486 ' PHILOSOPHICAL TRANSACTIONS. [anNO 1774. 



Dr. W. is sensible, that it may be thought strange, that none of the observers, 

 who had looked at the solar spots with so much attention, should ever have taken 

 notice of the gradual changes above described. This partly may be accounted 

 for from the following considerations. We have found that conjectures, con- 

 cerning the nature of the sun, were early indulged in the course of this inquiry. 

 His body was thought to be an immense globe of fire, which was for ever raging 

 with the most fervent heat. Hence the first observers, reflecting on the perpe- 

 tual generation, changes, and decay of the spots, and that through so wide an 

 extent of his surface, very naturally imagined, that they could consist of nothing 

 but smoke and grosser exhalations, or such transient and perishable materials. 

 This hypothesis had at least the air of being supported by a very plausible ana- 

 logy. The minds of men being carried away by such prepossessions, it would 

 less readily occur, that successful observations were only to be made, by an accu- 

 rate and critical attention to those minute changes, v/hich the spots sometimes 

 imdergo. But what v^fould still more conduce to this oversight, was the method 

 which most of them followed, in making their observations. This was by the 

 camera obscura, which both Scheiner and Hevelius often used, and which we 

 find greatly extolled by them, and described at great length in their writings. But 

 spots, when seen in this way, have nothing of that distinctness, which is so re- 

 markable, and so pleasing, when they are viewed directly through a good tele- 

 scope armed with an helioscope, or glass properly smoked. 



It appears then, that the solar spots are immense excavations in the body of the 

 sun; and that what hitherto has been called the nucleus, is the bottom, and 

 what has been called the umbra, the sloping sides of the excavation. It also 

 appears, that the solar matter, at the depth of the nucleus, does not emit light, 

 or emits so little, as to appear dark compared to that resplendent substance at 

 the surface; that this beauteous substance is at the surface mostfulgid; and 

 when any of it is seen below the general level, forming the sides of an excava- 

 tion, that then its lustre is some how impaired, so as to give the appearance of 

 a surrounding umbra. Here our induction ends. To proceed further would be 

 to carry it beyond its true limits, and to intermix with conclusions, which are 

 certain and manifest, the suggestion of hypotheses, which at best are precarious 

 and liable to error. But from what we have now seen, many curious specula- 

 tions do naturally present themselves. By what mysterious process is it, that 

 those astonishing excavations are at first produced? What is the nature of that 

 shining substance, which is thereby perpetually disturbed? To what are we to 

 ascribe the darkness of the nucleus, and the diminished lustre of the umbra ? 

 And what conceptions are we to form of the many strange changes, and at length 

 of the final decay of all these appearances, by which those regions of the sun, 

 that were so hurt and disfigured, again undergo a renovation? 



