ON THE PERIODICITY OF THE AURORA BOREALIS. 



327 



In Tables LXXVI. and LXXYII. the number placed against each year of spot- 

 maximum or minimum represents the number of auroras observed in that year, added 

 to the numbers observed in the preceding and following years. In this more general 

 mode of comparison the coincidences are more numerous, and the discrepancies are 

 not so glaring, between the fluctuations of the solar spots and of the aurora, as when 

 single years are used instead of small groups. But it seems to me doubtful whether, 

 in any case, the agreement is sufficiently exact and frequent to justify the conclusion 

 that the two classes of phenomena are associated, either as cause and effect, or as 

 common effects of some remote and unknown cause. On the supposition of such an 

 affiliation as Wolf advocates, it is wonderful that the long secular period in which the 

 number of auroras slowly oscillates between such wide extremes of maxima and 

 minima should be inconspicuous in the increments and decrements of the solar spots. 

 In 1787, and again about 1849, when the aurora-curve towers to unprecedented 

 heights, the solar spots scarcely respond to the summons which theory serves upon 

 them to do likewise. During the aurora-minimum of 1758, and throughout the dismal 

 blank which precedes and follows the aurora-minimum of 1812, the number of solar 

 spots, though somewhat diminished, indicates nearly the usual amount of activity on 

 the sun's surface. 



LXXVI. NUMBER OF AURORAS FOR EACH THREE YEARS WHICH CENTRE ABOUT A MAXIMUM OR 



MINIMUM OF SPOT-FREQUENCY. 



