May si, 1883] 



NA TURE 



107 



power, for our means are not sufficient to throw projectiles 

 with several thousand metres velocity ; and it is very re- 

 markable that this experiment comes at the same time as 

 the interesting experiment of Prof. Lemstrdm, showing 

 that electric currents are able to give a development 

 of light in our atmosphere, possessing the same number 

 of undulations in a second as the auroral light. Now our 

 meteoroid being a part of an aurora, it gives a stronger 

 proof of the origin of that phenomenon than Prof. Lem- 

 Strom's experiment, the greatest attraction of which is that 

 we are able to repeat it arbitrarily and with our own means. 

 Further, I have always maintained that electricity, excited 

 easily by friction, must be one of the causes of the auroral 

 light (" Thcorie Cosmique de l'Aurore Polaire," Journal 

 des Spectroscopistes Italiens, 1S7S, vol. vii. chap, ii.), 

 and it seems to me very plausible that cosmic matter, 

 approaching the earth, induces electric currents through 

 the air. Therefore I think that the results of Prof. Lem- 

 strom are in full harmony with the idea of a cosmic origin 

 of aurorae. 



The orbit found does not reach the surface of the earth, 

 being at its nearest approach still I23"9 kilometres (1 mile 

 = 1609/3 metres ; 1 German geogr. mile = 7420'4 metres) or 

 167 geogr mi'es from that surface. The length of the orbit 

 from the Utrecht perpendicular line to the Utrecht hori- 

 zon is 1,483,070 metres, and this line being run over in 60 

 seconds, 1 the mean relative velocity was 24.673 metres, 

 I5'3 miles, or more than 3 German geogr. miles. 



The dimcn-Uons of the "cosmic cloud " (length 40', 

 width 5 , as seen from Ipswich) are: length = 182,594, 

 width =21921 metres. By these dimensions, probably 

 too great from irradiation, it must show at Utrecht an 

 apparent length of 50 ; but the extremities were tapered 

 and therefore the length strongly influenced bj the trans- 

 parency of the air. It is therefore not strange that the 

 apparent length at Utrecht was during some few seconds 

 90 degrees. 



To conclude, I will remark that the proved existence 

 of a cosmic cloud, preserving its pretty sharp sides during 

 so long a path as that from Sweden to the Atlantic Ocean, 

 notwithstanding its velocity of 247 kilometres, proves its 

 particles to be nearly spherical. Otherwise these particles 

 should necessarily have diverged sideways from the orbit 

 and spread into space. In connection with the fact ob- 

 served by Mr. P. Zeeman (p. 297), that auroral clouds 

 gave interference-phenomena, when coming before the 

 moon's disk, and these latter phenomena requiring (Dagnin, 

 " Traitd de Phys.," iv. p. 446) the presence of nearly 

 equal particles of dusty matter, Mr. Zeeman's observation 

 proves the same property in the particles of the auroral 

 cloud. Being nearly equal, but not perfectly, the tan- 

 gential atmospheric resistance must throw the smallest 

 particles backward, and this explains the oblong shape of 

 the cloud. 



In presenting my results and reasonings to the readers 

 of this journal, I hope that they will remember that this 

 paper has been written in a shorter time than the author 

 had wished. H. J. H. Groneman 



Groningen (Netherlands), April 7 



THE AURORA BOREALIS 3 

 II. 

 "T^HE Aurora Borealis at Sodankyla. — Although the 

 ■*■ aurora borealis often appeared with considerable 

 intensity, it did not boast many varieties. It began gene- 

 rally with a faint arc in the northern sky, which soon de- 

 veloped into a sharp arc, with streamers and a kind of 

 luminous " drapery " spreading from east to wist. The 

 colour of this luminous drapery was not very changeable, 

 so that the spectroscope only returned the usual yellow- 



1 This number is stated also by the sharp determination given by the 

 Astronomer Royal, Prof Christie. 

