AUEOEA BOKEALIS. 225 



Summary of auroras observed at New Haven and Boston. 



These numbers exhibit an increase from 1742 to 17S6-'89, with certain ex- 

 ceptions which will be noticed hereafter. The middle of the period of maximum 

 abundance may be fixed at 1789, the average number of auroras for four years 

 amounting to 48. From this date the number declined rapidly, with slight 

 interruptions, to near 1820. The middle of the period of minimum frequency 

 may be assigned for 1816, when the average number of auroras did not exceed 

 one per year. From 1827 the numbers increase, and after 1837 the number is 

 very remarkable, the average for five years from 1837 to 1841 being 42 per 

 year. Then for three or four years there is a marked decline, and a subse- 

 quent revival, which is most decided in 1848 and 1852. Kegarding this as a 

 single period of maximum abundance, tjie middle of the period occurred not far 

 from 1845, making thus an interval of 58 years from the maximum in 1787 to 

 that of 1845. 



It is, then, established beyond question that during the past century the fre- 

 quency of auroras in New England has been subject to an inequality bearing 

 considerable resemblance to an astronomical periodicity, the period being about 

 58 years ; but to enable us to decide whether this period is ixniform would re- 

 quire observations continued for a much longer interval of time. 



For the purpose of making the comparison for a longer period, I have sought 

 for auroral observations from dift'erent parts of Europe. At first, I intended to 

 make the comparison with Bone's catalogue,* which gives a very extensive 

 collection of auroras from 500 B. C, to 1S5G, but after mature deliberation 

 decided to abandon it. This catalogue, although very extensive, is still quite 

 incomplete. Professor Wolf has given two supplementary cataloguest amount- 

 ing in the aggregate to about 1,000 auroras ; and I have, myself, found a number 

 of cases nut enumerated iu either catalogue. Moreover, by combining indis- 



* Sitzungsberichte der Akacl. der Wiss., Wicn, b. 22, pp. 1-74. 

 t Vierteljalirs Schrift der Nat. Gts. iu Zurich, 1857, pp. 83 and 401. 



15 s 



