PORTION OF THE COSMOS. THE PLANETS. 381 



delicate observations. Jupiter has been seen without any 

 of his satellites by Molyneux, in November 1681 ; by Sir 

 William Herschel on the 23d of May, 1802 ; and lastly by 

 Griesbach, on the 27th of September, 1843. Such a non- 

 visibility of the satellites refers, however, only to the space 

 external to the planet's disk, and does not oppose the 

 theorem that all the four satellites can never be eclipsed or 

 occulted at the same time. 



Saturn. 



The sidereal or true period of revolution of Saturn is 

 29 years, 166 days, 23 hours, 16 minutes, and 32 seconds. 

 Its mean diameter is 15507 German, or 62028 English geo- 

 graphical miles, equal to 9'022 diameters of the Earth. The 

 time of rotation of Saturn, derived from observations of 

 some dark spots (knot-like thickenings or condensations of 

 the streaks) ( 62 ), is 10 hours, 29 minutes, 17 seconds. To 

 this great velocity of rotation round the axis there corre- 

 sponds a great compression at the poles ; this compression 

 was determined by William Herschel, in 1776, at -j^. T . 

 Bessel, from observations more accordant with each other 

 and continued for three years, found, at the mean distance 

 of the planet from the Earth, the polar diameter 15"'381, 

 and the equatorial diameter 17"*053 ; giving an ellipticity 

 or compression of T V--2 ( 621 ) Saturn has also bands or 

 belts, but less marked and somewhat broader than those of 

 Jupiter. The most constant is a grey equatorial band : this 

 is followed by several others, which, however, have varying 

 forms, indicating an atmospheric origin. William Herschel 

 found that they were not always parallel to the ring ; and 



