250 TENTH ANNUAL REPORT, ETC. 



[A continuous series of photographic registers of the motion of the 

 magnetic needle is now kept up at the joint expense of the Coast Sur- 

 vey and this Institution, which will serve for comparison with any 

 observations which may h*? made on the aurora.] 



Prof. Olmsted, in a recent paper published by the Smithsonian In- 

 stitution, classifies different auroras as follows : 



" Class I. This is characterized by the presence of at least three out 

 of four of the most magnificent varieties of form, namely, arches, 

 streamers, corona, and waves. The distinct formation of the corona 

 is the most important characteristic of this class ; yet, were the corona 

 distinctly formed, without auroral arches or waves, or crimson vapor, 

 it could not be considered as an aurora of the first class. 



" Class II. The combination oitivo or more of the leading charac- 

 teristics of the first class, but wanting in others, would serve to mark 

 class the second. Thus the exhibition of arches and streamers, both 

 of superior brilliancy, with a corona, while the waves and crimson 

 columns were wanting, or of streamers with a corona^ or of arches 

 with a corona, without streamers or columns, (if such a case ever oc- 

 curs,) we should designate as an aurora of the second class. j 



" Class III. The presence of only one of the more rare character- \ 

 istics, either streamers or an arch, or irregular coruscations, but with- 

 out the formation of a corona, and with but a moderate degree of 

 intensity, would denote an aurora of the third class. 



" Class IV. In this class we place the most ordinary forms of the 

 aurora, as a mere northern twilight, or a few streamers, with none of 

 the characteristics that mark the grander exhibitions of the phenom- 

 enon." 



The same author remarks : 



''On the evening of the 27th of August, 1827, after a long ab- 

 sence of any striking exhibition of the aurora borealis, there com- 

 menced a series of these meteors which increased in frequency and 

 magnificence for the ten following years, arrived at a maximum 

 during the years 1835, 1836, and 1837, and, after that period, regu- 

 larly declined in number and intensity until November, 1848, when 

 the series appeared to come to a close. The recurrence, however, of 

 three very remarkable exhibitions of the meteor in September, 1851, 

 and of another of the first class as late as February 19th, 1852, indi- 

 cates that the close was not so abrupt as was at first supposed ; but 

 still there was a very marked decline in the number of great auroras 

 after 1848, and there has been scarcely one of the higher class since 

 1853. 



"A review of the history of the foregoing series of auroras appears 

 to warrant the conclusion that it constituted a definite period, which 

 I have ventured to call the "Secular Period," having a duration of 

 little more than twenty years ; increasing in intensity pretty regular- 

 ly for the first ten years, arriving at its maximum about the middle 

 of this period, and as regularly declining during thelatter half of the 

 same perio'd." 



If this view be correct, it would appear that but few brilliant dis- 

 plays of the aurora may be expected for a number of years to com.e. 



