Does there exist in the Sun (for it would evidently be useless to seek elsewhere) a phenomenon with a similar period? If such is the case, we shall perhaps have laid our hand on a simple relation of cause and effect. At all events, it will be a valuable hint and a sort of first indication of the track we ought to follow. Now, the spots on the Sun observe a precisely analogous period. Every eleven years they offer a strongly-marked maximum of frequency, followed, after an interval of several years, by a minimum, during which the Sun appears every day without a single accident or blemish on his brilliant surface. We are led, therefore, to investigate the case more closely.
More closely, in fact, we ought to look; for the coincidence may not be strictly exact. In that case, the present agreement of the two phenomena would be purely accidental; at the close of several periods it would disappear, and we should have been the dupes of a mere illusion. But M. Faye quotes a comparative table of the periods of the solar spots and of terrestrial magnetism, drawn up by M. Wolf, of Zurich, from which it appears that even the slight anomalies that occur, in respect to the average period of one of these phenomena, are faithfully reproduced by the other. This remarkable coincidence was almost simultaneously pointed out by General Sabine, Monsieur R. Wolf, of Zurich, and Monsieur Gautier, of Geneva.
Thus the spots on the Sun—those amplified whirlwinds which, by digging hollows in his surface here and there, introduce into his brilliant shell masses (more or less considerable) of the cooler hydrogen which envelops it—exercise on magnetism a daily action which is perfectly sensible to us. The problem of these mysterious variations, thus circumscribed, becomes henceforth more accessible.
True, the problem is not solved by that solitary circumstance. The advent of the spots determines two influences: first, they sensibly reduce the extent of the active surface of the Sun, and consequently of his radiations; secondly, they cause in the chromosphere, and far above it, gigantic hydrogenous eruptions, whose effects we are unable to appreciate. But that is precisely the point on which we have to concentrate our means of investigation; it is exactly there that Science may hope to seize the word of the enigma.
And, since there is an enigma, we are all the more strongly urged to solve it, because the daily variations of the magnetic needle are not alone in being affected by the variations of the photosphere. The same mode of reasoning connects them with the appearance of auroræ boreales. Here we are on familiar ground, discussing a phenomenon visible to and admired by all. Already Arago had remarked a sort of connection between the apparitions of the northern lights and disturbances of the magnetic needle. The concomitance was singular. And now we find that those auroræ present, exactly like the variations of the magnetic dip, a period agreeing with that of the
