228 ON THE PERIODICITY OF THE AURORA BOREALIS. 



and 17S7 auroras averaged twenty a year ; whereas, between 1832 and 1852, there 

 were only two a year, and in 1814 and 1815 none. 1 



The observations prior to 1700 are so meagre that, instead of taking each year singly, 

 I have made groups of ten years each, beginning with the year 1309. Neglecting the 

 observations made at the close of the seventeenth century by Grebner, at Breslau, about 

 which Forbes himself has great doubts, I can easily select the years 1575, 1625, and 

 1685, as approximate periods of recurring maxima, the successive intervals from one to 

 the other, and from the last to 1730, exhibiting a rough agreement with the perio- 

 dicity derived from observations of the last one hundred and fifty years. 



Passing next to still earlier records of the aurora, I may mention the years 585, 675, 

 805, 815, 995, 1115, 1195, 1355, and 1545 as representing decades in which the number 

 of recorded auroras ran ahead of the average. No weighty conclusion for or against 

 the interval, already assigned to the more recent maxima, can be drawn from these 

 conspicuous decades. For, by varying the interval to any value between fifty-two and 

 sixty-two years, as might be done consistently with the evidence, and then taking a 

 larger or smaller multiple of that interval, any decade whatever could be suited 

 to a maximum. From 1575 back to 1545 is a short interval of only thirty years. 

 Between 1545 and 1355 there are one hundred and ninety years, which are consist- 

 ent with three intervals, each of about sixty-three years. From 1355 to 1195 are one 

 hundred and sixty years, which will divide into three intervals of about fifty-three years 

 each. Between 1195 and 1115 are eighty years, which might be divided into two inter- 

 vals of forty years each. Between 1115 and 995 are one hundred and twenty years, 

 which may contain two intervals of sixty years. From 995 to 845 are one hundred 

 and fifty years, or three intervals of fifty years. From 845 to 805 there is only one 

 short interval of forty years. From 805 back to 675 are two intervals of sixty-five 

 years. From 675 to 585 are two intervals of only forty-five years. 



Although the earliest as well as the latest recorded observations of the aurora give 

 unmistakable evidence of the intermittent character of the phenomenon, it is obvious 

 that science must rely exclusively upon the observations of the last three hundred 

 years, and chiefly upon the observations of the last century and a half, for its best deter- 

 mination of the precise interval between successive maxima. Facts as well as analogy 

 favor the suspicion that the interval itself may vary from one century to another, though 

 a variation between forty and sixty years may seem an extravagant supposition. But 

 for the law of this variation, and, indeed, for any certainty as to the fact of the varia- 



i Annates de I'Obserr. Royale de Brux. XI. 42. 



