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.

_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.