5·88 | 5892 |Very bright double line (Na). | - | 6·67 | 5680 |Very bright line. | - | 7·20 | 5550 |Faint line. | - | 9·00 | 5180 |Very bright line. | - | 9·79 | 5047 |Fine faint line. | - | 10·03 | 5008 |} Very bright double line. | - | 10·07 | 5002 |} | - | 11·43 | 4803 |Faint confused line. | - | 12·69 | 4651 |} | - | 12·84 | 4633 |} Faint line not sharply defined. | - | 13·04 | 4612 |} | - | | | | - |From 14·61 | 4453 |} Confused band of light, which ends with | - |to 15·88 | 4444 |} a broad washy line. | - +-----------+------------+------------------------------------------+ - -Here follow several other lines. - -Rarefied air saturated with aqueous vapour. - - +----------+------------+----------------------------------------------+ - | Screw. |Wave-length.| Remarks. | - +----------+------------+----------------------------------------------+ - | 3·97 | 6562 |Moderately bright line. | - | (5·88)| 5892 |Bright double line (Na). | - | (6·25)| 5789 |Bright line (H). | - |From 7·03 | 5591 |}Broad dull band of light; near 7·03 a | - |to 7·55 | 5470 |} somewhat brighter line. | - | 7·59 | 5461 |Bright line (H). | - | 8·72 | 5231 |Dull stripe. | - | 8·96 | 5187 |Broad misty stripe.} On a dull steady ground. | - | 10·07 | 5002 |Faint line. } | - | 11·05 | 4859 |Very bright line. | - | 12·21 | 4709 |Moderately bright line.} On dimly lighted | - | 12·75 | 4644 |{Line fainter than the } ground, becoming | - | | |{ preceding. } fainter towards | - | 13·28 | 5585 |Very faint line (H). } the violet. | - | (15·71)| 4358 |Very bright line (H). | - | 15·90 | 4341 | ” ” | - +----------+------------+----------------------------------------------+ - -Here follow several more lines. - -In the first observations, the electric spark, about 1 centim. in length, -was allowed to pass between platinum points in ordinary air. - -The sodium-line near 5·88 appeared continually. The bright double line at -10·03 and 10·07, with a weaker current or longer spark, was no longer to -be recognized as a double line, but appeared as a broad somewhat confused -line, of which the brightest part was near 10·05. No lines belonging to -the platinum spectrum appeared. Ordinary rarefied air, under a pressure -of 25 to 30 millims., and which was enclosed by mercury in a tube 8 -millims. wide, showed exactly the same lines as Plücker’s nitrogen-tube -(_b_), except that some lines belonging to the spectrum of mercury also -appeared. - -This observation may be regarded as a confirmation of the conjecture -above expressed as to the condition of Plücker’s tube III. (nitrogen). -In the observations described under _b_, the air saturated with aqueous -vapour was under a pressure of 22 millims. Besides the sodium-lines, -lines of the mercury-spectrum appeared at 6·25, 7·59, and 15·71. The -spectrum of rarefied air under similar pressure was found to accord -completely with the spectrum of the light in the broad part of Plücker’s -tube. - -III. (Nitrogen _b._)—A comparison of the spectrum of rarefied air -saturated with aqueous vapour with the former shows the striking -alterations in the spectrum which are brought about by the presence of -the aqueous vapour. - - -3. _Comparison of the Aurora-Spectrum with the Spectra of Atmospheric -Gases and of Inorganic Substances._ - -In the next place, I turn to the comparison of the observed spectra -of different gases and of the air with the spectrum of the Aurora. -The first band of light in the red part of the Aurora-spectrum most -probably coincides with the first system of lines in the spectrum of -nitrogen (_a_). Probably only the bright part of this group of lines -is perceptible, on account of the extreme faintness of the Aurora; and -as in nitrogen the increase of the brilliancy of the spectrum takes -place towards the violet end, the absence of the intermediate spectrum -towards this direction would find its explanation. The most intense line -of the Aurora-spectrum at 7·12 is to be also found in the spectrum of -nitrogen (_a_)—as a very faint line, however. That this line appears -in the Aurora by itself, and with intensity relatively great, need not -appear strange, considering the great alteration of the gas-spectra under -different conditions of pressure and temperature. The third line of the -Aurora-spectrum, very vaguely defined on account of its great faintness, -coincides in the same way with a nitrogen-line. - -The line at 8·71 is met with in the nitrogen-spectrum (_c_), as well -as in the air-spectrum (_b_). The third line of the oxygen-spectrum at -8·95, which _seems to appear under very different conditions_, is found -again, as the fifth line in the spectrum of the Aurora. _Moreover, the -sixth line in the Aurora at 10·06 coincides very exactly_ with the known -nitrogen-line appearing in