ON THE PERIODICITY OF THE AURORA BOREALIS. 233 



these observations indications of a decennial period, and pointed out the years numer- 

 ically poorest or richest in solar spots. Wolf changed the length of the period to 

 11.11 years. Some observations, recently brought to light by Thiele, prove that as 

 early as 1775 Horrebow had a glimpse of the existence of some law of periodicity in 

 the frequency of solar spots. 1 



Since this first announcement many eyes have watched sedulously the spots upon 

 the sun, and the archives of science have been ransacked by Wolf and his co-laborers for 

 old observations. More than eleven hundred volumes, printed or manuscript, have been 

 searched, and from their pages Wolf 2 has gathered 23000 observed spots, extending over 

 one hundred and twelve years, between 1749 and 1860. Observations of the solar spots 

 have been accumulated on 22493 days, viz., on 2143 days in the 17th century, on 

 5490 days in the 18th century, and on 14860 days in the 19th century. 



From these multiplied materials Wolf has been able to write out an almost consecutive 

 history of the transactions on the sun's surface from the year 1610 to the present moment. 

 The interval between successive maxima varies between the extremes of 8.2 years and 

 15.5 years; the same period returning once in 79 or 85 years. The mean length of 

 the period, derived from the minima alone, is 11.17 years ; and from the maxima alone 

 11.02 years. The observations anterior to 1750 require a period of 11.15 years. The 

 great difference between the particular and the mean values of the period made the 

 year 1788 the year of the real maximum, and also the year of the mean minimum. Wolf 

 thinks that there is reason to suppose that unusual activity in the forces which develop 

 the solar spots results also in a diminution of the period between successive maxima. 8 



Table LIV. exhibits the years of maxima and minima which Wolf 4 has deduced 

 from the materials in his hands, and the intervals between them. He calculates that 

 the formula, — 



E x = 1732.823 + z. 11.119, 



satisfies best the observed minima, on the presumption that the period is constant. 

 But the difference between the calculated and observed years of minima amounts 

 sometimes to more than three years. He substitutes, therefore, for the first formula 

 another, which recognizes the variability of the period itself 5 : — ■ 



1 Astronomische Nachrichten, L. 257. 



2 Vierteljahrsschrift der Naturforsch. Gesellsch. in Zurich, VI. 175. Biblioth. Univ. Archiv. de Nat. et Phys. Sci. X. 260. 



3 Astron. Nachr. LIV. 346. Vierteljahrsschrift der Naturforsch. Gesellsch. in Zurich, VI. 192. Pogg. Annalen der Physik 

 and Chemie, CXVH. 505. 



4 Pogg. Annalen der Chemie und der Physik, CXVTI. 505. Astronomische Nachrichten, LIII. 338, and LIV. 257. 

 'Vierteljahrsschrift der Naturforsch. Gesellsch. in Zurich, III. 136, IV. 215-17, V. 247-250, VI. 188-190, and X. 355-58. 



VOL. X. 30 



