SIDEREAL ASTRONOMY. 337 



variation differ as much as in the ratio of 1 to 250. The 

 period of ft Persei, about 69 hours, is the shortest ; one of 

 495 days the longest yet ascertained. In some stars the 

 periods of increasing and decreasing brightness are equal; 

 in several the light increases more rapidly than it diminishes. 

 In certain stars (as Algol, Mira Ceti, and ft Lyra) the pe- 

 riods themselves undergo a periodical variation ; and the last- 

 named star is remarkable from having a double maximum 

 and minimum in each of its periods of 13 days. Among the 

 variable stars are some very familiar to us, as the Polar Star 

 and two or three of the Great Bear. One of the most splendid 

 examples yet observed is that of 77 Argus in the southern 

 hemisphere, described by Sir John Herschel in his Observa- 

 tions at the Cape of (rood Hope. The peculiarity and 

 grandeur of the changes in this extraordinary star raising 

 it, though at irregular intervals, from the fourth magnitude 

 to the vivid brightness of Sirius or Canopus are recorded 

 by a pen well capable of such delineation. 



The facts thus briefly cited are prolific of speculations of 

 the same character as those applied to the new stars, and 

 equally incapable of present solution. The general inference 

 of rotation on an axis or revolution about a centre, rendered 

 probable from other sources of evidence, is very directly 

 suggested to the mind, especially in the case of the stars of 

 periodical variation. The phenomena themselves, though 

 far more remarkable in variety and degree, are not wholly 

 without analogy to the conditions of our own central lumi- 

 nary. The solar spots, as more accurately observed by 

 modern astronomers, indicate changes in the state of the 

 Sun or of the luminous envelope surrounding it which 

 visibly affect the amount of light emitted, and might alter 

 more or less its brilliancy, if seen from remote distances as a 

 star. But we cannot carry the argument beyond a bare 

 suggestion, for we are hitherto ignorant of the cause of these 



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