The Aueora Boeealis. Ixxxiii 



third quarter, when the moon does not rise till midnight, it is also evident that the number of faint aurorae 

 rendered invisible must be very small. From the beginning of the fourth quarter, therefore, till conjunction, 

 the numbers seen will obey nearly the true law of frequency ; and as the visible maximum occurred before the 

 end of the third quarter, the true maximum must have occurred even nearer to opposition. On the whole, it 

 appears very certain, that the hypothesis of an actual maximum of frequency at opposition and minimum 

 at conjunction, is satisfied by the previous numbers of auroree, seen under the conditions of the varying duration 

 of moonlight for the hours of maximum frequency. This hypothesis is in unison with the law of magnetic 

 disturbance, which is a maximum at opposition, and a minimum at conjunction. 



Note on the Theory op the Aurora. 



175. Although temptations to frame hypotheses have been avoided hitherto, I cannot refrain from repeating 

 here, the opinion, that the phenomena of the aurora borealis are chiefly optical.- After watching the various 

 phases of the aurora for some years, the hypothesis of self-luminous beams and arches appeared to me unsatis- 

 factory, and the strongest argument in its favour, that obtained from the computed height of the auroral arches, 

 seemed of a very doubtful character. I was quite prepared, therefore, to adopt the idea, first I believe pro- 

 posed by M. Morlet to the French Academy, in May 1847, that the auroral arch is an optical phenomenon of 

 position. M. Morlet has pointed out that the arch appears generally as a segment of a circle, whereas, in these 

 latitudes, it ought invariably to appear as the segment of an ellipse, if the hypothesis be true, of a real lumi- 

 nous ring, with its centre on the continuation of the magnetic pole. He has also, among many other very 

 obvious objections to that hypothesis, shewn that the summit of the arch is generally in the magnetic meridian 

 of the place, the plane of which rarely passes through the magnetic pole, and seldom passes through the same 

 point, for three different places. I have, however, felt even more persuaded, that the aurora is, partly at least, 

 an optical phenomenon, from a consideration of that phase of the aurora constituting the corona borealis, a 

 persuasion that I stated, in the Literary Gazette of the time, in giving an account of the beautiful corona of 

 October 24, 1847. Mairan and, more lately, Dalton, have explained this phase of the aurora by a hypothesis of 

 polar beams, long fiery rods of solar atmosphere, according to the one, of red-hot ferruginous particles accord- 

 ing to the other, seen in perspective, as they lie in the direction of the magnetic force. A little acquaintance 

 with the phenomenon — the rushing and tilting of the beams against each other, one beam occasionally rising 

 from the horizon, passing through the centre of the crown and beyond it — would shew the improbability of this 

 hypothesis. I am persuaded, that the phenomenon of the corona borealis is produced in a narrow horizontal 

 stratum of the earth's atmosphere. Thanks to the discoveries of Dr Faraday, we do not now require a ferruginous 

 sea, in order to have polarized particles ; the watery crystals that inhabit the upper regions of the atmosphere 

 can themselves assume a polar state, determined by the passage of electric currents ; and we have only to com- 

 plete this fact by a hypothesis of luminous electric discharges seen refracted by these crystals, the position of 

 visibility of the refracted rays depending on the angles of the crystals, and the deflections from the direction of 

 magnetic force, which they suffer by the electric currents. Such a hypothesis, which occurs at once when an 

 optical phenomenon has to be accounted for, would explain these remarkable auroral clouds, so often seen in 

 connection with the aurora itself ; it would also serve to explain the appearance of the arch at certain alti- 

 tudes, lower for lower altitudes, determined by the position of the source of light, direction of the magnetic force 

 at the place, and the effect of the electric current in deflecting the crystals. The crystals successively deflected 

 by electric currents, would also exhibit the rushing pencils or beams. It need scarcely be remarked that dif- 

 ferently formed crystals might give rise to different phases of the phenomenon, while reflection might be com- 

 bined with refraction in certain cases, especially in the case of arches seen south of the anti-dip. Such a hypo- 

 thesis evidently assumes a source of light, independent of these optical resultants, and the pulsations seen in 

 many auroras may be real luminosities. It is hazardous, in the present ill-arranged state of auroral observa- 

 tion, to ofifer so rude a sketch of a new hypothesis, although we may suffer a considerable defeat in very good 

 company. 



Since the previous note was written, I find that M. Morlet has published a theory of the auroral arch 

 (Ann. de Ch., t. xxvii., 3me Serie). The ideas above were stated by me two years ago, to different persons. 



MAG. AND MET. OBS. 1845 AND 1846. 



