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VOL. LXiv.] Philosophical transactions- 483 



the distance of the plane of the umbra, when in a line with the eye, must have 

 been about l' 35" from the sun's eastern limb; from which we may safely con- 

 clude, that the nucleus of the spot was at that time not less than a semidiameter 

 of the earth below the level of the sun's spherical surface, and formed the bottom 

 of an amazing cavity, from the surface downwards, whose other dimensions were 

 of much greater extent. 



Being thus persuaded of the depression of this great spot below the surface, 

 he immediately set about examining smaller ones, in order to discover if they 

 were of the same kind. With this view, he began a course of observations, 

 that from them he might either make the inference universal, or limit it, as the 

 phenomena should point out. Dr. W. was not long engaged in this pursuit, 

 before he perceived in them the same changes of their umbrae, which have been 

 described above. This was manifest in spots of any considerable size, when the 

 air was favourable, and the telescope well adjusted for distinct vision. In ge- 

 neral, he found that the umbra thus changes, when a spot is about a minute 

 distant from the limb, at a medium. From all these observations, may we not 

 safely conclude, that every spot consisting of a nucleus and surrounding umbra, 

 as defined by Scheiner and Hevelius, is of the same kind with those above de- 

 scribed. 



In the course of the foregoing observations. Dr. W. had occasion to remark, 

 5 different times, another extraordinary circumstance of the spots, which he had 

 not seen mentioned by any one who has written on the subject. It consists of 

 changes which seem to arise from a disturbing force, when one spot breaks out 

 in the neighbourhood of another. The first case of this sort which he met with, 

 was on Nov. Qth, 1770, when the umbra of a spot, though a great way from 

 the limb, was deficient towards the right hand, at which side, and very near it, 

 there lay another spot much smaller. In like manner, 2 other spots, which lay 

 very near each other, had each of them that side of its umbra, which faced the 

 other, taken away. But it was remarkable, that 3 days after, the one spot had 

 nearly vanished, when the side of the umbra of the other spot, which faced it, 

 began now to dilate. Another spot had its umbra flattened on opposite sides, by 

 3 small spots on one hand, and one on the other. Again, 2 other spots had 

 their umbrae deficient, by the intervention of some small spots, that lay be- 

 tween them. 



Now it must here be particularly remarked, that though a spot, when undis- 

 turbed, will, when near the sun's limb, exhibit the change of the umbra before 

 mentioned ; yet it is plain that a case may now and then occur, when this change 

 will be counteracted, by means of the phenomenon just now described. And 

 accordingly, in the course of the observations, he in reality met with 3 cases, 

 when this change did not take place. 



