394 VARIABLE STARS. SECT. XXXVI. 



stance 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 variable 

 stars are too interesting not to be given in his own words. " A 

 periodic change existing to so great an extent 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 possibility of a change in the lustre of our sun 

 itself, which were first put forth by my father. If there be really 

 a community of nature between the sun and the 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 tem- 

 perature of the surface of our globe would of course be propor- 

 tionally greater. I speak now not of periodical, but secular 

 changes. But the argument is complicated with the consideration 

 of the possible imperfect transparency of space, which may be 

 due to material non-luminous particles, diffused irregularly in 

 patches analogous to nebula, but of great extent to cosmical 

 clouds, in short, of whose existence we have, I think, some indi- 

 cation in the singular and apparently capricious phenomena of 

 temporary stars, and perhaps in the recent extraordinary increase, 

 and hardly less sudden diminution, of 77 Argus." Mr. Hind has 

 come to the same conclusion with Goodricke and Sir John 

 Herschel, that the changes in the variable stars are owing to 

 opaque bodies revolving round them ; indeed there are strong 

 reasons to believe that there are solar systems analogous to our 

 own in the remote regions of space. Our sun requires nine 

 times the period of Algol to perform a revolution on its 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 in some of the variable 

 stars may arise from large spots on their surface, like those occa- 

 sionally seen in the radiant fluid masses on the surface of the 

 sun. One of these spots which was measured by Sir John Hers- 

 chel on the 20th of March, 1836, with its penumbra, occupied an 

 area of 3780 millions of square miles ; and the black central part of 