 3 Continued from p. 63. 



green line. Generally it was of a yellow-whitish colour, 

 with a slight shade of green. 



There was, however, an observation made of far greater 

 interest, viz. that the spectroscopic"reaction," 1 z>. X = 5569, 

 on several occasions was returned from every quarter of the 

 horizontal plane, even from the zenith, without any aurora 

 being visible. As this reaction was obtained while the 

 ground was still bare, there can be no question of its being 

 a reflection, but that this place was at the moment within 

 the sphere of an auroral discharge, but of such a weak 

 character that it did not appear in the form of aurora 

 borealis. This observation was therefore precisely similar 

 to the one made in 1871 in Lapland, described above. 



These observations were chiefly made by my assistant, 

 Herr Biese, who made another remarkable discovery. 

 Nearly due south-east from the Observatory, he received 

 on several occasions a spectral reaction from a narrow 

 belt of the sk> , although no aurora was visible, This ob- 

 servation, which was very difficult to effect, as the eye 

 had to be kept entirely away from all light for fully five 

 minutes before the reaction could be traced, I had myself 

 several opportunities of corroborating. In this direction 

 were situated some mountains 300 metres high, about 30 

 kilometres distant, and in my opinion the reaction was 

 due to t 1 e above-described phosphorescent flames, which- 

 were seen around the mountain-tops in Lapland and 

 Spitzbergen. All observations were of course made after 

 every trace of daylight had disappeared. 



The Phosphorescent Luminosity. — On several occasions 

 the attention of travellers in the Arctic regions has been at- 

 tracted to a peculiar soft light or " shine " during the 

 night. But, as the change from day tonight is very gradual 

 in the Polar regions, as compared with that of southern 

 climes, a certain amount of exertion of the mind is re- 

 quired in order to take cognisance and retain the fea- 

 tures of this phenomenon. As, however, attention has 

 been once drawn to the same, it will always be observed. 

 Already in ( ctober I noticed it at Sodank>l;i, and directed 

 the attention of my assistants to it. 1 give subjoined 

 some extracts from my diary concerning this pheno- 

 menon : — 



December 9, 1882. — The Polar night shows sometimes a 

 peculiar phosphorescent " shine " or diffused luminosity, 

 which possesses several phases, but the general character 

 of which is a luminosity of a yellow-white colour, which 

 renders the night as light as' the moon with a thick hazy 

 air. I take here the appearance and disappearance of 

 the light on two nights when its intensity was greatest. 



On December 6 1 was on a journey between C rajarvi 

 and Sodankyla. The phenomenon became then apparent 

 at 7 o'clock p.m. When daylight had completely disap- 

 peared, there seemed to remain a faint light in which the 

 outlines of objects around could only with difficulty be 

 discerned. At 7.40 this increased, however, so rapidly that 

 in a few moments every object around stood out clearly 

 in a yellow-white hazy phosphorescent luminosity of 

 quickly-shifting intensity. I had unfortunately no photo- 

 meter by me by which I could determine the same. It 

 lasted in this form until about 10 o'clock. 



December 8, at 5 p.m., I walked from the Observatory 

 to the church near it, in order to observe from its 

 steeple some fire-signals from Oratunturi. On the way, I 

 noticed that a yellow-white luminosity of shifting intensity 

 filled the entire horizon, while twenty minutes after it had 

 increased greatly in intensity, and was now strongest 

 in the north, whence it gradually faded to the south, 

 where it had least intensity. Near the horizon it was 

 difficult to discern the stars. Higher up it was, however, 

 easier, and from 6o c to the zenith the sky was clear, of a 

 mauve colour. It was exceedingly interesting to compare 

 the light with the Milky Way. The yellow-white light 



' [By this term Prof. Lemstrcim of course refers to the characteristic tint 

 in the spectrum of the aurora. The term might be justified by analogy with 

 the '" reactions" characteristic of the presence of the various chemical 

 elements. — Ed. 



