310 Intelligence and Miscellaneous Articles. 



the less luminous portions of the photosphere projected on to the 

 penumbral stratum. At the interior edge of the penumbra we gene- 

 rally find an appearance (especially when spots are enlarging) very 

 similar to what I have noticed above as being found at the interior 

 edge of the umbra — namety, an increase of brightness as compared 

 with the rest of the penumbra — the light decreasing from its inner 

 to its outer edge, where it is almost always perceptibly less than in 

 any other part. That this is not the effect of contrast with the 

 adjacent far brighter photosphere is proved by excluding that stratum 

 from a small field of view. The impression is the same as in the 

 case of the umbra — that the inner edge of the penumbra is heaped 

 up, and thicker than any other part of it. 



The third envelope, or photosphere, affords the'same kind of evidence 

 of being heaped up, or rolled back on itself, at its interior edge adjoin- 

 ing the outer edge of the penumbra. As I have stated above, it is 

 not unfrequently seen to be so when spots of considerable size are 

 near the sun's limb. The elongation of the brighter self-luminous 

 masses, as seen projected on the penumbra, and on some special 

 occasions at its outer edge, in the formation of luminous bridges, 

 has been adverted to in my paper recently presented to the Society. 



These remarks on the peculiar characteristics of the three enve- 

 lopes will suffice to show on w r hat evidence my conclusions rest 

 respecting the manner in which these profound spots are formed, and 

 which may be thus described. 



An immense volume of some non-inflammable gas, discharged 

 with prodigious force from the body of the sun by volcanic or some 

 similar agency, bursts through the cloudy stratum, rolling back on 

 all sides the displaced portion of that stratum, and producing the 

 heaped-up appearance at its inner and lighter edge. The black hole 

 produced in the stratum by this volcanic eruption forms the nucleus 

 of the spot. 



Having passed through the cloudy stratum, the evolved gas comes 

 within the influence of the heating power of the self-luminous pe- 

 numbral stratum ; and being greatly expanded thereby, its increased 

 volume removes a far larger area of this second stratum than of the 

 first, thus laying bare a considerable portion of the upper surface of 

 the cloudy stratum, and producing the umbra of the spot. Here, 

 too, the rolling back of the removed portion causes a heaped-up and 

 brighter appearance at the inner edge of the penumbra. Being still 

 further heated and expanded by approaching the photosphere, a similar 

 effect is produced upon this upper stratum, but to a far greater ex- 

 tent ; and a much larger portion of the photosphere is thrown off on 

 all sides, which being, as before, rolled back upon the rest, gives the 

 appearance of a heaping up of the luminous masses at the exterior 

 edge of the spot. 



Such is the process, or something like it, by which, as I have been 

 led to conclude,, these " profound" spots are formed. And it may 

 here be remarked, that an additional argument for the smaller den- 

 sity of the less luminous masses seen in the photosphere (and before 

 referred to) is found in the fact that these seem to be almost entirely 



