ON THE PERIODICITY OF THE AURORA BOREALIS. 323 



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Germany seventeen auroras were seen in 1750, but not so many as in 1748, 1749, and 

 1752. In Holland there is a maximum in 1750, although two more auroras were seen 

 in 1747 than in 1750. In Zwanenburg there is a strong maximum in 1750. At Upsal 

 twenty-seven auroras were seen in 1750, twenty-two in 1751, but fifty in 1746, only 

 one year after the last spot-minimum. At St. Petersburg, the five years which include 

 the maximum do not supply as many auroras as the five years which include the pre- 

 ceding minimum. At Abo there is a maximum, but larger in 1752. At Cambridge 

 there is a conspicuous maximum. 10. Next comes the spot-minimum of 1755. In 

 Holland twenty-two auroras were seen in 1755, 1756, and 1758, forty-two in 1754, and 

 only sixteen in 1753. In Germany none were seen either in 1755 or at the next 

 spot-maximum in 1761, and indeed only two between 1753 and 1768. At Zwanenburg 

 there is a satisfactory minimum. At Upsal a small number were seen in 1755, but 

 none in 1757 and 1758, probably on account of a break in the continuity of the series. 

 At St. Petersburg only four auroras were seen in the six years from 1753 to 1758 

 inclusive. But this may have been the effect of the secular minimum which came 

 about this time. At Abo there is a satisfactory minimum in 1756. The Cambridge 

 observations accord well with Wolf's theory, no auroras being seen in 1754, 1755, and 

 1756. 11. In the year 1761 is another spot-maximum. Few auroras were observed 

 about this time, except in high latitudes. In Zwanenburg eight auroras were seen in 

 1759, and there the series ends. In Upsal forty-eight auroras were seen in 1761, but 

 rather more in 1760 and 1759. In 1762 a clear maximum is made out for St. Peters- 

 burg. At Abo it comes exactly upon 1761. At Cambridge the year 1761 is midway 

 between two years when no auroras were seen, but itself is distinguished by fewer 

 auroras than the eight years which precede and follow it. 12. In the year 1766 is a 

 spot-minimum. In the same year was a decided aurora-minimum at St. Petersburg. It 

 came a year later at Abo. In other parts of Europe auroras were very scarce, not 

 only this year, but for the ten preceding years. 13. The next spot-maximum is in 

 the year 1769. In Germany the aurora rose suddenly to a maximum in 1770. In 

 Berlin and St. Petersburg it was later by one year. At Abo; it came in 1769. At 

 Cambridge and New Haven it coincided with the spot-maximum. 14. In the year 

 1775 is another spot-minimum. There was an apparent minimum of the aurora in 

 France, although as many auroras were seen that year as in 1769 and 1771, and twice 

 as many as in 1770. In Boningen, Jena, Berlin, and Germany in general, there is a 

 satisfactory minimum. In St. Petersburg there is a minimum in 1776, but a strong 

 maximum in 1774. At Abo the number of auroras in 1775 and 1776 was less than in 

 the four preceding years, but in 1774 more auroras were observed than in any pre. 



