Oct. 3, 1878] 



NATURE 



607 



While in low latitudes the arch is one of the most frequent of 

 auroral forms, in the polar regions this post is occupied by the 

 ribbon or streamer {Band), which resembles a torn and irregular 

 arch, and may take any position or direction, one or both the 

 ends being usually visible above the horizon. It takes the most 

 varied and ever-changing forms, forcibly suggesting a pennon 

 floating on the breeze, doubling and curling on itself in graceful 

 and translucent folds, or again appearing as if torn into shreds 

 blown by the wind. Weyprecht is inclined to believe that the 

 wind actually has some influence on the northern lights, and notes 

 that, after severe and widely-extended storms, the streamers are 

 especially ragged and broken. While in the arch the light intensity 

 ■changes but slowly, and simultaneously over the whole bow, 

 the reverse is the rule with streamers, in which it varies irre- 

 gularly, lightening in one place and fading in another. The 

 rapidity of these changes is very varied. Sometimes a band 

 will drift slowly across the northern or southern sky, without 

 change of form or colour, for a long time together, and then 

 suddenly flame up with rapid movement across the zenith to 

 form a crown, to renew its play, or to vanish on the other side. 

 Frequently pieces break off and form new bands, which spread 

 over the whole heavens, and then again fade dovfn to a single 

 band of a new form, or perhaps carry on their game until the 

 dawn drives them firom the sky. In most cases the light of the 

 streamers has a peculiar motion, resembling waves which roll 

 ■continuously from one end of the band to the other. These 

 waves are more intense and move more rapidly in proportion 

 to the activity of the bands themselves. The streamers vary 

 in their appearance, sometimes seeming to consist of a uniform 

 light-material like most of the arches, at others of closely- 

 packed perpendicular rays, with the intervals filled with the 

 same light-material. Between these two extremes are all 

 intermediate gradations. When the rays are visible each 

 brightens as the wave passes over it, but does not change its 

 position except by a slight lengthening, which gives the hopping 

 or dancing motion which, no doubt, is the origin of the term 

 *' merry dancers." When, on the other hand, the streamers 

 consist of simple "light -material," the waves cause a brightening 

 and slight undidatory motion of the edge. 



Admirable illustrations of these streamers, as they are occa- 

 sionally seen in lower latitudes, may be found in Prof. Piazzi 

 Smyth's Edinburgh astronomical observations, under the name 

 of "multiple arcs," but they are already approximating to 

 regular arches, from which they probably differ only in distance 

 and altitude. This work also contains most valuable plates and 

 observations of the auroral spectrum, as compared with that of 

 twilight and the zodiacal light. The threads (Fddeit, rayons) 

 are fine rays of light directed from the magnetic zenith to the 

 horizon, but not quite reaching either. They occur sometimes 

 singly or in bundles, sometimes pretty uniformly distributed, 

 and are very variable in length. Their breadth rarely exceeds 

 one minute of arc, and they are separated from each other 

 by dark spaces. Their motion is peculiar, and seldom 

 rapid. They lengthen and shorten upwards or downwards, 

 giving the impression that already existing threads are lighted 

 up or fade. They also move slowly from west to east or east 

 to west, and not unfrequently it seems rather that the light is 

 transferred gradually from one thread to another than that the 

 threads themselves actually move. 



The threads are evidently in intimate connection with the 

 streamers. Often they stretch from near the magnetic zenith 

 like a fan or a veil of gold or silver threads, of which the 

 streamer forms the broad lower border ; singularly, however, 

 they rarely actually reach it, but seem to fade away near 

 its edge, only to reappear with greater brightness in the streamer 

 itself. This phenomenon is seen in the most beautiful way 

 where two bands appear at once, one over the other, and each 

 mth its proper fringe of threads, like a silvery veil, falling in the 

 most graceful folds. It is, however,^ only occasionally that 

 threads and streamer occur in combination, and in the feebler 

 displays it is more usual to see fans of threads, or streamers 

 alone than both at once. 



The crown, as is well known, is the perspective effect pro- 

 duced round the magnetic zenith, towards which the auroral 

 beams, following the lines of magnetic force, seem to convei^e. 

