232 



ON THE PERIODICITY OF THE AURORA BOREALIS. 



In 1851 Lamont 1 announced the discovery of a ten year period in the amplitude of 

 the diurnal oscillation of the magnetic needle. This discovery reposed on the nice 

 observations which had been instituted at the Magnetic Observatory of Munich. Ten 

 years afterwards 2 he assigned the length of this periodical change with greater precis- 

 ion, making it 10.43 years. The maxima or minima occur in the following years : — 



Maximum, 1786.5 I Maximum, 1837.5 

 Minimum, 1817.0 | Minimum, 1843.0 



Maximum, 1848.8 

 Minimum, 1855.0 



Maximum, 1859.5 



Sabine 3 soon discovered that this decennial period appeared in the observations made 

 between 1843 and 1848 at the magnetic observatories of the British Colonies, viz., at 

 Toronto and Hobarton 3 ; and was applicable not only to the changes in the amplitude 

 of the diurnal oscillation but to the frequency of the irregular disturbances. Wolf recog- 

 nized the same law in the magnetic observations made in Prague and Kremsmunster. 

 Afterwards, Bache 4 discussed the magnetic observations made for two or three years at 

 the Girard College, in Philadelphia, and detected traces of the decennial period not only 

 in the changes of the declination but also in those of the luorizontal component of the earth's 

 magnetic intensity. 



The decennial period is approximately the same for all parts of the earth. Hansteen 5 

 saw it confirmed in his own long series of twenty-five years of magnetic observations 

 at Christiania ; and, -with Sabine and Lamont, he extended it to the inclination and the 

 intensity. Hansteen computed the period of the magnetic changes, and found its length 

 to be 11.33 years. Observations on Terrestrial Magnetism are " defective between the 

 years 179S and 1813, while Europe was disturbed by war. Still, no doubt remains in 

 regard to the fact of a nearly decennial fluctuation. The precise period of the oscillation 

 may be hereafter slightly corrected, and may itself suffer a secular variation. 



In the mean while Wolf, 6 now of Zurich, was engaged in studying into the laws of 

 periodicity of the solar spots. He availed himself of the long and faithful observations 

 which Schwabe 7 had been making, at Dessau, since 1826. Schwabe himself saw in 



1 Sitzungsberichte der Konigl. Bayer. Akad. der Wissensch. za Munchen, 1862, II. Heft II. 71. Pogg. Ann. der Physik und 

 Chemie, LXXXIV. 572, LXXXV. 412-20, and LXXXVI. 88. 



2 Pogg. Ann. der Chemie und Physik, CXVI. 607. Biblioth. Univ. Axchiv. de Nat. et Phys. Sci. XIX. 306. 

 s Phil. Trans. Roy. Soc. London, 1851, 2, and 3. 



4 U. S. Coast Survey Reports, 1859 and 1862. Proc. American Assoc, for the Advancement of Science, XIII. 251. 

 6 Astronomische Nachrichten, XLV. 195. 



6 Compt. Rend. XXXV. 364. Astronomische Nachrichten, XXXIV. 159 and XXXV. 369. Mittheil. der Bcrner Naturforsch. 

 Gesellsch. No. 245, p. 183. 



7 Astronomische Nachrichten, XXI. 235. 



