SATURN. 171 



a mean distance the polar diameter was 1 5" -381 ; the equato- 

 rial diameter 17"'053, consequently a flattening of T . T .* The 

 body of the planet has also ribbon-like stripes, which are, how- 

 ever, less perceptible, though, at the same time, rather broader 

 than those of Jupiter. The most constant of them is a gray 

 equatorial stripe. Next to this follow several others, but 

 with variable forms, indicating an atmospheric origin. Will- 

 iam Herschel did not always find them parallel to the rings, 

 neither do they extend as far as the poles. The region round 

 the poles presents 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 as- 

 cribed to the temporary formation of ice and snow, or to an 

 extraordinary accumulation of clouds,f 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 ^J^T-? ; 

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

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

 small density decreasing toward the surface. If the density 

 were quite homogeneous ( f Y ? of that of 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 great- 

 er than that of Saturn itself, and the outer ring is still brighter 

 than the inner.! The division of the ring seen by Huygens 

 in 1655, as a single one, was indeed observed by Dominique 



* Laplace (Expos, du Syst. du Monde, p. 43) estimates the flattening 

 at y l y- The extraordinary deviation of Saturn from the spheroidal fig- 

 ure, according to which William Herschel, after a series of laborious 

 observations, made with very different telescopes, found the major axis 

 of the planet, not in the equator itself, but in a diameter which inter- 

 sected the equatorial diameter at an angle of about 45. was not con- 

 firmed by Bessel, but found to be incorrect. 



t Arago, Annuaire for 1842, p. 555. 



t This difference in the luminous intensity of the outer and inner 

 rings was also stated by Dominique Cassini (M6m. de V Academic des 

 Sciences, annee, 1715, p. 13). 



Cosmos, vol. ii., p. 323. The public announcement of the discovery, 

 or, rather, the complete explanation of all the phenomena which are 

 presented by Saturn and his ring, did not take place until the year 

 1659, four years afterward, in the Systcma Saturnium. 



