,^7^ Transactions of tee American Institute. 



appearance; but the boundaries of the penumbra are sharply defined 

 and its color is tolerably uniform throughout. To meet this diffi- 

 culty the German astronomer Bode, assumed a second envelop of a 

 cloudy nature, supported by an atmosphere, and situated between the 

 true body of the sun and the atmosphere, as the outer light-giving 

 envelop is called. The reflection of the photosi)here from this sur- 

 face would account for all the light of the penumbra, while the 

 nucleus of the spot would be the body of the sun as seen through 

 the opening in this second envelope. Twenty years later, the great 

 William Hersehel added the idea that the transparent, elastic atmo- 

 sphere in which the stratum of clouds must be suspended, at a height 

 of not less than 4,000 miles, likewise supported, and extended beyond 

 the photosphere. Emanating from the true surface of the sun, this 

 gaseous atmosphere streamed upward, displacing the material of the 

 surrounding cloudy stratum and of the thinner photosphere. It is a 

 curious fact that a hypothesis almost identical with this theory of 

 Hersehel had been propounded as early as the middle of the fifteenth 

 century, before the existence of the spots was known to astronomers ; 

 yet this must be regarded rather as a fortunate guess than as a scien- 

 tific theory, for the evidence by which alone such a view can be sup- 

 ported was not then known. 



The general aspects of solar spots will be seen by the representa- 

 tions in the diagrams. The enormous magnitude of some spots has 

 been already mentioned, huge chasms which cover an area of some 

 two billions of square miles, and whose mouths would receive at once 

 forty or fifty globes as large as our earth. Their continual and rapid 

 changes of form and size, make it peculiarly difficult to settle many 

 interesting questions concerning them, but decided indications of 

 rotary motion have been observed in many of them, which would 

 imply that they are turning like huge whirlpools around their own 

 centers. The nucleus, although we speak of it as black, and although 

 it appears intensely so in contrast with the glowing radiance of the 

 surrounding portions, is in itseLf by no means devoid of brightness. 

 It has been well said by Winnecke, that were the light of the whole 

 sun to be extinguished, excepting the portion radiating from the 

 nucleus of a spot, our eyes would scarcely be able to endure the 

 dazzling beams. Herschel's estimate has generally been considered 

 too low, yet it would give the dark nucleus of a spot a luminous 

 intensity, nearly 2,500 tiipes greater than that of the full moon. 



A cuiious and frequqnt appearance is that of so-called bridges, 



