SATURN. 519 



but with variable forms, indicating an atmospheric origin. 

 William Herschel did not always find them parallel to the 

 rings ; neither do they extend as far as the poles. The region 

 round the poles prcspnts a very remarkable phenomenon, a 

 change in the reflection of light which is dependent upon 

 Saturn's seasons. This region is more brightly luminous in 

 winter, a phenomenon which calls to mind the variable snow- 

 region of Mars, and did not escape the penetration of William 

 Herschel. Whether such an increase of luminous intensity 

 is to be ascribed to the temporary formation of ice and snow, 

 or to an extraordinary accumulation of clouds, 78 it is still 

 indicative of the action of changes in temperature, and of 

 the existence of an atmosphere. 



We have already stated the mass of Saturn to be 3-5- Jy. 7-; 

 it, together with the enormous volume of the planet (its 

 diameter is |- of the diameter of Jupiter), leads us to infer a 

 very small density decreasing towards the surface. If the 

 density were quite homogeneous (-^o" ^ ^ na ^ ^ water), the 

 flattening would be still greater. 



The planet is surrounded in the plane of its equator with 

 at least two fully suspended and extremely thin rings, both 

 situated in the same plane. Their luminous intensity is 

 gi eater than that of Saturn itself, and the outer ring is still 

 brighter than the inner. 79 The division of the ring seen by 

 Huygens in 1655, as a single one, 80 was indeed observed by 

 Dominique Cassini in 1675, but first accurately described by 

 William Herschel in 1789-1792. Since Short's time the 

 outer has been found to be streaked by numerous fine stripes, 



78 Arago, Annuaire for 1842, p. 555. 



79 This difference in the luminous intensity of the outer 

 and inner rings was also stated by Dominique Cassini (Mem. 

 de V Academic des Sciences, annee 1715, p. 13). 



80 Cosmos, vol. ii. p. 705. The public announcement of the 

 discovery, or rather the complete explanation of all the phe- 



