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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.”
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.
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.
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).
It may here be mentioned that the Zodiacal Light is usually described as, in these latitudes, of a golden yellow or pale lemon tinge.
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.
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.
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.
In anticipation of the total eclipse of the Moon on the 27th February, 1877, several articles appeared in the leading journals of the day describing, for the public benefit, the appearances which might be expected during the occurrence of the phenomenon.
Among these was one by Mr. R. A. Proctor, in which the following passage occurs:—“That dull, or occasionally glowing red colour, shown by the moon when she is fully and even deeply immersed in the shadow of the earth, is a phenomenon whose explanation is not without interest. Formerly it was thought that the moon possessed an inherent light, or perhaps was illuminated by auroral light, which only became discernible at the time of total eclipse. Indeed even Sir W. Herschel fell into the mistake of supposing this the only available explanation, having miscalculated the efficiency of the true cause.”
This passage was only pointed out to me by a friend after the eclipse had actually taken place, and I had sent him some notes of what I then saw. My notes on the occasion comprised, amongst others, the following remarks:—“The tints of colour also during partial eclipse, owing, no doubt, to the moon’s considerable altitude, were singularly bright and well contrasted. Silver-grey, dusky copper-red, and the same tint clearer and brighter were ranged side by side with a lovely jewel effect. We noticed also at times a crimson-scarlet tint, deeper and less mixed with yellow than the copper colour. This last tint reminded me much of a crimson glow common to the Aurora, and which I also once distinctly remarked (of course in a weaker degree) in the zodiacal light” (antè, p. 68).
On the occasion of the eclipse of August 23-24, 1877, we were favoured at Guildown, in common with many other places, by a singularly clear sky during the progress of the moon’s obscuration and subsequent clearing. In the early part of the evening, however, the moon, from some cause (possibly atmospheric vapour), seemed to have, as the earth’s shadow advanced on its disk, an unexpectedly misty and indistinct appearance, which lasted up to and including total obscuration. Golden yellow, yellow copper, dull copper, ruddy copper, and dull red were successively the principal colours observed at different times and at various portions of the moon’s surface.
After referring to some spectroscopic appearances, my notes then ran on thus:—“As the shadow began to pass off, and the bright sharp crescent of the illuminated portion of the moon to appear, the general aspect of the moon’s disk seemed to me to greatly change. The certain amount of indistinctness noticeable during approach and continuance of totality, gave way to a considerable sharpness of the moon’s features as seen through the shadow. The shadowed part glowed with a richer copper tint, on which were seen dark, almost black, spots and patches.” Then follows a description of these; and the notes continue:—“Two features here struck me—the one a continuation of the upper limb of the illuminated crescent, so that it seemed to form a bead of light just on the centre of the upper edge of the moon; the other two patches of crimson light, similar to those I described as having been seen in the last total eclipse. One of these, quite a small one, was just under the elongated bead before described; the other, a much larger and more diffused one, was seen towards the south-west limb of the moon, about midway between it and the centre. The spots or patches were of a decidedly crimson-red, in contrast to the ordinary copper-red of the disk, and were noticed by my friend as well as by myself.”
These were eye observations. The patches were quite well seen (but not so brightly as with the eye) with a double achromatic field-glass. With a 3¼-inch Cooke refractor and low power, they seemed lost in the general moon tint; but they were then diminishing in brightness. From a comparison of my two sketches, the patches seem to have gradually deepened in tint, and we considered them to have disappeared in a like gradual manner.
My first sketch was taken shortly after end of total phase; the second about ten minutes later. I have reproduced the original sketches in preference to any drawing prepared from them (Plate IV. figs. 2 and 3).
The patches did not last long, but were lost as the shadow swept off the moon. I saw nothing of the sort during the approach of or pending totality, nor until a small crescent of the moon began to appear behind the shadow.
I have looked for other accounts of these patches, but cannot find any. Most observers have described the deeper colour of the shadowed moon by the word “copper.” Some extend this colour to red; but there is probably much in the state of the atmosphere affecting this.
At Avignon, December 3, 1703, the moon appeared, pending eclipse, “extraordinarily illuminated and of a very bright red,” while other and different features were seen at Montpelier.
On March 19, 1848, observers in England, Ireland, and Belgium described the moon’s disk as “intensely bright coppery red.” On the occasion of August 23-24, 1877, before mentioned, an article in one of the public papers described the moon’s disk, during totality, as of a “dull copper colour.”
Mr. Henry Keye, in the Engadine, at a height of 4500 feet above sea-level, and with the purest air, saw the partially covered moon (before totality) as a “dull copper colour.”
