Auroræ: Their Characters and Spectra. J. Rand Capron

      Plate VIII.

      Herr Carl Bock’s Lapland Aurora, 3rd October, 1877.

      Herr Carl Bock’s Lapland Aurora, 3rd Oct. 1877. Lapland Auroræ generally of the yellow type.

      In January 1878 I had the pleasure to meet, at the Westminster Aquarium, Herr Carl Bock, the Norwegian naturalist, who accompanied four Laplanders, two men and two women, with sledges, tents, &c., on their visit to this country. The Laplanders (as mentioned elsewhere) did not confirm the accounts of noises said to have been observed by Greenlanders and others during the Aurora. Carl Bock mentioned to me that the displays he saw in Lapland were most brilliant, but generally of the yellow type (the Laplanders called the Aurora “yellow lights”). He saw only one red Aurora. He kindly lent me a picture (probably in its way unique), an oil-painting of an Aurora Borealis, entirely sketched by the light of the Aurora itself.

      A picture painted by light of the Aurora. Movement of the rays. Inner edge of arc fringed with rays.

      The painting is remarkable for the tender green of the sky, an effect probably due to a mixture of the ordinary sky colour with the yellow light of the Aurora. This picture was taken at Porsanger Fjord, in lat. 71° 50´, on 3rd October, 1877. It lasted from 9 P.M. till about 11 P.M. The rays kept continually moving, and certain of them seemed in perspective and behind the others. It will be noticed that the inner edge of the arc is fringed with rays, contrary to the sharp and definite margin which is usually presented. Probably two Auroræ or auroral forms were seen—a quiescent arc in front, and a set of moving streamers beyond. Two larger and brighter patches of light are seen at each extremity of the arc, as in the case of the Aurora seen by me at Guildown, February 4th, 1874, which, indeed, the display much resembles. A reduced facsimile of Herr Bock’s excellent picture is given on Plate VIII.

      Aurora of longitudinal rays.

      Herr Bock also acquainted me that on the following day he saw an Aurora in which the lines of light, instead of being vertical, were longitudinal, and were continually swept along in several currents. They were not so strong as in the former case.

      Rev. T. W. Webb’s Aurora.

      Rev. T. W. Webb’s Aurora. Arc resolved into sets of streamers moving in opposite directions.

      The Rev. T. W. Webb has described to me in a letter an Aurora very like that seen by Carl Bock in Lapland, and apparently the prevailing type in those regions. An arc similar to that figured by Carl Bock appeared in the N.W., and seemed to resolve itself into two sets of streamers moving in opposite directions (or the one set might be fixed and the other moving), like the edges of two great revolving toothed wheels. This lasted but for a few seconds; but during that interval the tints were varied and brilliant, including blue and green.

      The English Arctic Expedition 1875–76, under Capt. Sir George Nares.

      English Arctic Expedition, 1875–76. Instructions for use of officers. Appendix B. Capt. Sir G. Nares’s report. True Auroræ seldom observed, and displays faint. Citron-line observed on only two displays. Appendix C.

      In anticipation of the starting of this Expedition, some instructions for the use of the officers in connexion with the hoped for display of brilliant Auroræ were prepared:—as to general features of the Auroræ, by Professor Stokes; as to Polarization, by Dr. William Spottiswoode; and as to Spectrum work, by Mr. Norman Lockyer and myself. As these instructions were somewhat elaborate, and will apply to all Auroral displays, I have supplied a copy of them in Appendix B. They were unfortunately not brought into requisition, for want of the Auroræ themselves. Capt. Sir George Nares has reported to the Admiralty, under date 5th December, 1877, as follows:—“Although the auroral glow was observed on several occasions between 25 October, 1875, and 26 February, 1876, true Auroræ were seldom observed; and the displays were so faint, and lasted so short a time, and the spectrum observations led to such poor results, that no special report has been considered necessary. Although the citron-line was observed occasionally, on only two displays of the Aurora was it well defined, and then for so short a time that no measure could be obtained.” (For Sir George Nares’s further Report see Appendix C, containing extracts from blue-book, ‘Results derived from the Arctic Expedition, 1875–6.’)

      Aurora Australis.

      Aurora Australis. Mr. Forster’s description. Long columns of white light spreading over the whole sky.

      In an article on Auroræ in high Southern latitudes (Phil. Trans. No. 461, and vol. liv. No. 53), we find that Mr. Forster, who as naturalist accompanied Capt. Cook on his second voyage round the world, says:—“On February 17th, 1773, in south latitude 58°, a beautiful phenomenon was observed during the preceding night, which appeared again this, and several following nights. It consisted of long columns of a clear white light shooting up from the horizon to the eastward almost to the zenith, and gradually spreading over the whole southern part of the sky. These columns were sometimes bent sideways at their upper extremities; and though in most respects similar to the northern lights of our hemisphere, yet differed from them in being always of a whitish colour, whereas ours assume various tints, especially those of a fiery and purple hue. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point.” This account agrees very closely in particulars with Capt. Maclear’s notice of Aurora Australis [after referred to], and especially in the marked absence of red Auroræ.

      The height of the barometer does not appear to be mentioned, the temperature being apparently much the same as in the more recent cases.

      Capt. Maclear’s Aurora Australis, 3rd March, 1874. Light of pale yellow tint only.

      In a letter dated from H.M.S. ‘Challenger,’ North Atlantic, April 10th, 1876, Capt. Maclear was good enough to communicate to me some particulars of an Aurora Australis seen 3rd March 1874, in lat. 54° S., long. 108° E. The letter is mainly descriptive of the spectrum (which will be described in connexion with the general question of the spectrum of the Aurora). It states that the red line was looked for in vain, and that the light appeared of a pale yellow, and had none of the rosy tint seen in the northern displays.

      Capt. Maclear’s Auroræ described in ‘Nature.’

      Capt. Maclear has since contributed to ‘Nature,’ of 1st November 1877, a description of four Auroræ seen from the ‘Challenger’ in high southern latitudes (including the one communicated to me). He speaks of the opportunity of observing as not frequent, either from the rarity of the phenomena, or because the dense masses of cloud prevalent in those regions prevented their being seen except when exceptionally bright. There were four appearances described:—

      Feb. 9, 1874.

      (1.) At 1.30 on the morning of February 9th, 1874, preceded by a watery sunset, lat. 57° S. and long. 75° E., bar. 29·0 in., ther. 35°; brilliant streaks to the westward. Day broke afterwards with high cirrus clouds and clear horizon.

      Feb. 21, 1874.

      (2.) At 9.30 P.M., February 21, 1874, lat. 64° S., long. 89° E., bar. 28·8 in., ther. 31°; one bright curved streamer. The