ELECTRICAL APPARATUS. 53 



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 exhibi- 

 tions of the phenomenon." 



The same author remarks : — 



"On the evening of the 2Hh of August, 182T, after a long 

 absence of any striking exhibition of the aurora borealis, there 

 commenced a series of these meteors, which increased in fre- 

 quency and magnificence for the ten following years, arrived at a 

 maximum during the years 1835, 1836, and 1837, and, after that 

 period, regularly declined in number and intensity until Novem- 

 ber, 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, indicates 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 regularly for the first ten years, arriving at 

 its maximum about the middle of this period, and as regularly 

 declining during the latter half of the same period." 



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

 displays of the aurora may be expected for a number of years to 

 come. 



