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LXVII. On the Duration of a Solar Spot. 

 By W. Pringle, Esq. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IT is stated by Dr. Dick and other writers that " no spot 

 has been known to last longer than one that appeared in 

 the year 1676, which continued upon the sun for seventy days." 

 I am inclined to think that a large spot now going off the sun's 

 disc (21st Nov.), has lasted still longer. 



On the 19th of August last I observed a double spot, or 

 two adjacent spots, near the sun's eastern verge, which as they 

 advanced increased in size, and on the 23rd were visible to 

 the naked eye as one spot. Their longest diameters were 

 27,000 and 31,000 miles respectively, being each about a third 

 less in breadth, embracing the penumbras. They were about 

 10,000 miles apart, though for the first two or three days they 

 appeared to be linked together by a curved chain of minute 

 spots or shallows at their northern extremities, but which be- 

 came eventually absorbed into the penumbra of the largest of 

 the two. They were both of an oval form, and were followed 

 till the 28th or 29th, when the nucleus of one had divided 

 into several parts. They probably disappeared on the 31st 

 of August. 



On September ] 6 I remarked a large spot (at 4 p.m.) so far 

 advanced on the sun's eastern limb as distinctly to exhibit 

 several black nuclei within the edges of the penumbrse, but no 

 great nucleus in the centre; and on the 17th, when it had 

 come more round, this peculiarity became more apparent, the 

 interior sides being studded with small dark spots, while the 

 centre showed nothing but one vast clouded space or shallow. 

 As I had been looking for the reappearance of the two spots 

 of August, it occurred to me that they might have become 

 conjoined, the conjunction of the two penumbrae forming the 

 central space of the spot now advancing. There being no 

 other traces of them, I conceived I was justified in this con- 

 clusion. The smooth contour of the two oval spots had be- 

 come since the contact a vast irregular polygon, as if by the 

 concussion of two forces the penumbrae had been dashed or 

 shaken into angular protrusions. The entire spot on the 

 17th measured about 50,000 miles in its longest dimensions. 

 A very fair engraving of its outline appeared in the Illustrated 

 London News of October 7th. Its size on the 21st was esti- 

 mated at 60,000 miles : I made it somewhat more afterwards. 

 It was quite perceptible to the naked eye for nearly a week 



