Auroræ: Their Characters and Spectra

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:—

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.

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).

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.

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.

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.

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.

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.

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.

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.

The comparison by Prof. Fritz of M. Moberg’s Finland observations has been lately reviewed in ‘Nature’ (March 8, 1878) and the result arrived at that, “After ten years, in spite of the vastly accumulated material of careful observations, there appears no necessity to change Herr Fritz’s system of curves in any essential detail; indeed certain parts of the same, which were at first only based on probability and supposition (the part of the principal zone between the north of Norway and Nishen Kolynisk as an instance), we now know with perfect certainty to be correct.” It has been remarked that the local occurrence of Auroræ is not in accordance with the hypothesis of the phenomenon being one of a cosmical nature.

The winter of 1870 was remarkable for brilliant displays; and the displays of October 24th and 25th, 1870, were remarkably brilliant in England and in America also, and the Aurora Australis was seen on the same days at Madras. These displays were seen in England and America in the daytime as patches or coronæ of white light, with streamers stretching upward from them.

CHAPTER VI.
THE AURORA IN CONNEXION WITH OTHER PHENOMENA.

Auroræ and Clouds.

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.)

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.

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.

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.

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.

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.

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.

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.

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.

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.

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:—

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.

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.

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.

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.

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:—

(1) The magnetic disturbances were of extraordinary magnitude and frequency.

(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.

(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.

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).

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.

On an occasion in 1859 great solar disturbances were observed, the Greenwich magnets were much disturbed, and a fine Aurora was visible.

Cipoletti, of Florence, remarks on the strong magnetic disturbances at Vienna and Munich during the Auroræ of 4th February, 1872, and 4th February, 1874.

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.

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.

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.

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.

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.

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.

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.”

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.

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.

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.

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.

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.”

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.

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.

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.

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.

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.