484 PHILOSOPHICAL TRANSACTIONS. [aNNO 1774. 



seen this day, was, that this spot might probably be a vast excavation in the hi- 

 minous matter of the sun; the nucleus, commonly so called, being the bottom, 

 and the umbra the shelving sides of the excavation : and that the umbra, next 

 the centre of the disc, though out of view, did still however exist, and was ren- 

 dered invisible by its present position only; and further, that the sudden altera- 

 tions, now discernible in the figure of the nucleus, were occasioned by some part 

 of it also being hid, by the interposition of the edge of the excavation, between 

 the nucleus and the eye. 



These considerations made him attentively wait its return. At last, on De- 

 cember nth, he again discovered it, on the opposite side of the disc, it having 

 by that time advanced a little way from the eastern limb, being distant from it 

 l' 30*. And now he could only perceive 3 sides of the umbra, namely, the 

 upper and under sides, and that towards the limb, which was the side that formerly 

 had vanished. The side towards the centre of the disc was not as yet visible; but 

 he concluded, on the same grounds as formerly, that it was hid from sight, by 

 its averted position only, and that, after the spot had advanced a little farther, 

 it would make its appearance. Accordingly, the next day, at 10 o'clock, it 

 came into view, and he saw it distinctly, though narrower than the other sides. 

 After this, his observations were interrupted by unfavourable weather, till the 

 17th, when the spot had passed the centre of the disc, the umbra now appearing 

 to surround the nucleus equally. All the foregoing appearances, when taken 

 together, and when duly considered, seem to prove in the most convincing man- 

 ner, that the nucleus of this spot was considerably beneath the level of the sun's 

 spherical surface. 



The next thing was, to think of some means by which he might form an esti- 

 mate of its depth. At the time of the observation on Dec. 1 2th, he had remarked 

 that the breadth of the side of the umbra, next the limb, was about 14"; but, 

 for determining the point in question, it was also requisite to know the inclina- 

 tion of the shelving side of the umbra to the sun's spherical surface. And here 

 it occurred, that in the case of a large spot, this would, in some measure, be 

 deduced from observation. For, at the time when the side of the umbra is just 

 hid, or begins first to come in view, it is evident, that a line joining the eye 

 and its observed edge, or uppermost limit, coincides with the plane of its decli- 

 vity. By measuring therefore the distance of the edge from the limb, when 

 this change takes placej and by representing it by a projection, the inclination 

 or declivity in some measure may be ascertained. Dr. W. had not an opportu- 

 nity, in the course of the foregoing observations, to see the spot at the time 

 when either of the sides of the umbra changed. It is however certain, that 

 when the spot came on the disc for the 2d time, this change happened some 

 time in the night between the nth and 12th of December; and he judged that 



