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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



thoughts any theory respecting periodic changes 

 in the corona — the series, namely, which is in- 

 cluded in the sixth chapter of my treatise on the 

 sun. Each of these observations I shall consider 

 in connection with the known condition of the 

 sun as to spots, and those results which seem to 

 bear clearly, whether favorably or unfavorably, 

 on the theory we are inquiring into, I shall bring 

 before the reader. 



Kepler, whose attention had been specially 

 drawn to the subject of the light seen round the 

 sun during total eclipse, by certain statements 

 which Clavius had made respecting the eclipse of 

 15G7, describes the eclipse of 1605 in the follow- 

 ing terms : " The whole body of the sun was 

 completely covered for a short time, but around 

 it there shone a brilliant light of a reddish hue 

 and uniform breadth, which occupied a consider- 

 able portion of the heavens." The corona thus 

 seen may fairly be assumed to have resembled in 

 extent that of 1871. A bright corona reaching, 

 like that seen during the eclipse of last July, to a 

 height of only about 70,000 miles from the sun's 

 surface, would certainly not have been described 

 by Kepler as occupying a considerable portion of 

 the heavens ; for a height of 70,000 miles would 

 correspond only to about a twelfth of the sun's 

 diameter, and a ring so narrow would be described 

 very differently. It seems, then, that in 1605 a 

 corona was seen which corresponded with that 

 observed when the sun has had many spots on 

 liis surface. Now we have no record of the con- 

 dition of the sun with regard to spots in 1605 ; 

 but we know that the year 1615 was one of many 

 spots, and the year 1610 one of few spots ; whence 

 we may conclude safely that the year 1605 was 

 one of many spots. This case, then, is in favor 

 of the theory we are examining. 



In passing, we may ask whether the observa- 

 tion by Clavius which had perplexed Kepler may 

 not throw some light on our subject. Clavius 

 says that the eclipse of 1567, which should have 

 been total, was annular. The usual explanation 

 of this has been that the corona was intensely 

 bright close to the sun. And though Kepler 

 considered that his own observation of a broad 

 reddish corona satisfactorily removed Clavius's 

 difficulty, it seems tolerably clear that the corona 

 seen by Clavius must have been very unlike the 

 corona seen by Kepler. In fact, the former must 

 have been like the corona seen last July, much 

 smaller than the average, but correspondingly 

 increased in lustre. Now, with regard to the 

 sun-spot period, we can go back to the year 1567, 

 though not quite so securely as we could wish. 



Taking the average sun-spot period at eleven 

 years, and calculating back from the minimum 

 of spots in the year 1610, we get four years of 

 minimum solar disturbance, 1599, 1588, 1577, 

 and 1566. We should have obtained the same 

 result if we had used the more exact period, 

 eleven and one-ninth years, and had taken 1610.8 

 for the epoch of least solar disturbance (1610.8 

 meaning about the middle of October, 1610). 

 Thus the year 1567 was a year of few sun-spots, 

 probably occupying almost exactly the same po- 

 sition in the sun-spot period as this present year, 



1878. Clavius's observation, then, is in favor of 

 our theory 



But another observation between Clavius's 

 and Kepler's may here be noticed. Jessenius, 

 who observed the eclipse of 1598, at Torgau, in 

 Germany, noticed that, at the time of mid-total- 

 ity, a bright light shone round the moon. On 

 this occasion, remarks Grant, the phenomenon 

 was generally supposed to arise from a defect in 

 the totality of the eclipse, though Kepler strenu- 

 ously contended that such an explanation was at 

 variance with the relation between the values of 

 the apparent diameters of the sun and moon as 

 computed for the time of the eclipse by aid of the 

 solar and lunar tables. The corona, then, must 

 have resembled that seen by Clavius; and, since 

 the year 1598 must have been very near the time 

 of fewest spots, this observation accords with the 

 theory we are examining. 



The next observation is that made by Wyberd 

 during the eclipse of 1652. Here there is a diffi- 

 culty arising from the strange way in which the 

 sun-spots behaved during the interval from 1645 

 to 1679. According to M. Wolff, whose investi- 

 gation of the subject has been very close and 

 searching, there was a maximum of sun-spots in 

 1639, followed by a minimum in 1645, about the 

 usual interval of about six years having elapsed ; 

 but there came a maximum in 1655, ten years 

 later, followed by a minimum in 1666, eleven 

 years later, so that actually twenty-one years 

 would seem to have elapsed between successive 

 minima (1645 and 1666). Then came a maxi- 

 mum in 1675, nine years later, and a minimum in 



1879, four years later. Between the maxima of 

 1639 and 1675, including two spot-periods, an 

 interval of thirty-six years elapsed. There is no 

 other instance on record, so far as I know, of so 

 long an interval as this for two spot-periods. In 

 passing, I would notice how little this circum- 

 stance accords with the theory that the sun-spots 

 follow an exact law, or that, from observations of 

 the sun, means can ever be found for forming a 



