272 FROM NEBULA TO NEBULA 



usual, and velocities of one thousand miles, and even more, are 

 by no means exceptional. 



The average life of a sun-spot may be taken as two or three 

 months; the longest yet on record is that of a spot observed in 

 1840 and 1841, which lasted eighteen months. There are cases, 

 however, where the disappearance of a spot is very soon followed 

 by the appearance of another at the same point, and sometimes 

 this alternate disappearance and reappearance is several times re- 

 peated. While some spots are thus long-lived, others, however, 

 endure only for a day or two, and sometimes only for a few hours. 



As to the causes of sun-spots Doctor Abbot says 

 (The Sun, p. 188) : 



The causes which produce sun-spots, being as yet doubtful, 

 or perhaps it is better to say entirely unknown, the causes of their 

 periodicity and of the irregularity of the periods are, of course, 

 also unknown. Attempts have been made to connect the period 

 with' the times of revolution of the planets, and, indeed, the mean 

 length of the sun-spot period is not far from the period of the 

 revolution of Jupiter (n.86 years). No satisfactory case for a 

 connection between these phenomena is yet made out. 



During his total eclipses the sun is seen to be sur- 

 rounded by a pale luminous veil, called the corona, which 

 during sun-spot periods is fairly regular all around, but 

 in the quiescent periods is confined to his equatorial 

 regions. It has been remarked of the corona, as matters 

 requiring explanation; first, that it appears to partake 

 of the uniformity of the sun's rotation, and, second, that 

 comets which have been observed to pass through it suf- 

 fered no retardation. 



The chromosphere is a layer of incandescent gases 

 having a distinctly reddish tint, and comes next below the 

 corona; but whereas the latter is never observable except 

 during total eclipses, the former can be seen at any time 

 with the aid of telescope and spectroscope combined. Hy- 

 drogen is its chief component. 



The so-called prominences or protuberances are up- 

 ward projections in the chromosphere, but exist only 

 temporarily. They are of two kinds, eruptive, which 

 arise from some violent explosive action, and quiescent, 

 which assume graceful tree-like forms and seem rather to 

 evolve than spring. 



