382 SPECIAL RESULTS IN THE URANOLOGICAL 



they do not extend to the poles. The parts of the disk 

 adjacent to the poles present a very remarkable phenomenon, 

 consisting in a variation in the reflected light, dependent 

 on the seasons of the year in Saturn. The polar regions 

 shine more brightly in their respective winters, a phseno- 

 menon which reminds us of the varying snowy regions of 

 Mars, and which did not escape the keen-sightedness of 

 William Herschel. Whether this increased luminous inten- 

 sity may arise from the temporary formation of snow and ice, 

 or whether it may be attributed to an extraordinary accumu- 

 lation of clouds, it would equally indicate effects produced 

 by changes of temperature and the presence of an atmo- 

 sphere ( 622 ). 



We have already stated the mass of Saturn to be 3 6 1 1 : 

 from this amount, and from its great comparative volume 

 (its diameter is % of that of Jupiter), we infer a very small 

 degree of density, diminishing towards the surface. If the 

 density, (which is -^-^ of the density of water), were homo- 

 geneous throughout, the compression at the poles would be 

 still greater than it is observed to be. 



This planet is surrounded in the plane of its equator by 

 at least two detached exceedingly thin rings, situated in one 

 and the same plane : they have a greater intensity of light 

 than the planet itself, and the outer ring is brighter than 

 the inner one ( 623 ). The division of what had been recog- 

 nised, in 1655, by Huygens, as a single ring ( 624 ), was seen, 

 indeed, as early as 1675 by Dominique Cassini, but was 

 first described with exactness by William Herschel (1789 

 1 792). Since they were first remarked by Short, finer lines 

 or divisions in the outer ring have been repeatedly observed ; 

 but these lines or streaks have never appeared very constant. 



