THE SUN'S CORONA AND EIS SPOTS. 



545 



trustworthy system of weather prediction, even 

 if we assume (which has always seemed to me a 

 very daring assumption) that terrestrial weath- 

 er is directly dependent on the progress of the 

 sun-spot period. But here the irregularity of the 

 spot-changes affects us only as preventing us from 

 determining, or even from guessing, what may 

 have been the condition of the sun's surface in 

 the year 1652. This year followed by seven 

 years a period of minimum disturbance, and 

 preceded by three years a period of maximum 

 disturbance; but it would be unsafe to assume 

 that the sun's condition in 1652 was nearer that 

 of maximum than that of minimum disturbance. 

 We must pass over Wyberd's observations of the 

 corona in 1652, at least until some direct evi- 

 dence as to the sun's condition shall have been 

 obtained from the papers or writings of the 

 observers of that year. I note only that Wyberd 

 saw a corona of very limited extent, having, in- 

 deed, a height not half so great as that of many 

 prominences which have been observed during 

 recent eclipses. If the theory we are examining 

 should be established beyond dispute, we should 

 be led to infer that the year 1652 was, in reality, 

 a year of minimum solar disturbance. Perhaps, 

 by throwing in such a minimum between 1645 

 and 1666, with, of course, a corresponding maxi- 

 mum, the wild irregularity of the sun-spot changes 

 between 1645 and 1679 would be to some degree 

 diminished. 



We are now approaching times when more 

 satisfactory observations were made upon the 

 corona, and when, also, we have more com- 

 plete records of the aspect of the sun's sur- 

 face. 



In 1706 Plantade and Clapies saw a bright 

 ring of white light extending round the eclipsed 

 sun to a distance of about 85,000 miles, but 

 merging into a fainter light, which extended no 

 less than four degrees from the eclipsed sun, 

 fading off insensibly, until its light was lost in 

 the obscure background of the sky. This cor- 

 responds unmistakably with such a corona as we 

 should expect only to see at a time of many sun- 

 spots, if the theory we are examining is sound. 

 Turning to Wolff's list, we find that the year 1705 

 is marked as a year of maximum solar disturb- 

 ance, and the year 1712 as that of the next mini- 

 mum. Therefore, 1706 was a year of many sun- 

 spots — in fact, 1706 may have been the year of 

 actual maximum disturbance, for it is within the 

 limits of doubt indicated by Wolff. Certainly a 

 corona extending so far as that which Plantade 

 and Clapies saw would imply an altogether ex- 



107 



ceptional degree of solar disturbance, if our the- 

 ory is correct. 



In 1715 Halley gave the following description 

 of the corona : 



41 A few seconds before the sun was all hid, 

 there discovered itself round the moon a luminous 

 ring about a digit" (a twelfth) "or perhaps a 

 teuth part of the moon's diameter in breadth. It 

 was of a pale whiteness or rather pearl-color, seem- 

 ing to me a little tinged with the colors of the Iris, 

 and to be concentric with the moon." 



He added that the ring appeared much whiter 

 and brighter near the body of the moon than at a 

 distance from it, and that its exterior boundary 

 was very ill-defined, seeming to be determined 

 only by the extreme rarity of the luminous mat- 

 ter. The French astronomer Louville gave a 

 similar account of the appearance of the ring. 

 He added, however, that "there were interrup- 

 tions in its brightness, causing it to resemble 

 the radial glory with which painters encircle the 

 heads of the saints." The smallness of the co- 

 rona on this occasion corresponds with the de- 

 scription of the corona seen last July ; and though 

 Louville's description of gaps is suggestive of a 

 somewhat different aspect, yet, on the whole, the 

 corona seen in 1715 more closely resembles one 

 which would be seen at a time of minimum solar 

 disturbance, if our theory can be trusted, than 

 one which would be seen at a time of maximum 

 disturbance. Wolff's list puts the year 1712 as 

 one of minimum disturbance, with one year of 

 doubt either way, and the middle of the year 1817 

 as the epoch of maximum disturbance, with a 

 similar range of uncertainty. The case, then, is 

 doubtful, but on the whole inclines to being un- 

 favorable. I may remark that, because of its 

 unfavorable nature, I departed from the rule I 

 had set myself, of taking only the cases included 

 in my treatise on the sun. For the corona of 

 1715 is not described in that treatise, as indeed 

 affording no new information respecting this so- 

 lar appendage. The evidence given in this case 

 is probably affected 'in somd degree by the unfa- 

 vorable atmospheric conditions under which Hal- 

 ley certainly, and Louville probably, observed the 

 eclipse. In any case, the evidence is not strong ; 

 only I would call attention here to the circum- 

 stance that if, as we proceed, we should come to 

 a case in which the evidence is plainly against 

 the theory we are examining, we must give up the 

 theory at once. For one case of discordance doe3 

 more to overthrow a theory respecting associa- 

 tion between such and such phenomena, than a 