the spectra of some of the nebulæ. Lastly, as -to the broad band of light in the Aurora-spectrum from 12·33 to 12·88, -several lines are found in this place in the spectrum of nitrogen as well -as the air-spectrum (_a, b_); so that here, too, a coincidence between -the spectra may be regarded as probable. - -The observations show with some certainty that at least one line at 10·06 -of the Aurora-spectrum coincides with the maximum brilliancy of the -air-spectrum, and that the other lines appear with great probability in -the spectra of atmospheric gases. - -In the very great difference of the gas-spectra under varying conditions -of pressure and temperature, it would indeed be difficult to succeed -in producing artificially a spectrum which should resemble that of the -Aurora in all parts. Moreover, it must be admitted, under the hypothesis -that the Auroræ are electric discharges in rarefied air-strata, that -these strata, qualified for the transmitting of electricity, will have a -very considerable thickness. - -In this case the conditions of pressure on these air-strata are -themselves so different that, within certain limits, each will yield its -own peculiar spectrum; but we shall see the sum of collective spectra, so -to speak, spread out behind each other; and therefore the impossibility -of attaining a perfect agreement between the Aurora-spectrum and the -artificially exhibited spectra of mixed gases is evident. - -A comparison of the Aurora-spectrum with the spectra of inorganic -substances may be easily worked out by the help of the above-quoted -wave-lengths of the single lines of the former, with due regard to -probable errors, and with the aid of Ångström’s Atlas of the Solar -Spectrum. Here the perfect harmony of the brightest Aurora-line (which -was fixed with an exactitude of about one seventh of the separation of -the sodium-lines) with the lines of the iron-spectrum is especially -striking. The wave-lengths in the above-cited observations of the bright -Aurora-line vary between 556·9 and 557·3, whilst, according to Ångström, -two lines of the iron-spectrum are situated at 556·85 and 557·17. - -Iron-lines corresponding to the other Aurora-lines, within certain limits -of accuracy, are also to be found, as will be seen from the following -comparison:— - - +--------------+--------------------+----------------------+ - |Aurora-lines. | Lines of the | Remarks. | - | | iron-spectrum. | | - +--------------+--------------------+----------------------+ - | | { 630·08 } | Moderately bright. | - | 629·7 | { 629·85 } | | - | | | | - | | { 539·60 } | | - | | { 539·92 } | Mostly very faint. | - | 539·0 | { 539·05 } | | - | | { 538·85 } | | - | | | | - | | { 523·43 | Very faint. | - | 523·3 | { 523·21 | Moderately bright. | - | | { 522·90 | Very faint. | - | | | | - | | { 519·79 | ” | - | | { 519·40 | ” | - | 518·9 | { 519·16 | Moderately bright. | - | | { 519·06 | ” ” | - | | { 518·51 | Very faint. | - | | | | - | | { 500·65 } | | - | | { 500·52 } | | - | 500·4 | { 500·49 } | Very faint. | - | | { 500·30 } | | - | | { 500·20 } | | - | | | | - | From 469·4 } | - | to 462·9 } 3 stronger and 4 very faint iron-lines. | - +----------------------------------------------------------+ - -Yet this agreement, though remarkable, can only be considered as -complete proof of the presence of iron-vapour in the atmosphere when we -shall have succeeded in showing by observation analogous modifications of -the relative conditions of brilliancy in the iron-spectrum by alterations -of temperature and density; and in this way explain the appearance of -relatively very faint iron-lines in the Aurora-spectrum, or, on the other -hand, the absence of the most intense lines. - -It will meanwhile remain far more in accordance with probability to -regard the _Aurora-spectrum as a modification of the air-spectrum_; -since we are already aware, in the case of gases, of the alteration of -the spectra by conditions of temperature and pressure; and an agreement, -at any rate, quite as certain between the spectrum in question and the -spectra of atmospheric gases has been proved above. - - [I am indebted to Miss Annie Ludlam for a translation from the - German of the above Memoir.