482 PHILOSOPHICAL TRANSACTIONS. [aNNO \7Q5. 



6i times less on the sun, than on our equatorial regions; and as an elevation 

 similar to one of 3 miles on the earth would not be less than 334 miles on the sun, 

 there can be no doubt but that a mountain much higher would stand very 

 firmly. The little density of the solar body seems also to be in favour of the 

 height of its mountains; for, caeteris paribus, dense bodies will sooner come to 

 their level than rare ones. The difference in the vanishing of the shelving side, 

 instead of explaining it by mountains, may also, and perhaps more satisfactorily, 

 be accounted from the real difference of the extent, the arrangement, the height, 

 and the intensity of the shining fluid, added to the occasional changes that may 

 happen in these particulars, during the time in which the spot approaches to the 

 edge of the disc. However, by admitting large mountains on the surface of 

 the sun, we shall account for the different opinions of two eminent astronomers; 

 one of whom believed the spots depressed below the sun, while the other sup- 

 posed them elevated above it. For it is not improbable that some of the solar 

 mountains may be high enough occasionally to project above the shining elastic 

 fluid, when, by some agitation or other cause, it is not of the usual height; and 

 this opinion is much strengthened by the return of some remarkable spots, which 

 served Cassini to ascertain the period of the sun's rotation. A very high country, 

 or chain of mountains, may oftener become visible, by the removal of the ob- 

 structing fluid, than the lower regions, on account of its not being so deeply 

 covered with it. 



In the year 1791, I examined a large spot in the sun, and found it evidently 

 depressed below the level of the surface; about the dark part was a broad margin, 

 or plane of considerable extent, less bright than the sun, and also lower than 

 its surface. This plane seemed to rise, with shelving sides, up to the place 

 where it joined the level of the surface. In confirmation of these appearances, 

 I carefully remarked that the disc of the sun was visibly convex; and the reason 

 of my attention to this particular, was my being already long acquainted with a 

 certain optical deception, that takes place now and then when we view the moon; 

 which is, that all the elevated spots on its surface will seem to be cavities, and all 

 cavities will assume the shape of mountains. But then, at the same time the 

 moon, instead of having the convex appearance of a globe, will seem to be a 

 large concave portion of a hollow sphere. As soon as, by the force of imagi- 

 nation, you drive away the fallacious appearance of a concave moon, you restore 

 the mountains to their protuberance, and sink the cavities again below the level 

 of the surface. Now, when I saw the spot lower than the shining matter of 

 the sun, and an extended plane, also depressed, with shelving sides rising up to 

 the level, I also found that the sun was convex, and appeared in its natural 

 globular state. Hence I conclude that there could be no deception in those ap- 

 pearances. 



