VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS, 481 



Jong dark night. Nay, we have pretty good reason to believe, that probably all 

 the planets emit light in some degree; for the illumination which remains on the 

 moon in a total eclipse cannot be entirely ascribed to the light which may reach 

 it by the refraction of the earth's atmosphere. For instance, in the eclipse of the 

 moon, which happened October 22, 1790, the rays of the sun refracted by the 

 atmosphere of the earth towards the moon, admitting the mean horizontal re- 

 fraction to be 30' 50". 8, would meet in a focus above ISQ thousand miles beyond 

 the moon; so that consequently there could be no illumination from rays re- 

 fracted by our atmosphere. It is however not improbable, that about the polar 

 regions of the earth there may be refraction enough to bring some of the solar 

 rays to a shorter focus. The distance of the moon at the time of the eclipse 

 would require a refraction of 54' 6", equal to its horizontal parallax at that time, 

 to bring them to a focus so as to throw light on the moon. 



The unenlightened part of the planet Venus has also been seen by different 

 persons, and, not having a satellite, those regions that are turned from the sun 

 cannot possibly shine by a borrowed light ; so that this faint illumination must 

 denote some phosphoric quality of the atmosphere of Venus. In the instance of 

 our large spot on the sun, I concluded from appearances that I viewed the real 

 solid body of the sun itself, of which we rarely see more than its shining atmos- 

 phere. In the year 1783, I observed a fine large spot, and followed it up to the 

 edge of the sun's limb. Here I took notice that the spot was plainly depressed 

 below the surface of the sun; and that it had very broad shelving sides. I also 

 • suspected some part at least of the shelving sides to be elevated above the surface 

 of the sun; and observed that, contrary to what usually happens, the margin of 

 that side of the spot, which was farthest from the limb, was the broadest. 



The luminous shelving sides of a spot may be explained by a gentle and gradual 

 removal of the shining fluid, which permits us to see the globe of the sun. As 

 to the uncommon appearance of the broadest margin being on that side of the 

 spot which was farthest from the limb when the spot came near the edge of it, 

 we may surmise that the sun has inequalities on its surface, which may possibly 

 be the cause of it. For when mountainous countries are exposed, if it should 

 chance that the highest parts of the landscape are situated so as to be near that 

 side of the margin, or penumbra of the spot, which is towards the limb, it may 

 partly intercept our view of it, when the spot is seen very obliquely. This would 

 require elevations at least 5 or 6 hundred miles high; but considering the great 

 attraction exerted by the sun on bodies at its surface, and the slow revolution it 

 has on its axis, we may readily admit inequalities to that amount. From the cen- 

 trifugal force at the sun's equator, and the weight of bodies at its surface, I com- 

 pute that the power of thi'owing down a mountain by the exertion of the former, 

 balanced by the superior force of keeping it in its situation of the latter, is near 



VOL. XVII. 3 Q 



