ON THE PERIODICITY OF THE AURORA BOREALIS. 229 



tion itself, we must wait for centuries of additional observations, unless some happy 

 generalization, as to the physical cause of auroral displays, shall illuminate and render 

 transparent the obscure and scanty records of the past 



When Mairan published the first edition of his Traite de FAurore Boreale, in 1733, he 

 pointed out what he supposed to be twenty-two returns (reprises) of the aurora. But 

 his deductions were based on only 229 auroras, all but 59 observed since 1707. Twenty 

 of his great returns were therefore predicated upon only 59 recorded appearances, and 

 were the results of negative rather than of positive evidence. Indeed, in his catalogue, 

 one observation or a small cluster of observations stamped a year as one of maximum 

 frequency, the intermediate years being unrepresented in any record. The intervals 

 between these returns varied from ten to two hundred years. Mairan 1 made no 

 attempt at discovering a specific interval between the recurring maxima. He says: — 



" Depuis le milieu du XII me siecle jusqu'au milieu du XLY 1 ™, je ne trouve rien qui puisse etre pris certainement pour 



l'Aurore Boreale Ce qui donneroit, depuis 1157, une interruption au Phenomene de pres de 200 ans 



s'il n'etoit a craindre que ce ne soient seulement les Observateurs et les Historiens qui nous manquent" 



And again (page 185), Mairan says : — 



" Depuis 1621 jusqu'en 1686, c'est-a-dire, dans l'intervalle de plus de 60 anne'es, je ne trouve aucune observation bien 

 marquee de l'Aurore Boreale, et Ton sait cependant quels Astronomes et quels Observateurs il y a eu pendant ces temps- 

 la. De sorte que Ton peut compter cette interruption du Phe'nomene comme une des plus longues, entre celles qui sont 

 le mieux constatees." 



In the second edition of Mairan's work, published in 1754, are added 1441 auroras, of 

 which 302 were prior to 1707. Moreover, between the years 1157 and 1351, where at 

 first he supposed there was a dismal blank, he has collected from other sources four 

 examples, and between the years 1621 and 16S6, where he thought there was a per- 

 fect vacancy, he afterwards placed 81 auroras. Indeed, between the years 1622 and 

 1634 inclusive there are 49 auroras, and the single year 1629 is a conspicuous 

 maximum. If Mairan had revised his reprises, in view of the much larger materials he 

 had in his hands when publishing his second edition, he would no doubt have modified 

 the epochs to suit his enlarged knowledge; but he had no special occasion to reconsider 

 that part of his general subject. 



Our distinguished foreign associate Hansteen, after noticing the frequency of the 



' Traite' Physique et Historique de rAurore Boreale, p. 179-83. 



