OF TIIE AURORA BOREALIS. 39 



other similar periods, we obtain corresponding results. Thus the return immedi- 

 ately preceding the recent ones, lasted from 1760 to 1783, a period of 23 years; 

 and the next preceding that lasted from 1716 to 1740, 1 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 aug- 

 mented 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 

 40 years ; during which time the phenomenon is far less remarkable both in fre- 

 quency 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. 



While I feel assured of the general fact of the existence of such great returns of 

 the aurora borealis, as those which I have denominated secular periods, yet in this 

 first attempt to determine the intervals between them and to fix the average dura- 

 tion of each, I am sensible of the want of more precise historical data than we at 

 present possess, and therefore submit the foregoing conclusions to the candid inquirer 

 after truth with much diffidence. Philosophers justly regard with some distrust 

 attempts to trace numerical relations, in natural phenomena, since these coinci- 

 dences are often entirely imaginary. Examples of this are familiar to all who are 

 conversant with the biography of Kepler; but that such attempts sometimes conduct 

 to valuable discoveries, is also evinced in the labors of the same illustrious astro- 

 nomer. 



ORIGIN AND CAUSE OF THE AURORA BOREALIS. 



At the next return of the secular period of the aurora (which I anticipate will 

 commence about the year 1890, and be at its maximum about the beginning of the 

 succeeding century), we may justly expect that, from the more advanced state of 

 the natural sciences, and from the accumulation of more accurate data, philosophers 

 will be able to arrive at more correct conclusions respecting the cause of these 

 m3 T sterious exhibitions than we of the present age can attain. The record contained 

 in the preceding pages, of the facts as observed during the recent period, and of the 

 laws as far as they are already ascertained, may nevertheless, should this record 

 endure so long, afford to the observers of that day some useful examples for com- 

 parison, and possibly some valuable hints for reflection, even should our main 

 conclusions be set aside for those which are more definite and just. And I enter- 

 tain the hope that the inquirers after truth will regard with indulgence any efforts 

 to clear up the mystery which environs this subject, even should they fail to 

 acquiesce in the conclusions to which I have been conducted. 



The leading inquiries involved in this discussion are the following: What is 



Mairan, p. 426. 



