14 ON THE PERIODICITY OF THE AURORA BOREALIS. 



it is impossible to confound with the irregular oscillations by which it is sometimes 

 affected, and in the recurrence of which Sabine has discovered a striking periodicity. 

 These strong and irregular disturbances are repeated over the whole line of Magnetic 

 Observatories from St. Petersburg to Nertchinsk, so that a table, derived from the 

 registers of St. Petersburg, is likewise adapted to the other observatories. In many 

 localities the heavens may be shut in by clouds and the auroras not visible, when their 

 presence is suspected from the oscillations of the needle. 



Idelcr, in his " Meteorologia Veterum Gfrcecorum ct Romanorum, p. 49," devotes a few 

 pages to the investigation of "Aurora? borealis apud veteres vestigia." Ideler makes 

 the following criticism upon the supposed allusions of Aristotle to the aurora : 



•• Aristoteles describit phenomena, qua? xaafiara, (ioQwovc, nai aiuarudt) xpupaTa voeat. Quale intelligenduni sit pheno- 

 menon, re vera dictu est difficillirnurn. Itaque, priusquam do ipso agimus, quantum fieri potest accuratam descriptionem 

 ex ipsius verbis eruere conemur. Phenomena nou nisi node serena eerni, incensionem flarmna? ardori similem esse, com- 

 paravi posse splendorem cum flamma per fumum visa, dieit ; denique non longo temporis spatio in eoelo remanere. Colo- 

 res gigni refractione luminis in erassiore aijre eenset, exemplum aflerens rubicunduni stellaram orientium et occidcntium 

 colorem, potissimum vero cminere puniceum et pm-pureum, quia crcruleus cum atro confundatur et vix discerni queat. 

 Multi de aliis rebus eogitaverunt, nos nou dubitamus quia auroras boreales intellexcrit Aristoteles (quarum fortasse nul- 

 lam suis ipse oculis eonspexit) imprimis verbis eommoti : Omnino in teneoris albus color tot prabel variolates, ut ftammam per 

 fumum adspicere tibi videaris: quie ad borealis aurora? speciem proxime aecedunt. Diffieultates tamen remanent plures; 

 prior, cur Aristoteles non dixerit, ejus modi phenomena in boreali tantum plaga oriri, immo in libro de mundo (si omnino 

 illius auetor est, qua iu re video dissentire grammaticos) dieat id rarissime tantum versus meridiem et septemtriones oriri ; 

 altera deinde iu verbis : Interdiu Sol cohibet, noctu vero, excepto una colore puniceo, reliqui propter eundem fere cceli colo- 

 rem non apparent, cum tamen boreales aurora? omnes prismaticos colores prrcbeant." ' 



Ideler quotes the following lines from Lucan's Pharsalia, I. 526 : — 



" Ignota obscura? viderunt sidera noctes 

 Ardentemque polum flammis, cseloque volantes 

 Obliquas per inane faces, crinenque tremendi 

 Sideris, et terris mutantem regna cometen. 



He also adduces the following passage from Tacitus : 2 — 



" Sonum insuper audiri, formas deorum et radios capitis adspiei persuasio adjicit." 



The lines quoted in the Annals of Philosophy, IX. 250, do not appear to me to refer 

 to the aurora. 



Armorum sonitum toto Germania caelo 

 Audiit. 3 



1 See Aristotle's Meteorol., Lib. I. Cap. 5. Also, Ideler's Aristotelis Meteorologicorum Libri, I. 374 and II. 330. 

 8 Germania, Cap. XLV. s Virgil, Georgic. I. 474. 



