104 ON THE SECULAR PERIODICITY OF THE AURORA BOREALIS. 



shadowed in what has already been said, was illustrated and confirmed by the com- 

 prehensive review of the subject which Mairan made in his Traite Physique ei Historique 

 de VAurore Boreale, published by the French Academy first in 1731, and a second edi- 

 tion in 1754. Mairan was incited to his great labor by the remarkable aurora of 

 October 19, 1726. The first edition contained the record of 229 auroras, scattered 

 over the long interval between the years 500 and 1731. These appearances were 

 divided into 22 distinct groups, separated from each other by unequal intervals of 

 time, themselves barren in auroral displays. But in the Echireis semens of the second 

 edition of his work, Mairan had accumulated 2137 recorded observations, or, subtract- 

 ing the duplicates, 1441 independent exhibitions of the aurora between the years 583 

 and 1751. With this large accession of new materials, Mairan would doubtless have 

 modified the number and epoch of his groups, had he pursued the subject further in 

 that direction. If the whole interval between 1541 and 1751 is divided into seven 

 periods of thirty years each, there are, according to Mairan's catalogue, 38 appear- 

 ances of the aurora in the first period, or between 1541 and 1571 ; 66 appearances in 

 the second period, or between 1571 and 1601 ; 57 appearances in the third period, or 

 between 1601 and 1631; 18 appearances in the fourth period, or between 1631 and 

 16G1 ; 26 appearances in the fifth period, or between 1661 and 1691 ; 195 appearances 

 in the sixth period, or between 1691 and 1721 ; and 908 in the seventh period, or be- 

 tween 1721 and 1751. In the year 1732, one hundred auroras were observed; and in 

 1730, one hundred and sixteen. The general progress of physical science and the 

 multiplication of observers may partly account for the large numbers in the sixth and 

 seventh cycles ; but they fail to explain the inferiority of the fourth and fifth cycles 

 to the second and third. Hansteen* distinguishes twenty-four periods between 502 

 B. C. and the present epoch, particularly the ninth, which is from 541 to 603 ; the 

 twelfth, which is from 823 to 887; the twenty-second, which is from 1517 to 1588; 

 and the twenty-fourth, which is from 1707 to 1788. He assigns the maximum of the 

 latter period to 1752; but it is much nearer a minimum, as I shall show hereafter. 



The weight of evidence which Mairan has accumulated in favor of the periodicity of 

 the aurora has not been universally held conclusive. Bertholon t has argued against 

 what he calls t\* pretended interruptions in the occurrence of this phenomenon. He main- 



that there really 



the auroral displays, and explains away the 



apparent cessation by accidental circumstances, as the lack of observatories, the scarcity 

 ot observers, their want of experience, their bad geographical position, or their inability 



XXII 



t Encyc. Method. Phys., I. 347 - 349. 



°gy 