Prof. Pritchard, writing from the Oxford University Observatory, says that at 12h 10m (about the time my sketches were taken) there was a good deal of light on the moon’s following limb, and the colour was “more red than copper,” and apparently redder than it had been at a similar distance of time before totality. Mons. Faye reported to the French Academy of Sciences that “a striking phenomenon not previously noticed was that the reddish tinge, resembling that of a fine sunset, was deepest at the margin of the disk, a circumstance which he could not explain.” Dr. Allnatt, writing from Frant, says:—“At totality the moon’s disk presented a most extraordinary appearance: the western limb was comparatively transparent, but the main body appeared as though enveloped in a semi-opaque clot of coagulated blood, through which the lunar features were dimly visible.”
The observations of Prof. Pritchard and Mons. Faye point more immediately to redness; and this is the nearest approach I can find to the patches I noticed. These patches do not seem to me easy of explanation. They could not well be colours or details due to the actual surface of the moon itself. The moon, we are aware, has only a certain portion of the visible disk slightly tinted. The Mare Serenitatis is certainly of a slight green tinge; and to the Palus Somni and certain other districts is attributed a pale red or pink; but these tints could hardly have sufficed to produce the effect seen, as the patches were conspicuous for a bright and decided colour. The positions, moreover, did not correspond; while the ease with which other details of the surface were seen at the time would, if the tints had arisen from the surface itself, probably have enabled the circumstance to be detected.
The refraction of the sun’s rays by passage through the earth’s atmosphere is, too, not a satisfactory explanation. This, as judged by the appearance of the covered moon immediately before and at totality, gives a disk of shadow deeper in tone in the centre and lightening towards the edges, but in other respects fairly uniform, so that the whole disk seems to partake of the same tint and its graduations; and this is what might have been expected under the circumstances. The patches, on the other hand, were quite local.
The theory of the moon’s possessing no atmosphere whatever is now very generally, but perhaps too readily, received (mainly upon the evidence of the spectroscopic observations of occulted stars[9]), as there still seems a reasonable doubt whether our satellite may not possess an atmosphere, possibly rarefied, but yet sufficiently dense to permit of the formation of cloud or vapour.
A curious case, in which a patch of vapour or cloud was supposed to be detected on the moon’s surface, is reported by the Rev. J. B. Emmett in a communication to the ‘Annals of Philosophy’ (New Series, vol. xii. p. 81). It is dated “Great Ouseburn, near Boroughbridge, July 5, 1826,” the observation being made with “the greatest care with a very fine telescope.”
On the 12th April 8h, while observing the part of the moon called Palus Mœotis by Nevelius, with an excellent Newtonian reflector of 6 inches aperture, at a particular part of the Palus, which he minutely describes, he saw, with powers 70 and 130, “a very conspicuous spot wholly enveloped in black nebulous matter, which, as if carried forward by a current of air, extended itself in an easterly direction, inclining a little towards the south, rather beyond the margin of Mœotis.” April 13th 8h to 9h, the cloudy appearance was reduced both in extent and intensity, and the spot from which it seemed to issue had become more distinctly visible. On April 17th scarcely a trace of the nebulous matter remained; but so long after as June 10th 8h “a little blackness” remained about the spot. Mr. Emmett suggested “smoke of a volcano or cloudy matter.” A copy of the drawing annexed to the paper is given on Plate X. fig. 10 (black patch on moon). If this observation was (as it certainly appears to be) critical and exact, there must have been a disturbance of the moon’s surface, indicating some sort of cloud- or vapour-supporting atmosphere; and probably, for the purposes of Auroræ, an atmosphere of a very rarefied condition would suffice[10].
According to the ‘New York Tribune,’ at a recent semi-annual meeting of the American Academy of Sciences, Professor Alexander “brought forward a variety of evidence tending to indicate some envelope like an atmosphere for the moon. The evidence was principally drawn from observations during eclipses. The explanations usually offered for the bright band seen around the moon at such times was fully considered, and shown to be inadequate, though good as far as they would apply. The ruddy band of light is much too broad to be the sun’s chromosphere. It was most apparent in those instances where the moon was nearest the earth. It would best be accounted for by supposing an atmosphere to the moon, a thin remnant of ancient nebulosity, comparable to that which accompanies the earth and gives rise to the appearance of the Aurora Borealis.” Is it not, however, possible that the appearance might have arisen from Auroræ in action within the region of the earth’s own atmosphere during the passage of the sun’s rays through it at the time of the eclipse? The whole subject is difficult of explanation, and should be one of the points for attention on the occasion of the next total Lunar eclipse. It seemed to me appropriate for introduction into the present history of the Aurora, whatever its solution may ultimately be.
