OS THE PERIODICITY OF THE AURORA BOREALIS. 241 



endlich 'las Nordlicht in einem innigen Zusammenhange und parallalen Gauge mit der Sonnenfleekenbildung steht 

 und zwar in der Weise, dass zur Zeit der reichsten Flcckenbildung das Nordlicht am h'aufigsten auftritt und 

 umgekehrt die Minima Zusammenstimmen und dass, wShrend bci den Sonnenflecken die Hauptniasima sieh weniger 

 auszeichnen, dies bei den Nordlicbtern weit entschiedener der Fall istu" 



Herschel 2 pays a just tribute to the magnitude and the value of these researches of 

 Wolf and Fritz and recognizes the fact that the secular period of fifty-six years suits 

 the observations upon the aurora better than that of sixty-five years. 



If the isochronism supposed to exist between the disturbances of the sun's atmos- 

 phere, the fluctuations of terrestrial magnetism, and the displays of the aurora were 

 satisfactorily demonstrated, the question naturally arises which of the three is cause 

 and which is effect, or whether all three are not independent effects of some general 

 cause. The spots upon the sun have been called mirrors in which the reflex influence 

 of the planets upon that luminary could be seen. An English writer, Grey, has said 

 that he called upon a friend to ascertain by computation whether there was any pe- 

 riod in the movements or conjunctions of the heavenly bodies which could throw light 

 on the eleven-year period of the solar spots. His friend responded that Mercury and 

 Venus would come into conjunction with each other on the same side of the sun once 

 in every 11.08 years. 3 Reichenbach has alluded to the fact that Jupiter's period is 

 nearly equal to the spot period, and that the maxima of solar disturbance approximate 

 to the times of Jupiter's passage through the aphelion. 4 



If there is an annual periodicity in the frequency of the sun's spots, suggesting the 

 earth's variable action upon the sun at different seasons of the year, may not the 

 other planets also exert a similar influence. Wolf has asked and answered this 

 . question. Expressing the comparative influence of a planet over the sim by 5, he 

 computes the following weights as exponents of the activity of the most important 

 planets in this direction : — 



Mercury = 0.4 7 

 Venus = 1.68 

 Earth = 1.00 



Mars = 0.06 

 Jupiter = 12.53 

 Saturn = 1.12 



Uranus = 0.04 

 Neptune = 0.04 



The ordinate of the curve which represents the disturbance produced upon the sun's 

 surface by these planets is calculated by this formula : — 



M =A -f B (1.68 sin. 585°.26t -f 1.00 sin. 360°t + 12.53 sin. 30°.35t -f 1.12 sin. 12*.22t). 



1 Vierteljalirsschrift der Naturforsch. Gessellsch. in Zurich. X. 258. 



? The Quarterly Journal of Science. I. 229. 3 Phil. Mag. Fourth Series, XX. 246 and 271. 



•* Amer. Journ. Sci. Second Series, XXV. 296. 



VOL. X. 31 



