228 



AURORA BOEEALIS. 



lowing tabic shows the dates of these periods of maximum and of minimum 

 frequency, as far as they can be gathered from the American and European 

 observations independently : 



It will be observed that the two series of observations accord pretty well 

 with each o^her, and they show a mean intervjil of eleven years between the 

 successive maxima. It should be remarked, however, that from 1792 to 1802 

 the number of observed auroras is small, and there is an appearance of another 

 maximum in 1797. If we count this as a period of maximum abundance, then 

 the mean interval between the successive maxima will be reduced to ten years, 

 and the evidence at present appears to be in favor of this conclusion. Thus the 

 observations of the aurora seem to indicate a maximum every ten years, and a 

 madimum viaximorum every fifty-nine or perhaps sixty years. If any doubt 

 should still remain whether this phenomenon exhibits a true astronomical pe- 

 riodicity, it will probably be removed when we discover its connexion with the 

 movements of the magnetic needle. 



Disturbance of the magnetic needle. — The aurora is ordinarily accompanied 

 by a considerable disturbance of the magnetic needle. This magnetic influence 

 of the aurora has been known for more thau a hundred years, and within the 

 last thirty years it has been studied with great care. When an aurora consists 

 merely of a bank of light like the dawn, and rises but little above the horizon, 

 the disturbance of the magnetic needle is slight, while the effect increases with 

 the brilliancy and extent of the aurora. Auroral beams generally cause a 

 disturbance of the needle, particularly when the beams themselves are in 

 active motion. Auroral waves or flashes, especially if they extend to the zenith, 

 cause a violent agitation of the needle, consisting ©f an irregular oscillation on 

 each side of its mean position. During the aurora of September 2, 1859, the 

 entire range of the needle at Toronto was 3° 45', and at Rome was 4'^ 13'. 



These extraordiary deflections of the needle prevail almost simultaneously 

 over large portions of the globe, even where the aurora itself is not visible ; and 

 they have been termed, by Baron Humboldt, magnetic hurricanes. On the 25th 

 of September, 1841, an extraordinary disturbance of the magnetic instruments 

 Avas observed at Greenwich. This disturbance affected both the direction and 

 intensity of the magnetic needle. The changes in the direction of the needle 

 were by sudden impulses ; after each impulse the needle was stationary for a 

 fi'W seconds, then it was jerked to another position and was again stationary.* 

 On the same day a remarkable disturbanca of the magnetic instruments was 



* Greenwich Magnetic Observations, 1841, p. 48. London and Edinburgh Philosophical 

 Magazine, December, 1841, p. G05. 



