BO UK A LIS. Ill 



imate to a just estimate of the numerical value of this periodicity. I will state his 

 conclusion in his own words. 



"From the foregoing, and many similar inductions, I think it may he inferred, with 

 considerable probability, that the greatest secular periods of the aurora boreal is occur 

 at intervals of about sixty-five years, reckoning from the middle of one period to the 

 middle of another, although returns of a less remarkable character arc probably inter- 

 spersed among these. 



"The duration of one of these great periods appears to be from 21 to 25 years. That 

 which we have recently passed through commenced in 1827, and if we consider it as 

 completed in 1848, when there was almost a cessation of the phenomenon in its higher 

 forms for two years, its duration was 21 years. The occurrence of three exhibitions 



■ 



of the first class in September, 1851, and of one in February, 1852, throws some doubt 

 on this point. Although the greatly diminished intensity since 1848 would incline 

 me to consider the period as terminating then, yet these later exhibitions indicate a 

 duration of 25 years. If we examine into the duration of other similar periods, we 

 obtain corresponding results. Thus the return immediately preceding the recent one 

 lasted from 1760 to 1783, a period of 23 years; and the next preceding that lasted 

 from 1716 to 1740, another period of 24 years. On the whole, therefore, I conclude 

 that the aurora borealis is subject to periodical returns, during which it is exhibited in 

 extraordinary frequency and greatly augmented splendor and magnificence ; that these 

 periods are at intervals of about 65 years; that they last for a period not exceeding 

 25 years, and, consequently, that from the end of one visitation to the beginning of 

 another is an interval of nearly 10 years, during which time the phenomenon is far 

 less remarkable both in frequency and intensity. 



"Probably similar periods occur in the polar regions, since travellers differ much in 

 their account of the numbers and degrees of splendor of these exhibitions at different 

 times." * 



I propose to publish immediately, in a second memoir, a complete catalogue of 

 auroras, and then to renew the discussion in regard to the periodicity of the phe- 

 nomenon with more ample materials. 



* 



Pages 38, 39. 



