is assigned to the solar equator. If we assume that the depth of the solar atmosphere is really greater at the solar equator, then we cannot but admit as an inevitable sequel that this atmosphere checking, as it does, (and as the theory itself requires) the radiation of heat from the solar equatorial regions to a greater extent than that from the polar regions, would cause the former regions as observed by us to appear deficient in heat-rays (and presumably in light-rays also). The deficiency must certainly become observable if the amount of heat retained in this way were adequate to produce the effects described by Sir John Herschel.

I am not endeavouring, be it understood, to negative the general conclusion to which Herschel has been led, that a difference of condition really prevails between the equatorial and polar regions to a degree sufficing to account for the spot zone as a zone of solar cyclones. Very probably this difference of temperature subsists, and almost certainly, whatever the cause may be, the spot zone is a zone of solar tornadoes. But the imagined action of a deep atmospheric layer over the solar equator seems incompatible with the observed appearance of the solar disc.

We come next to a most important series of observations directed by Schwabe, of Dessau, to the determination of the laws according to which the number and size of the solar spots may vary from time to time. Such at least was the character which Schwabe's researches eventually assumed. At first they were directed to less important objects, though from the beginning the

observation of the Sun on every day when its face can be seen formed a part of Schwabe's plan.*

seen.

His observations began in the year 1826, during which many spots were visible from time to time upon the Sun's face; indeed, on the 277 days during which the weather permitted Schwabe to observe the Sun, there were but twenty-two on which no spots could be In the next year there were even more spots, and only two days when none were seen. In the next two years the Sun's face was not on any day seen without spots. In 1830 only one day occurred on which no spots could be seen; in 1831, only three. But in 1832 there were no less than forty-nine days (out of 270 on which observations were made) during which no spots were seen. In 1833 there were 139 such days out of 267; in 1834, 120 out of 273; in 1835, 18 out of 244; and then followed four years during which not a single observing day occurred on which no spots were visible.

Schwabe recognised even at this early stage of the inquiry (only twelve years having as yet elapsed since he began his researches) that a certain periodicity marks the occurrence of Sun-spots, or, rather, the recurrence of years rich and poor in such phenomena. It is true that as he had noted but one full period, it might seem that he had absolutely no evidence on which to ground such a view. But in the observation of periodic variations, there are other features besides

* 'I went out like Saul,' he afterwards said, 'to seek my father's asses, and lo! I found a kingdom.'

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194

is assigned to the solar equator. If we assume that the depth of the solar atmosphere is really greater at the solar equator, then we cannot but admit as an inevitable sequel that this atmosphere checking, as it does, (and as the theory itself requires) the radiation of heat from the solar equatorial regions to a greater extent than that from the polar regions, would cause the former reges as cserved by us to appear deficient in heat-rays and presumably in light-rays also). The deficiency is certainly become observable if the amount of heat rained in this way were adequate to produce the es described by Sir John Herschel. I am a caring, be it understood, to negative the generacision to which Herschel has been led,

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4

the periodic return of maxima or minima to guide the experienced observer. The progression towards and from the maximum or minimum is as instructive when carefully watched as the recurrence of many maxima. The observer requires only to assure himself that this progression possesses, certain characteristics, to feel assured that he is dealing with no accidental relations, but with true periodic changes. These characteristics are, chiefly, a steady (not uniform) progression from maximum to minimum, and vice versa, a rapidity of change midway between maximum and minimum as compared with the rate of change near either of these stages, and a certain uniformity of character in the progression towards each stage (though the progression from minimum to maximum may not resemble in character the return towards the minimum). These characteristics Schwabe noted, and he felt satisfied that the numerical relations of the solar spots vary in a truly periodic manner.

But he felt that further observations were necessary to convince the scientific world-always slow to recog nise new truths, or to accept results not rendered palpable, so to to speak, by an accumulation of evidence. He laboured on therefore for twenty more years, tracing the gradual increase and diminution of spots in frequency and in their general dimensions, until he had completed the observation of no less than three complete oscillations from maximum frequency through minimum back to maximum again. It began to be felt that Schwabe was establishing his case, and accordingly