ON THE PERIODICITY OF THE AURORA BOREALIS. 329 



There is reason for thinking that the abnormal disturbances in the sun's crust are 

 coincident with unusual magnetic perturbations. On September 1, 1859, Carring- 

 ton and Hodgson were watching the sun at different observatories, and noticed a 

 spot crossing the surface, between ll h 18 m and IP 23 m a. m. Almost at the same 

 moment, viz. at ll h 20 m , the self-registering magnetic instruments at Kew recorded 

 the violent jerks to which they were subjected, and a magnetic storm extended 

 widely through the earth. Great stress is laid upon the fact that there was on the 

 same day a signal display of aurora, which was extensively seen, and so potent as to 

 interfere with telegraphing, and set on fire the telegraphic offices. Upon this coinci- 

 dence Mr. Carrington makes the following judicious comment: "While the cotem- 

 porary disturbance deserves noting, he would not have it supposed that he even leans 

 towards hastily connecting them. One swallow does not make a summer." * It is not 

 a serious objection to the supposed connection between the solar spots and the aurora, 

 that, in this particular case, the passage of the spot was confined to a few minutes, 

 while the aurora lingered for many days ; for a cause may be sudden and short-lived, 

 while its effect is more lasting. But the fact that the effect anticipated the cause, 

 that the aurora was brilliantly displayed as early as the 28th of August, and more or 

 less conspicuously on the three following nights, does not admit of as easy explana- 

 tion. If the outbreak of special spots upon the sun can be associated, in time, with 

 extraordinary motions in the instruments which register the earth's magnetic condi- 

 tion, the foundation is laid for the hypothesis that sun-storms and aurora-storms are 

 remotely, if not intimately, linked to one another. For one of the first fruits of the 

 magnetic crusade originated by Humboldt was the verification, by abundant examples, 

 of what had been distinctly pronounced before, viz., the synchronism of auroral 

 storms and magnetic storms. In the last ^century the Abbe Mann wrote : 2 " I have 

 frequently observed the irregular magnetic variations during great auroras, as in 

 1767." Arago, watching his needles in Paris, saw by his intellectual vision what Parry 

 beheld with the natural eye. And Kupffer was so confident that auroras, which might 

 even be shrouded from the upper world by clouds, were faithfully announced to 

 underground observers throughout the whole line of Russian Magnetic Observatories, 

 as to think that the direct observation of the aurora might be neglected even when 

 the weather was propitious. 



The two following Tables have been prepared to facilitate the comparison between 

 the Maxima and Minima of solar spots and auroras. 



1 Month. Notices of the Roy. Astron. Soc, XX. 13-15. 2 Mem. de l'Acad. Brux., I. 265. 



VOL. X. 42 



