ON THE PERIODICITY OF THE AURORA BOREA1IS. 239 



The exclusion of the observations gathered in Arctic expeditions makes the anomalous 

 maximum of the year 1821 disappear from this curve. He then draws another curve 

 from the Swiss observations alone, which he thinks conforms still more exactly to the 

 secular curve of the solar spots. He quotes the Swiss Chronicles of Vogel to the effect 

 that in the year 677 the aurora was seen in that country for ten successive nights : so 

 extraordinary an event for that locality that Wolf thinks there can be no doubt that 

 that year was one of the grand maxima. Also, between the years 1560 and 1583 the 

 aurora was seen very frequently. The whole interval between the years 1788 and 

 1568 contains four periods of fifty-five years and twenty periods of eleven years each. 

 The whole interval between the years 1568 and 677 contains sixteen intervals of 

 55.69 years, and eighty intervals of 11.14 years each. The long interval between 

 1788 and 677 contains twenty intervals of 55.55 years, and one hundred intervals 

 of 11.11 years. On the other hand, Olmsted's period of 65 years would introduce 

 a minimum instead of a maximum into the year 1568. As Wolf's period is about 

 six-sevenths of Olmsted's period, the two will concur in making the same year a maxi- 

 mum once in about three hundred and ninety years; for example, in the years 1788 

 and 1398. Fritz 1 argues for another grand maximum in the year 1505 or 1516 : but 

 the evidence is far from being conclusive. 



After Fritz 2 had extended his catalogue of auroras to eighty-four hundred he arranged 

 them in three separate groups according to the geographical position of the observer, 

 and then compared the number for each year between 1824 and 1861 with Wolf's rela- 

 tive numbers of solar spots. Though a general parallelism between the two phenomena 

 prevailed, the alternations in the aurora lagged behind the corresponding ones in the solar 

 spots. As soon as Fritz's 3 catalogue of auroras had reached ninety-five hundred, embrac- 

 ing 40000 observations, he again made a table of comparisons between the number of 

 auroras in each year and the relative number of solar spots, extending the comparison 

 to the interval between the years 1700 and 1861 : and making it, not only for the 

 observations in the aggregate, but also separately for the auroras observed in distinct 

 zones of latitude. He finally settles down upon the years 1730, 1788, and 1848, as 

 those of the grand maxima, and the years 1700, 1758 or 1766, and 1811 or 1812, as 

 the years of the grand minima. The secondary maxima and minima which he de- 

 duces from those series of observations which he considers most complete are ex- 

 hibited in Table LVL. 



1 Vierteljahrsschrift der Naturforsch. Gessellsch. in Zurich. X. 162. 



2 Vierteljahrsschrift der Naturforsch. Gessellsch. in Zurich. IX. 122 



3 Vierteljahrsschrift der Naturforsch. Gessellsch. in Zurich. X. 232-5. 



