VOL. LXIV.3 PHILOSOPHICAL TRANSACTIONS. 533 



it is only by long and uninterrupted trials, that any judgment can be formed 

 concerning the cause of errors. 



The first is a series of observations on the going of the clock, which gradually 

 gains by the heat of the season, and loses by the cold again. Then a register of 

 the barometer, thermometer, and hygrometer. Next, occultations of stars by 

 the moon. Then eclipses of Jupiter's satellites, after the manner of M. Bailly. 

 Since the reading of a paper, communicated last year to this society by Dr. 

 Wilson, professor at Glasgow, on the spots of the sun ; who mentions some ap- 

 pearances when they approach the limb, which I thought I had now and then 

 observed, says Mr. W.; I have frequently turned my glass that way, as occasion 

 offered, to see whether those appearances were constant, or what might be dis- 

 covered to confirm the hypothesis laid down in the latter part of that paper. Dr. 

 Wilson, I hope, will excuse me when I say, that the appearance he mentions 

 when the spots approach the sun's limb, as if they were in a cavity on his sur- 

 face, is not constant. They generally indeed have appeared so to me, I confess. 

 But as they sometimes have not, and as I have very frequently seen them almost 

 in contact with the limb, that is, not 4 a second of time distant in passing a 

 wire; I think they can scarcely be in such a hollow, below his surface, as the 

 Doctor describes. To me indeed, by the brighter light often adjoining to them 

 when near the limb, they have rather put on the appearance as if they were in 

 the crater of a volcano on the top of an eminence, which then turned its side 

 towards us; and if so, the spot would appear somewhat nearer to the limb than 

 it actually was. I have indeed never seen any protuberance on either limb of the 

 sun, as I have on the moon; but I have often observed, near the eastern limb, 

 a bright facula just come on, which has the next day shown itself as a spot; 

 though I do not recollect to have seen such a facula near the western one, after 

 a spot's disappearance. Yet I believe both these circumstances have been ob- 

 served by others, and perhaps not only near the limbs. 



As to the nebulae, they are certainly not always, though they usually are, quite 

 round each spot, or each cluster of spots; neither are they always externally 

 convex. Nothing therefore can be concluded from that circumstance. Besides 

 spots are sometimes quite without any nebulae at all; or none that I could per- 

 ceive with any power of my glass. What the spots, or their nebulae, are, I pre 

 tend not to guess. To me they appear as if they were adjoining to the surface: 

 though that is doubted by better astronomers, who have calculated their motions. 

 The circumstance of the faculae being sometimes converted into spots, I think I 

 may be sure of. That there is generally, perhaps always, a mottled appearance 

 over the face of the sun, when carefully attended to, I think I may be as certain. 

 It is most visible towards the limbs; but I have undoubtedly seen it in the centre: 

 yet I do not recollect to have observed this appearance, or indeed any spots, to- 



