THE SIDEREAL UNIVERSE. 743 



testable fact, yet their real nature is as yet very imper- 

 fectly explained. Some astronomers maintain that they 

 are only holes in the luminous envelope of the sun, which 

 allow us to see its dark strata. Others think they are 

 clouds of vapour which wander over the surface of this 

 immense globe of fire. However this may be, it is to the 

 ' observation of these spots that we owe tlie discovery of 

 the rotatory movement of the sun — a movement which 

 takes place in twenty-five days.^ 



The solar heat is so powerful that we can only form a 

 veiy imperfect idea of it. The greatest combustion in our 

 blast-furnaces, pushed to a wdiite heat, cannot for a moment 

 be compared with it. An attempt, however, has been made 

 to estimate the temperature of this formidable furnace. 

 "Let the sun," says Camille Flammarion, "be considered 

 as a globe as large as 1,400,000 terrestrial globes, and com- 

 pletely covered Avith a layer of coal seven leagues in thick- 

 ness. Then the heat furnished by the combustion of all 

 this coal would be equal to what the sun annually projects 

 into space." 



And yet great and incomprehensible as may be the heat 



^ It seems probaljle from tlie solar observatious recently made by De La Eiie, 

 Stewart, and Loewig, "On the Nature of Sun Spots," that the spots are colder 

 than either the photosphere or the sun ; that this greater cold is not due to the 

 general body of the sun at the bottom of a spot being of a lower temperature 

 than the photosphere, and is not pjroduced by any chemical or molecular process, 

 but by matter coming from a colder region, and that when a spot is formed 

 there is a down-rush and melting of photospheric matter. In a i>aper by M. 

 Faye in the Comptes Rendus, the author deduces from Mr. C'arrington's re- 

 .searches the conclusions that sun spots are depressions beneatli the surface of the 

 sun's photosphere fr<jm 20,000 to 40,000 miles in de]jth ; that many of the ap- 

 parent irregularities of their motion, attributed to cyclones, are pi-obably ex- 

 plicable by the continued variation in the motion jiroper to each successive 

 parallel of the photosphere, and that the great regularity of their motions seems 

 incompatible with any hypothesis of mere superficial or local movements in the 

 photosphere, and rather points to some more general action arising from the in- 

 ternal mass of the sun. — Tr. 



