224 



OH THE PERIODICITY OF THE AURORA BOREALIS. 



Wolf made an elaborate comparison, month by month and year by year, for the wbole 

 period between the years 1826 and 1848 inclusive, of the number of solar spots seen 

 on days when the aurora was observed as compared with the general average. If the 

 evidence was not decisive, it at any rate leaned to the opinion of Mairan tbat the 

 aurora was concomitant with the development of solar spots more frequently than oth- 

 erwise. He alludes emphatically to the spots and the aurora of the last of August and 

 the first of September, 1859. 



III. Pliny, after describing appearances in the heavens which evidently refer to the 

 aurora borealis, adds these words : " Atque hajc ego statis temporibus naturae, ut caetera 

 arbitror existere." Ideler 1 quotes this passage with the following comment ; •'• Novitne 

 Plinius statis period is auroram borealem apparere?" I have stated in an earlier me- 

 moir 2 the general argument for a secular periodicity in the recurrences of the aurora 

 borealis. An appearance which is now so familiar was so strange in 1719 that the 

 aurora of that year caused such terror throughout New England as to suspend " all 

 amusements, all business, and even sleep, there being a general expectation of the ap- 

 proach of the final judgment." 3 In the early part of the present century the inhabi- 

 tants of the Shetland Islands complained of the loss of this useful light which was so 

 common in the last quarter of the preceding century; and the necessity of substituting 

 artificial illumination for ordinary farming purposes. 4 The experiences and traditions 

 of the Icelanders are of the same kind. Henderson 5 said in 1815: "I had an opportu- 

 nity of contemplating them (auroras) almost every clear night during the whole win- 

 ter." Compare this with the accounts of Anderson for the period between 1730 and 

 1747, and of Horrebows in 1749 and 1750. 



Although there can be no doubt in regard to the fact of the secular periodicity of the 

 Aurora Borealis, there may be great difficulty in ascertaining, with even an approxima- 

 tion to certainty, the period of this secular change: partly, because the period is long, 

 and demands centuries of careful observations before it can be unmasked ; partly, 

 because the secular change is evidently entangled with other changes of smaller 

 periods which there may be no means of eliminating. I have drawn the secular curve 



1 Meteorologia Veterum Grrecorum et Romanornm, p. 52. a Mem. of the American Academy of Arts and Sciences, IX. 101. 

 8 Regent's Reports of the Senate of Xew York, for 1836, p. 228. 'Edinburgh New Phil. Journ. XVI. 35. 

 6 Iceland or the Journal of a Residence in that Island, I. 357, and Nouvelle Description de l'Islande par Horrebows, trad, de 

 l'Allenaand. 



