364 VARIABLE STARS. SECT. XXXVII. 



ing to Hevelius, it did not appear at all for four years. 

 y Hydrae also vanishes and reappears every 494 days : 

 and a very singular instance of periodicity is given by 

 Sir John Herschel, in the star Algol or /3 Persei, which 

 is described as retaining the size of a star of the second 

 magnitude for two days and fourteen hours ; it then 

 suddenly begins to diminish in splendor, and in about 

 three hours and a half is reduced to the size of a star 

 of the fourth magnitude ; it then begins again to increase, 

 and in three hours and a half more regains its usual 

 brightness, going through all these vicissitudes in two 

 days, twenty hours, and forty-eight minutes, a Cassi- 

 opeia? is also periodical, accomplishing its changes in 225 

 days : the period of the star 34 Cygni is 18 years ; and 

 Sir John Herschel has discovered very singular varia- 

 tions in the star y of the constellation Argo. It is sur- 

 rounded by a wonderful nebula, and from a star of little 

 more than the second magnitude it suddenly increased 

 between the years 1837 and 1838 to be a first-rate star 

 of the first magnitude. At the latter period it was equal 

 to Arcturus, and its brilliancy was then so great as to 

 obliterate some of the details of the surrounding nebula. 

 Afterward it decreased to the first magnitude, and then 

 began to increase again. Sir John has also discovered 

 that a Orionis may now be classed among the variable 

 and periodic stars, a circumstance the more remarkable, 

 as it is one of the conspicuous stars of our hemisphere, 

 and yet its changes had never been remarked. The 

 inferences Sir John draws from the phenomena of vari- 

 able stars are too interesting not to be given in his own 

 words. " A periodic change existing to so great an ex- 

 tent in so large and brilliant a star as a Orionis, cannot 

 fail to awaken attention to the subject, and to revive the 

 consideration of those speculations respecting the possi- 

 bility of a change in the lustre of our sun itself which 

 were put forth by my father. If there really be a com- 

 munity of nature between the sun and fixed stars, every 

 proof that we obtain of the extensive prevalence of such 

 periodical changes in those remote bodies adds to the 

 probability of finding something of the kind nearer home. 

 If our sun were ever intrinsically much brighter than at 

 present, the mean temperature of the surface of our 



