136 ROCHESTER ACADEMY OF SCIENCE. [May 9, 



That this relation of auroras and magnetic storms to the location 

 of disturbed areas at the eastern limb is not adventitious is shown also 

 by the manner of recurrence. At each return the beginning is abrupt 

 and strong, and the subsequent decline gradual. This is apparent in 

 the case of auroras, but is best seen in the tracings from the magnet- 

 ographs which record magnetic storms automatically. For example, 

 the outbreaks of February 13th and March 12th, 1892, were distin- 

 guished by phenomenal characteristics, and began suddenly and 

 violently at the exact interval of twenty-seven and one-quarter days. 

 Consequently the originating solar disturbance could not have under- 

 gone any change of location whatever on the sun's surface, its 

 magnetic effect recurring at an interval differing but a few minutes 

 from the synodic period obtained as the result of the present research. 

 If in such a case the originating disturbance were at the meridian, or 

 elsewhere than at the eastern limb, it is possible that there might be 

 abruptness and violence of beginning but there could not be such 

 exactness of periodicity corresponding to the time of a synodic revolu- 

 tion of the sun. 



The tables to which reference has been made show also that 

 these recurrences of the aurora are not continuous, but are best 

 defined near the equinoxes, and almost disappear near the solstices. 

 In numerous instances single series of recurrences at one equinox do 

 not reappear in the same location in the tables until the return of the 

 corresponding equinox in the year following. This happens even in 

 years in which the disturbed condition is constant, so that disturb- 

 ances even when large and active fail to exercise their full magnetic 

 effect at all seasons of the year. No hint as to the explanation of 

 this was secured until the latitude of the disturbed areas was taken 

 into the account, whereupon it became apparent that proximity to the 

 plane of the earth's orbit is requisite. Spots and faculje are not 

 scattered promiscuously upon all parts of the sun but are confined to 

 narrow belts at a distance of several degrees north and south from 

 the equator. Because of the inclination of the sun's axis to the 

 plane of the earth's orbit, it is only at the equinoxes that the earth 

 approaches the heliocentric zenith of one or the other of these belts. 

 Whatever may be the explanation it is only when thus in range that 

 the earth experiences the full magnetic effect. Thus when a dis- 

 turbed area is upon one side of the sun's Equator solely, its effect 

 reaches the earth only while the latter is opposite the corresponding 

 hemisphere. In the paper upon the Zodiacal Light presented to the 

 Academy last year, evidence was adduced which tends to show that 



