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HISTOKY OF PHYSICAL ASTRONOMY.

It gives no account, however, of the numerous faculte which frequently areseen in the vicinity of a spot previous to its disappearance. It is alsomanifestly defective in many other particulars. Derharn was of opinionthat the spots are volcanos in the sun, that the nucleus represents thesmoke, and that the faculse, which subsequently appear, are the glowingflames of the eruption. A fatal objection to this hypothesis is, that theappearance of the solar faculas, for the most part, precedes the appearanceof the spots.

The theory of the solar spots, propounded by Dr. Wilson of Glasgow , isfar more worthy of consideration than any that we have hitherto noticed.This ingenious astronomer maintained that the sun is an opaque masssurrounded by a luminous atmosphere, and that the spots are excavationsin the luminous matter, by means of which the observer is enabled to seethe dark body of the sun. The reasoning by which he established theimportant fact that the spots are depressions below the luminous surfaceof the sun is of a purely inductive character, and is founded upon obser-vations of the great solar spot which appeared in the year 1769. The ac-count he gives of these observations is exceedingly interesting*. He firstperceived the spot on the 22nd November. It appeared below the equa-torial diameter, and was not far from the western limb. On the 23rd heobserved it again, and found that a remarkable change had taken place.The penumbra, which on the previous day was equally broad on all sides ofthe nucleus, was now very much contracted on the side which lay towardsthe centre of the disk, while the other parts retained nearly their formerdimensions. On the 24th he again observed the spot. The distance fromthe limb was now only 24", and the contracted side of the penumbra hadentirely vanished. The breadth of the nucleus on the same side, also ap-peared to be more suddenly impaired than it ought to have been by themotion of the sun across the disk. Dr. Wilson demonstrated, by strictgeometrical reasoning, that these are the appearances which would neces-sarily ensue on the supposition that the spot was a vast excavation, ofwhich the nucleus was the bottom, and the penumbra the sloping sides.If it really was an excavation, a similar succession of changes in a reverseorder should take place when the spot reappeared on the eastern limb.This was, in fact, what occurred to the observation of Dr. Wilson. On the11th December the spot appeared on the opposite side of the disk. Itwas then distant about 1' 30" from the eastern limb. The side of thepenumbra, next to the limb which formerly vanished, was now wdiollyvisible, while that turned towards the centre of the disk appeared to bewanting. On the 12th December it came into view, and he saw it dis-tinctly, although narrower than the other side. He did not see the spotagain until the 17th December, when it had passed the centre of the disk,and the penumbra now 7 appeared to surround the nucleus equally on all sides.

The interesting facts first announced by Dr. Wilson on this occasion,have been fully established by the observations of Sir William Herschel and all subsequent astronomers who have directed their attention to thephenomena of the solar spots. The conclusion to which they unavoidablylead cannot be any other than that which suggested itself to Dr. Wilson,namely, that the spots are not on the same level with the rest of the solarsurface, but are depressions below it, formed by the partial removal of theluminous matter which envelopes the dark body of the sun. This was au

* Phil. Trans., 1774.