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.