 The intensity of the appearance is very variable, and at times 

 becomes such that thousands of short broad flashes dart at once 

 from or towards the centre, while the whole firmament is covered 

 with widespread rays, which lengthen and shorten with a 

 flickering motion. Broad bright flames leap about the centre as 



if driven by a fiery whirlwind, and all the heavens seem in 

 flames. In general the more brilliant the appearance, the 

 shorter is its duration, and though it sometimes happens that 

 masses of light move for hours about the centre, yet in such 

 cases the brilliancy and the motion are alike feeble. 



As a rule a crown is formed whenever an aurora of pro- 

 nounced form passes the magnetic zenith, i.e., when the 

 beams of which it is composed are parallel with the observer's 

 line of sight. Faint aurorae, however, especially arches with 

 slow motion and no rays, may pass the zenith without forming a 

 crown, and, on the other hand, a feeble crown sometimes 

 becomes visible without the passage of any noticeable band or 

 arch. When, as sometimes happens, the streamer is formed of 

 uniform " light -material " without distinct rays, rays are also 

 absent from the crown, which consists merely of lambent flames 

 flickering round the centre, and resembling those of alcohol 

 burning on a flat surface. 



The haze {Dunst), as its name implies, resembles a faint mist, 

 from which, by moonlight, it is scarcely distinguishable, as its 

 light is never intense. 



With regard to colour, Weyprecht observed that in bands the 

 red always formed the lower and green the upper part, the 

 middle being whitish. The haze was frequently reddish, but 

 dull greenish seems to have been the prevailing colour, which is 

 compared to that of the electric spark. Most unfortunately the 

 expedition was unprovided with any spectroscope suitable for 

 such observations, so that on this most important point we have 

 no information to record. 



The intensity of the light was such that the smallest type was 

 frequently legible, and larger could be read easily. Weyprecht 

 proposes to measure the intensity of the avurora by observation of 

 the legibility of print of different sizes. 



With regard to the height of the northern lights above the 

 earth's surface, Weyprecht is strongly of the opinion that they 

 are much lower in the arctic regions than in lower latitudes, but 

 was not able to make any direct measurements. Then- brilliancy 

 and distinctness, and, above all, their rapid movements, give 

 the impression of nearness ; and the observations of Parry, Far- 

 quharson, and others, lead to the same conclusion. On one or 

 two occasions the avurora appeared to be below the light cirrus 

 clouds, which do not attain a maximum height of more than 

 8,000 metres ; but the observation was by no means certain. 



Aurorae were also repeatedly visible when the sun was so little 

 below the horizon that the height of the direct sun-rays in the 

 zenith was not more than from six to twelve miles. If, as has 

 been suspected, the auroral light depends on some form of 

 mist for its basis, this would have been rendered visible 

 had the height been such as to bring it into the sunlight ; but if, 

 as is generally supposed, it depends on the electrification of rare 

 and transparent gases, this would not be the case ; so that it 

 seems difficult to draw any conclusion from these facts. Ad- 

 mitting, however, the conclusion, which i> Jn itself probable 

 enough, that in high latitudes the appearances are lower than 

 with us, it will go far to explain many of the differences which 

 may be noted between Weyprecht's description and what we are 

 accustomed to see. Streamers such as he describes would, if far 

 enough off", and owing their altitude to great elevation, appear 

 like arches, as the lower edge is at an almost uniform height, 

 the windings would disappear with distance, and we should have 

 the appearance of a pretty regular arch of irregular brightness 

 and with beams shooting from it towards the zenith, while the 

 individual threads would be, as is indeed the case, rarely or 

 never visible. 



Weyprecht repeatedly observed clouds and mist which took 

 the same forms as aurora, and strongly resembled it ; but jhe 

 does not think a case is made out for any real connection 

 between the two phenomena. The stars, however, are decidedly 

 obscured by an intense aurora, and many observations seem to 

 point to some such connection. Several very interesting ones are 

 quoted by the present writer in the article " Aurora" in the last 

 edition of the Encyc. Brit. Weyprecht frequently uses the 

 phrase "light-material" in speaking of the aurora, and it is 

 evident that there must be some material basis to the phenomenon. 

 This, he suggests, may be the minute ice crystals, which are the 

 cause of mock suns and moons, appearances of daily occurrence 

 in the polar regions. It seems certainly possible that these may 

 be projected far above the earth, or even above the atmosphere 

 by electrical repulsion, and may serve as carriers of electrical dis- 

 charges, which would at once illuminate the particles and arrange 

 them in the lines of magnetic force. If the aurora is really, as 



