36 ON THE RECENT SECULAR PERIOD 



3. Making a comparison with respect to seasons of the year, the case stands 

 thus: — 



Former period. Latter period. Former period. Latter period. 



Winter months 71 51 Summer months 18 65 



Spring " 70 74 Autumnal " 74 71 



In the former period there was a great excess of the winter OA r er the summer 

 auroras, the numbers being as 71 to 18 ; l whereas, in the latter period, the disparity 

 is small, and the advantage on the side of summer in the ratio of 65 to 51. More- 

 over, the former gave 74 to autumn, and 70 to spring; while the latter gives 71 

 to autumn and 74 to spring. In regard, however, to intensity, the balance has 

 always been in favor of autumn, the greatest auroras having been most numerous 

 in September and November, 2 while they have never occurred in June ; but, in 

 respect to number, the balance between the seasons, of late years, has been just 

 the opposite of what it was a century ago, the minimum, instead of the maximum 

 number, having of late occurred during the winter months; and this is the more 

 remarkable, since the greater lengths of the winter nights would, of itself, lead us 

 to expect a greater number of auroras at this season of the year. 3 



3. Secular Periodicity. — The great returns of the aurora borealis, which we have 

 denominated secular periods, occur at intervals of from sixty to sixty-five years, reclaming 

 from the middle of one period to the middle of another, and last from twenty to twenty- 

 five years. It is, in general, an acknowledged fact, that there are long intervals 

 during which great auroras are seldom seen, and other periods of less duration 

 during which they occur with remarkable frequency and magnificence. Dr. Halley 

 had reached an advanced period of life before he had an opportunity of witnessing 

 a striking display of this phenomenon, when the great aurora of 1716 announced 

 the recurrence of one of the great periods, which afforded this eminent philosopher 

 numerous occasions for seeing the long desired spectacle; and, as has been before 

 remarked, for a long interval, from about the year 1783 to 1827, there was not, 

 perhaps, a single aurora of the highest class, and but few above those of the lowest. 

 We have seen that, in 1827, there commenced a period of unusual frequency, such 

 as to arrest attention both in Europe and in America; that beginning with ten per 

 annum the number increased by a pretty regular gradation to thirty, fifty, and even 

 seventy-five a year, reaching this latter number in the year 1840, and then 

 declining in number and still more in intensity, until, since 1852, scarcely an 

 example has occurred above the lowest forms of the meteor. 



Being assured of the fact, that the phenomenon in question is truly characterized 

 by long periods during which it occurs in unusual intensity and frequency, between 



1 Mairan institutes a labored comparison, to show that the number of winter greatly exceeds that of 

 summer auroras, and hence argues that auroras arise from the earth's coming within the sun's atmosphere 

 (as he took the zodiacal light to be), being, of course, most within that atmosphere at the perihelion; 

 but, from the above comparison, it appears that this argument has no weight. 



2 Great Auroras about the middle of November, in 1574, 1607, 1835, 1837, 1840, 1841, 1844, 1848. 



3 "We may hope that, by the labors of Mr. E. C. Herrick, at New Haven, and of Lieut. Lefroy, in 

 Canada, both of whom have, for some time, been keeping accurate records of all current auroras, we 

 shall soon have the means of determining more accurately than at present the nature of the annual 

 periodicity. 