In the case of Mars and Jupiter, whose atmospheres are sufficiently recognized, red- and scarlet-tinted patches are frequently noticed. In Mars this is generally attributed to the geological character of the surface of the planet itself; but I have observed on Mars’s surface during the recent opposition a local rosy tint of a more diffused and indefinite character; and in the case of Jupiter the appearances seem almost always connected with the clouds’ belts, as distinguished from the regions lying nearer to the planet’s surface.
Professor Dorna, of Turin, ascribed a flickering light seen on the reddened disk of the moon during the Lunar eclipse of February 1877 to the action of a Lunar Aurora, holding that the refraction of the sun’s rays within the cone of the earth’s shadow was not an adequate explanation (‘L’Opinione Nazionale,’ March 3, 1877).
The spectroscope might have afforded some information on the question; but my own telescopes (8¼ and 3¼ in.) were not of sufficient aperture to give a sensible spectrum of a portion of the moon’s eclipsed surface, and my observations were chiefly made on the entire disk with hand-spectroscopes without a slit. Mr. Christie, at the Royal Observatory, Greenwich, made a set of observations during totality, and also during subsequent partial phase, with a single-prism spectroscope. During totality a strong absorption band was seen in the yellow, and the red and blue ends of the spectrum were completely cut off, while the orange was greatly reduced in intensity. The yellow and green were comparatively bright, and seemed to constitute the whole visible spectrum. The absorption band became narrowed as the end of the total phase approached, and during partial phase was reduced to a mere line. The red end of the spectrum was cut off by a dark band commencing about halfway from D to C, in which a black line was suspected. The bands observed were characteristic of the spectrum of light which has passed through a thick stratum of air. In the description of the spectrum of the Aurora in Part II., it will be seen that the conspicuous red and green lines of the Aurora are either coincident with, or very close to, some of these atmospheric lines. It does not appear that Mr. Christie examined the crimson patches specifically, nor that he saw bright lines on any part of the moon’s eclipsed disk.
Mr. Henry Pratt has also kindly handed me for use his notes of the Lunar eclipse of August 23, 1877, as seen at Brighton on a splendid night. They were made as the phenomenon progressed, are 58 in number, and in many instances only a few minutes, or even seconds, apart. A selection of them is here given:—9h 13m 50s, first contact of shadow. 9h 30m, shadow very dark; no details of disk easily seen. 9h 40m, first appearance of red. 9h 50m, red all over disk, except margin bluish and S. part green tint. 10h 2m, a sudden brightening of whole disk, in strong contrast to two minutes previously. 10h 15m, E. limb much darker. 10h 35m, south pole decidedly brightest. 10h 44m, S.E. limb much brighter. 10h 48m, whole disk much darker. 10h 51m, S.E. limb brightening again. 11h 1m, N.E. limb brightening. 11h 3m, N.E. limb has darkened and brightened three times during last two minutes. 11h 20m, N. pole has darkened. 11h 21m, N. pole has brightened. 11h 24m 30s, N. pole darker red. 11h 35m, N. pole bright. 11h 35m 30s, same dark and red. 11h 42m, N.E. limb especially bright for a few seconds, and then reddened and shaded again. 11h 49m, S. pole reddened. 12h 1m, S.W. limb reddest part; S. pole red; N. pole paler red. 12h 3m 50s, first appearance of E. limb (my first sketch was made shortly after this, and my second about ten minutes later). 12h 21m, a bright patch on N.N.W. separated from N. pole. 12h 24m, S.W. region is reddest part of eclipse. 12h 40m, redness of shadow fading out.
With a small Browning star-spectroscope Mr. Pratt saw the red and blue ends of the spectrum cut off, but nothing else. Mr. Pratt adds that the red colour was not an effect of contrast or an optical delusion in any way, as was proved by using at times a limited field containing only the red portion under examination. In reference to the curious brightening and darkening of the disk, and the change from time to time of local colour, he says that with much experience he has seen nothing of the same marked character on other occasions, and that “the whole matter was at the time astonishing to me, but none the less real.” The local red patches seen by me seem also to have been observed by Mr. Pratt.
