SKCT.XXXVII.] VARIATION IN SUN*S LIGHT. 4Q5 



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 possibility of a change 

 in the lustre of our sun itself, which were put forth by my 

 father. If there really be a community 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 globe would of course be proportionally 

 greater. I speak now not of periodical but secular changes. 

 But the argument is complicated with the consideration of 

 the possibly imperfect transparency which may be due to 

 material non-luminous particles, diffused irregularly in 

 patches analogous to nebulae, but of greater extent to 

 cosmical clouds, in short of whose existence we have, I 

 think, some indication in the singular and apparently 

 capricious phenomena of temporary stars, and, perhaps, in 

 the recent extraordinary increase, and hardly less sudden 

 diminution, of rj Argus." Mr. Goodricke has conjectured 

 that the periodical changes in the stars may be occasioned 

 by the revolution of some opaque body coming between us 

 and the star, and obstructing part of its light. Our own 

 sun requires nine times the period of Algol to perform a 

 revolution on its own axis ; while, on the other hand, the 

 periodic time of an opaque revolving body, sufficiently 

 large to produce a similar temporary obscuration of the sun 

 seen from a fixed star, would be less than fourteen hours. 



It is possible that the decrease of light of some of the 

 variable stars may arise from large spots on their surface, 

 like those occasionally seen in the radiant fluid masses on 

 the surface of the sun. One of these spots, which was 

 measured by Sir John Herschel on the 29th of March, 

 1836, with its penumbra, occupied an area of 3780 millions 

 of square miles; and the black central part of a spot that 



