244 AUROEA BOEEALIS. 



The rel.-itive frequency of the solar spots exhibits a similar periodicity, and 

 the maximum number of spots corresponds in a remarkable manner with the 

 maximum value of the magnetic variation. Indeed, for the past forty years the 

 times of maxima and minima of the two phenomena have been almost absolutely 

 identical, and seem to favor the conclusion that the apparent anomalies occasion- 

 ally noticeable in the earlier observations are due mainly to the incompleteness of 

 the observations ; for it is only since 1826 that any one has undertaken to keep 

 a continuous record of all the spots visible at any time upon the sun's disk ; and 

 before 1818 the observations of the magnetic declination were, for the most 

 part, only occasional, and are generally insufficient to determine, in a perfectly 

 satisfactory manner, the mean annual range of the magnetic needle. The ob- 

 servations seem also to indicate that the successive maxima are not all equal to 

 each other, but are themselves subject to a periodicity ; one period extending 

 from 1779 to 1839, comprehending an interval of fifty-eight years 



We have found that auroras exhibit a periodicity, the last period extend- 

 ing from 1779-'80 to 1839-'40, embracing an interval of about sixty years ; 

 and during this period there have been indications of alternate maximum 

 and minimum abundance, corresponding in a remarkable manner with the 

 maxima and minima of the solar spots, if we except the period from 1800 to 

 18:20, during which auroras were too infrequent to afford the basis for a safe de- 

 duction. 



It seems, then, pretty well established that these tliree phenomena, the solar 

 spots, the mean daily range of the magnetic needle, and the frequency of auroral 

 exhibitions, manifest two distinct periods ; one a period of from ten to twelve 

 years, the other a period of from fifty-eight to sixty years. The first of these 

 periods corresponds to one revolution of Jupiter, and it has been suspected that 

 Jupiter might be the occasion of these periodical disturbances of the sun's sur- 

 face. If Jupiter does, indeed, exert such an influence, then it is to be presumed 

 that Saturn must exert a similar, though less powerful influence, which woiild 

 have a period of about thirty years ; and since five revolutions of Jupiter corre- 

 spond nearly to two of Saturn, embracing a period of fifty-nine and a half years, 

 at the end of this period the two planets return to nearly the same relative 

 positions, and their joint action should exhibit a period of fifty-nine and a half 

 years. 



If Jupiter and Saturn do, indeed, exert such an influence upon the sun's sur- 

 face, then it seems probable that each of the other plauets must also exert an 

 influence, (though perhaps inappreciable;) and the earth may exert a direct in- 

 fluence upon the sun's surfiice, causing an inequality in the solar spots, whose 

 period is one year ; and Venus and Mercury may exert a similar influence. 



If Ave inquire for the mode in which the planets might exert an influence upon 

 the sun's surface we are left almost entirely to conjecture. But one plausible 

 hypothesis is that there are continually circulating around the sun powerful 

 electric currents, which currents may possibly be the source of the sun's light ; 

 that these currents act upon the planets, developing in them electric currents, or 

 modifying the currents developed in them by the action of other forces ; that 

 these currents circulating around the planets react upon the solar currents with 

 a force depending upon the variable distance of the planet, and therefore having 

 periods corresponding to the times of revolution of the planets. These disturb- 

 ances of the solar electric currents may be one cause of the solar ajiots, and an 

 uiuisual disturbance of the solar currents may cause a simultaneous disturbance 

 of the electric currents of the earth's surface, giving rise to unusual manifesta- 

 tions of electric light — that is, to unusual displays of the aurora. 



There is also another cause Avhich may produce a change in the number of 

 aui-oras visible at one station, viz., the gradual change of position of the terrestrial 

 magnetic poles. The dip of the magnetic needle at Paris in 1671 was 75°, 

 while in 1864 it was only 66^ 3', showing a diminution of 8° 57' in 193 years, 



