VOL. LXIV.] PHILOSOPHICAL TRANSACTIONS. ' 483 



rence of circumstances, has sometimes effected more than could have been 

 expected from the most promising measures: a remark which, it is hoped, will 

 in some degree be found justified in the sequel of this paper. The observations 

 on the solar spots, now to be related, appear to be totally different from any 

 hitherto to be found, and such as seem to open a new and curious field of specu- 

 lation into the whole of this subject. 



Astronomers will remember, that a spot of an extraordinary size appeared on 

 the sun, in Nov. 1769. On the 22d, Dr. W. had a view of the sun through 

 an excellent Gregorian telescope, of 26 inches focus, which magnified Uptimes. 

 It was not far from the sun's western limb, and below his equatorial diameter. 

 The' atmosphere being very clear, and free from all tremor and undulation, it 

 was pleasant to see the nucleus of the spot, and the shady zone or umbra which 

 surrounded it, so very distinct. Next day he again saw the spot, having its 

 nucleus and umbra very sharply defined. He now found however a remarkable 

 change; for the umbra, which before was equally broad all round the nucleus, 

 appeared much contracted on that part which lay towards the centre of the disc, 

 while the other parts of it remained nearly of their former dimensions. This 

 change of the umbra seemed somewhat extraordinary, as it was the very reverse 

 of what he expected from the motion of the spot towards the limb. But next 

 day, at 10 o'clock, he had another observation, and discovered changes which 

 were still more unexpected. The distance of the spot from the limb was now 

 about 24". By this time the contracted side of the umbra had entirely vanished ; 

 and the figure of the nucleus was now remarkably changed, from what it had 

 been the precediiig day. This alteration of the figure appeared evidently to have 

 taken place on that side which had now lost the umbra, the breadth of the 

 nucleus being thereby more suddenly impaired than it ought to have been, by 

 the motion of the spot across the disc. 1 



Regarding these circumstances as new. Dr. W. began to consider what might 

 be the cause of them. One of two things seemed necessarily to be the case; 

 either, that they were owing to some physical alteration or wasting of the spot, 

 and of that part of it where the deficiency of the umbra was observed; or else, 

 that they were owing to the nearer approach of the spot to the limb, by the sun's 

 rotation on his axis. The last of these two ideas had no sooner struck him, 

 than he began to suspect that the central part, or nucleus of this spot, was be- 

 neath the level of the sun's spherical surface; and that the shady zone or umbra, 

 which surrounded it, might be nothing else but the shelving sides of the lumi- 

 nous matter of the sun, reaching from his surface, in every direction, down to 

 the nucleus: for, on this supposition, he perceived that a just account could be 

 given of the changes of the umbra, and of the figure of the nucleus, above 

 described. The opinion therefore which he ventured to form, from what he had • 



3 a 2 



\ 