As an addition to the instances of Tycho, Picard, &c., mentioned in the note on p. 73, Mr. Pratt has also sent me his notes of some observations by him, of “local obscuration of the floor of Plato.” As somewhat condensed, they are as follows:—1872, July 16. While in other parts of the floor spots and streaks were well visible, “the N.W. portion was in such a hazy condition that nothing could be defined upon it.” 1873, Nov. 1. 27 light streaks seen (7 new): the brightness of the streaks was in excess of their usual character, as compared with the craterlets; “an obliteration or invisibility of all the light streaks in the neighbourhood of craterlet no. 1 was very noticeable;” and also “a similar obliteration of the N. end of the streak called the Sector, near craterlet 3.” 1874, January 1. 18 light streaks seen, including 3 new, “some of which outshone other longer known ones. This was curious; for had they been as bright within the last two years as on this occasion I must have noticed them.” Mr. Pratt points out, as worthy of remark, that some months previous to November 1st, 1873, neither craterlets nor streaks on the floor of Plato “had maintained their previous characteristic brightness,”—a fact which he thinks ought to be considered together with the outbreak of brilliancy of both orders on that day, as well as the apparently sudden existence of new ones.
The ‘Observatory,’ March 1, 1879, p. 375, contains an account, by Mr. H. C. Russell, of some Astronomical Experiments made on the Blue Mountains, near Sydney, N. S. W. Among these it is noticed that on 21st October, 1878, at 9 A.M., when looking at the moon, Mr. Hirst found that a large part of it was covered with a dark shade, quite as dark as the shadow of the earth during an eclipse of the moon. Its outline was generally circular, and fainter near the edges. Conspicuous bright lunar objects could be seen through it; but it quite obliterated the view of about half the moon’s terminator, while those parts of the terminator not in the shadow were distinctly seen.
No change in the position of the shade could be detected after three hours’ watching. The observation is made, “One could hardly resist the conviction that it was a shadow; yet it could not be the shadow of any known body. If produced by a comet, it must be one of more than ordinary density, although dark bodies have been seen crossing the sun which were doubtless comets.” The diameter of the shadow from the part of it seen on the moon was estimated at about three quarters that of the moon[11].
Mr. Norman Lockyer, in his ‘Solar Physics,’ a work of 666 pages, gives but little space to the Aurora. The index comprises:—“Aurora Borealis, connexion with sun-spots, pp. 82-102.” “Affirmed coincidence of spectrum with that of the corona, pp. 244, 256.”
Page 82. After referring to Gen. Sabine as having shown that there are occasional disturbances in the magnetic state of the earth, and that these disturbances have a periodical variation, coinciding in period and epoch with the variation in frequency and magnitude of the solar spots as observed by Schwabe, the author proceeds to state, “and the same philosopher has given us reason to conclude that there is a similar coincidence between the outburst of solar spots and of the Aurora Borealis.”
Page 102. “We have also shown that sun-spots or solar disturbances appear to be accompanied by disturbances of the earth’s magnetism, and these again by auroral displays.”
Page 243. “What, then, is the evidence furnished by the American observers on the nature of the corona (solar)? It is bizarre and puzzling to the last degree. The most definite statement on the subject is that it is nothing more nor less than a permanent Solar Aurora! the announcement being founded on the fact that three bright lines remained visible after the image of a prominence had been moved away from the slit, and that one (if not all) of these lines is coincident with a line (or lines) noticed in the spectrum of the Aurora Borealis by Professor Winlock.” Mr. Lockyer then adds, that amongst the lines he had observed up to that time, some forty in number, this line was among those which he had most frequently recorded, and was, in fact, the first iron line which made its appearance in the part of the spectrum he generally studied, when the iron vapour is thrown into the chromosphere.
Hence he thought he should always see it if the Aurora were a permanent solar corona, and gave out this as its brightest line, and on this ground alone should hesitate to regard the question as settled.
Page 256 is an extract from a communication by Prof. Young to ‘Nature,’ March 24, 1870, in which the Professor refers to the bright line 1474 as being always visible with proper management. He also thinks it probable that this line coincides with the Aurora line reported by Prof. Winlock at 1550 of Dr. Huggins’s scale, though he is by no means sure of it. He had only himself seen it thrice, and then not long enough to complete a measurement. He was only sure that its position lay between 1460 and 1490 of Kirchhoff.
For this reason he did not abandon the hypothesis, which appeared to have other elements of probability, in the general appearance of the corona, the necessity of immense electrical disturbances in the solar atmosphere as the result of the powerful vertical currents known to exist there, as well as the curious responsiveness of our terrestrial magnets to solar storms; yet he did not feel in a position to urge it strongly, but rather awaited developments. Father Secchi was disposed to think the line hydrogen, while Mr. Lockyer still believed it to be iron.