ON THE PERIODICITY OF THE AURORA BOREALIS. 225 



for each place or each combination of places at which sufficiently long series of ob- 

 • serrations have been made ; and, also, the mean curve for all the observations united. 

 From this mean curve it has been necessary to exclude the occasional observations made 

 in polar voyages, and in very high latitudes. The almost daily frequency of the aurora 

 at such places would unduly inflate the curve for special epochs of observation unless 

 balanced by continuous observation at the same places during the time of at least one 

 secular period. For example : the series of observations on the aurora made at Moose 

 Factory, Cumberland House, Athabasca Lake, Fort Eeliance on Great Slave Lake, Fort 

 Enterprise, Fort Confidence, Point Barrow, etc., which furnish from 100 to 150 auroras 

 a year, are necessarily excluded, as the observations continued only two years or less. 

 The unusual activity displayed by meteorologists during the few years that the Me- 

 teorological Society of the Palatinate was in operation put on record a large number 

 of auroras which would otherwise have escaped, and which give an undue preponder- 

 ance to that brief period. 



Mairan, in his unrivalled discussion of the periodicity of the aurora, has exercised a 

 wide discretion and exhibited the best judgment in the selection of his materials, in- 

 stead of making an indiscriminate use of all : — 



" Par exemple, tout pays oil l'arriere - saison et 1'Hiver ne presentent aux yeux de l'Observateur, pendant des mois 

 entiers, qu'un Ciel toujours couvert, qu'un temps pluvieux ou charge" de brouillard, doit etre exclus de nos Comparai- 

 sons et de nos calculs. Car de semblables pays ne pourroient nous fournir dans cette partie de l'annee que quelques 

 grandes Aurores Bore'ales, et il s'agit ici du nombre plutot que de la grandeur. Eh quel usage faire en ce cas d'une 

 Comparaison oil l'un des termes seroit si defectueux, et manqueroit quelquefois totalement ? Par une semblable raison 

 je rejetterai les observations faites dans des pays trop Polaires et oil le Soleil seroit des jours et des mois entiers sur 

 l'horizon, dans la saison opposee a la precedente ; car il n'y auroit encore alors que les tres-grandes Aurores Bore'ales 

 qui pussent s'y montrer, tandis qu'on en auroit observe un tres- grand nombre d'autres qui auroient paru en 

 Hiver." « 



For these reasons Mairan omits the observations made by Croyere in the North of 

 Russia, at Kola, at Kilduin, and on the shores of the frozen sea, two or three degrees 

 beyond the Polar Circle. 



Satisfactory observations on the aurora have been made only during the last one 

 hundred and fifty years. And even during this choicest period, the number of constant 

 and systematic observers in the field, as well as their station of observation, has varied. 

 Sometimes there have been none ; at other times not less than a dozen. These fluct- 

 uations would have affected the mean curve more injuriously were it not for the fact 

 that the zeal of observers died out when the auroras died out, and was rekindled again 



1 Traite Physique et Historique de rAurore Bore'ale, pp. 488 and 509. 

 VOL. X. 29 



