AUEOEA BOREALIS.' 



243 



Column fiftli shows tlie years iu wliicli auroras exhibited a maximum or a 

 minimum frequency. 



Upon inspecting this table we perceive that the mean annual range of the 

 magnetic needle exhibits periodical variations, and these changes show a re- 

 markable regularity during the last twenty-iive years, embracing a period during 

 which the needle has been observed with much greater care than formerly. The 

 interval from one maximum to another is a little more than ten years. We 

 also perceive that the successive maxima are not equal to each other, but ex- 

 hibit variations which indicate a periodicity, showing a tendency to attain their 

 greatest value after an interval of about five periods, or from fifty to sixty years. 



