334 Mr. J. Eand Capron on the Auroral 



others have, with more precision, claimed it to be a favourable 

 specimen of " cosmic dust " display. The best known and 

 most zealous advocate of this " cosmic dust " theory, which 

 not only touches the character of our " beam " but that of 

 auroral displays in general, is Prof. Groneman, of the Ecole 

 M. de l'Etat at Groningen (Pays-Bas). 



In my i Auroras' (p. 64) I have briefly referred to the Pro- 

 fessor's theory, of which I had only then seen a condensed 

 account ; but subsequently he very kindly sent me a full print 

 of his " Theorie cosmique de Faurore polaire " (Estratto dalV 

 Appendice alle Memorie della Societci degli Spettroscopisti 

 Italiani, 1878, vol. vii.). In chapter ii. (" l'hypothese fonda- 

 mentale ") of this work, the theory is expounded in connexion 

 with Schiaparelli's discovery of "poudre cosmique" circula- 

 ting round the central star of our system in elliptic rings, 

 meeting the earth in its orbit, occasioning shooting-stars, 

 meteors, &c, and, amongst other phenomena, luminous appa- 

 ritions such as auroras. 



This paper is worthy of attentive reading and consideration; 

 for while in its twenty chapters main stress is laid upon the 

 general connexion of the "cosmic dust" with auroras, with 

 the zodiacal light, and with the " Gegenschein " or antisolar 

 light, the particular phenomena of auroras are fully and elabo- 

 rately discussed in detail. 



Prof. Groneman has followed up this paper by letters in 

 1 Nature/ vol. xxiii. p. 195, and vol. xxvii. p. 388, and by an 

 article in the same volume, p. 296. The article last referred 

 to is particularly valuable, not only as supplying us with the 

 observations at Utrecht and at Zonnemaire before cited, but 

 as giving a drawing of an auroral arch seen at Groningen, 

 Nov. 2, 1871, resembling the beam, as well as some accounts 

 of other similar phenomena seen during auroras, and some sta- 

 tistics as to the heights of auroras, including those of Heis and 

 Flogel, 10 to 100 geographical miles (46 to 461 English miles), 

 Tromholt, 17 geogr. miles, Galle, 40 to 60 geogr. miles, and 

 Prof. Groneman himself, 59 geogr. miles. He considers the 

 course of the beam to have been a great circle cutting the 

 horizon and also the equator in two opposite points. On the 

 cosmic theory we may at once remark that in some respects 

 it is quite possible that auroras and clouds of meteorites circu- 

 lating within our atmosphere may resemble one another ; but 

 I confess I am not prepared to declare in favour of the " theorie 

 cosmique" as a satisfactory explanation of auroral pheno- 

 mena. One adverse point urged by Mr. Backhouse (' Nature,' 

 p. 315) is the fact that meteors are mostly given to fly in all 

 directions, while auroral arches and beams (of which this one 



