VOL. LXrV.] PHILOSOPHICAL TRANSACTIONS. ' 489 



nature, not lasting on the sun for a long space of time, does it not seem rea- 

 sonable to think, that their gradual decrease, as mentioned in article 4th, is 

 ccasioned by the luminous matter encroaching again on that part of the dark 

 globe, which had been uncovered ? And from this may we not infer, that the 

 luminous matter gravitates, and is in some degree fluid; for thus would it not 

 have a tendency to flow down, in all directions, and encroach, so as at last to 

 cover the nucleus? And do not these things appear further probable, when we 

 reflect on that uniform inclination, which the sides of the umbra or excavation, 

 have to the external surface of the sun's body? For does not this indicate a fluid 

 sort of matter gradually yielding to the force of gravity ? And again, is not this 

 notion further supported, when we consider the property mentioned in the 5th 

 article, namely, that the exterior boundary of the umbra never consists of sharp 

 angles or turnings, but is always curvilinear, and most frequently of a round 

 form : for we know that this boundary is nothing else but the lip of the excava- 

 tion, which, on supposition that the luminous matter possesses some degree of 

 fluidity, will not be disposed, either in enlarging or contracting, to become irre- 

 gular by sudden or sharp turnings ? 



On supposition that the surface of the dark globe of the sun is smooth and 

 level, it may be urged, that the nucleus of a spot, while on the decrease, should, 

 according to the present view of things, always acquire a figure at least nearly 

 circular, and that the luminous matter, continuing to flow down on all sides by 

 an equal gravity, should so encroach on the nucleus, as to make it retain that 

 figure, till at last it be entirely overflowed. But this not being the case, and 

 because it most frequently happens, that the encroachments of the umbra on the 

 nucleus are extremely variable, as mentioned in the 6th article, may we not from 

 this infer, that the surface of the internal dark globe of the sun, is by no means 

 smooth and level, but on the contrary very irregular, for, on this supposition, 

 if for example the area of the nucleus of a great spot were so diversified by moun- 

 tains and vallies, would not the encroachments of the luminous matter be con- 

 sequently irregular: and, according as it was more or less retarded or accelerated, 

 at different places, by being contiguous to prominencies or hollows, would not 

 all the alterations in the figure of the decreasing nucleus, how variable soever, 

 be thus plainly accounted for ? and because it often happens, that the nuc4eus of 

 a spot, while on the decrease, is gradually cut in pieces by a luminous zone or 

 zones, which wander across it, as mentioned in the 7th article, does not this 

 look like the gradual flowing in of the luminous matter, as it were, into deep 

 channels, which would thus appear to abound in the surface of the sun's dark 

 body ? If we reflect on the irregularities on the surface of this earth, and on the 

 enormous mountains and cavities in the moon, may we not, from sucli analogy, 



VOL. XIII. 3 R 



