ON THE PERIODICITY OF THE AURORA BOREALIS. 



235 



Secchi, and Gautier the conclusion rested on the comparison of recent observations both 

 of the solar spots and of the magnetic disturbances. Wolf has pursued the subject 

 farther, and coupled with his long list of years numerically rich or poor in solar spots 

 such observations on the amplitude of the diurnal oscillation of the magnetic needle as 

 he was able to hunt up. The result of his investigation is contained in Table LV. In 

 the case of the Declination and the Inclination, the maxima and minima of the magnetic 

 changes coincide nearly with the maxima and minima of the solar spots. But in the 

 variations of the Horizontal component of the Magnetic Force the maxima fall upon the 

 same years as the minima of the solar spots. 1 



TABLE LV. 



It will be seen from Table LV. that Wolf and Lamont are not agreed as to the exact 

 times of the magnetic maxima and minima. Wolf maintains that the periodicity of 

 the magnetic changes is the same as that of the solar spots, viz., 11.11 years ; while 

 Lamont insists on his original period of 10.43 years for the magnetic fluctuations. Wolf' 2 

 divides the whole interval between 1786 and 1860 into six periods of very unequal 

 length ; Lamont divides the same interval into seven periods of equal length. Hence 

 Lamont has a maximum in 1796.5, and a minimum in 1791.3, the maximum nearly 

 coinciding with Wolf's minimum of 1798.5. Sabine 3 concurs with Lamont in thinking 

 that a period of 10 years fits the magnetic movements better than a period of 11 years. 

 Hansteen, on the contrary, deduced from his own observations at Christiania a period 

 for the magnetic inclination and intensity which conformed to Wolf's period of 11.11 



l Vicrteljahrsschrift der Naturforseh. Gesellsch. in Zurich, V. 240, 1 ; Pogg. Annalcn der Chemie und Physik, CXVII. 505-8. 

 J Vierteljahrsschrift der Naturforseh. Gesellseh. in Zurich, II. 293, and X. 160. 

 'Magnetical and Meteorological Observations at St. Helena, II 136. 