—J. R. C.] - - - - -FOOTNOTES - - -[1] βόθυνος, a hollow. - -[2] πίθος, a cask. - -[3] χάσμς, a chasm. - -[4] M. Lemström (Swedish Expedition, 1868) concludes that the corona of -the Aurora Borealis is not entirely a phenomenon of perspective, but that -the rays have a true curvature, that they are currents flowing in the -same direction and attract each other. There is also an account [_antè_, -p. 16] of an Aurora at Melville Island (Parry’s first voyage), in which -two arches were seen curving towards each other. - -[5] A brilliant display in December 1870, on the east coast of Sicily, -was followed by very violent storms, with the overflow of the Tiber and -the flooding of Rome. - -[6] Some curious instances have been recently (January 1879) given in -the ‘Times’ of such electric phenomena, comprising, amongst others, gas -lighted by the finger in Canada, points of flame seen on the ironwork of -Teignmouth Bridge, and similar points seen on the alpenstocks and axes of -a party making a mountain ascent in Switzerland. - -[7] On the occasion of the Aurora of September 24, 1870, Dr. Allnatt -says, “the air seemed literally alive with the unwonted phosphorescence.” - -[8] See, however, Dr. Schuster’s article “On the Spectra of Lightning,” -Phil. Mag. May 1879, p. 316. - -[9] The proof from occulted stars merely goes to the fact that the moon -possesses no atmosphere _appreciable in that way_. It may still be a -question whether there does not exist something of the kind, lying low -and close to the surface, and possibly of a rarefied character, which -would scarcely make itself visible by its effects in occultations. -Cloud-vapour might form in an atmosphere of inconsiderable density. - -[10] This observation is not without a certain amount of confirmation by -more recent ones, in which certain lunar objects and regions have been -suspected of mist or vapour. Mr. Birt (‘English Mechanic,’ vol. xxviii. -no. 725) mentions two—the cloud-like appearance of the white patch west -of Picard, and the interior of Tycho, which at one time always misty and -ill-defined, is now become perfectly distinct and sharply defined. - -December 4, 1878, 4h 45m. I observed Klein’s crater as a dull dark spot, -larger than the true object; and while definition was good and other -objects were well defined, “the floor of Klein’s object, the oval spot -near, and also Agrippa (especially), all had _an odd misty look as if -vapour were in or about them_” (‘English Mechanic,’ vol. xxviii. no. -727). The mystery of different observers seeing and not seeing Klein’s -object on the same night is hardly to be accounted for by the angle of -illumination. - -[11] Some doubt has been cast on this observation, on the ground that -nothing unusual was seen, and that the appearances were only those -ordinarily presented by the moon at its then phase. I simply give the -account as it appears in the scientific journal in which it was published. - -[12] The question of a connexion between the waxing and waning of the -solar corona and the prevalence of sun-spots is now being mooted, and -may have an important bearing on the subject of the constitution of the -corona. It would seem that when the corona has been examined about the -time of minimum of sun-spots, it has proved fainter though more extended, -while the bright lines of the spectrum have been absent, indicating a -change or variance in the gaseous part of it at those periods. - -[13] There seems to be some confusion as to the W.L. here given; 5567 is -usually accepted as Ångström’s line, while Prof. Smyth refers to it as -5579. The position, too, when examined with a spectroscope of greater -dispersion, is not exactly over the citron-line of acetylene, both the -above referred to lines lying somewhat more towards the violet end of the -spectrum (see Plate V. fig. 7). - -[14] Ångström’s drawing, in giving this character to the two Aurora-bands -which are said to correspond with violet-pole bands about 47 and 43, is -incorrect, and calculated to mislead by giving the Aurora-bands a feature -corresponding to the violet-pole bands which they do not possess. I am -not aware of any Aurora-line or band which is described as distinguished -by degrading towards the violet. - -[15] The tubes generally seem marked Si Fl instead of the ordinary -notation Si F. Si Fl₆ is probably, in fact, Si F₄. - -[16] Dr. Schuster has found that while the line-spectrum of lightning is -attributable to N, it has also a band-spectrum, which he considers due to -O and a slight trace of CO₂ (Phil. Mag. 5th ser. vol. vii. p. 321). - -[17] _In these observations some suggestions made by Mr. Capron have been -incorporated._ - -[This was Mr. Lockyer’s note. In point of fact, the Author was -responsible for the verbatim paragraphs comprised between the letters _A_ -and _B_, and _C_ and _D_, in the instructions as now reprinted.] - -[18] Communicated by the author to the Royal Saxon Academy of Science, -1871. - -[19] Reports of the Royal Saxon Academy of Science, Oct. 31, 1871. - -[20] This red line was first noticed by Zöllner. - -[21] Recherches sur le Spectre Solaire, p. 42. - -[22] American Journal of Science, lxviii. 123. - - - - - -End of the Project Gutenberg EBook of Auroræ: Their Characters and Spectra, by -J. 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