66 SATUKN AND HIS RIXGS. SECT. IX. 



as four millions to one that all the motions of the planets, both 

 of rotation and revolution, were at once imparted by an original 

 common cause, but of which we know neither the nature nor 

 the epoch. 



The larger planets rotate in shorter periods than the smaller 

 planets and the earth. Their compression is consequently greater, 

 and the action of the sun and of their satellites occasions a 

 nutation in their axes and a precession of their equinoxes (N. 147) 

 similar to that which obtains in the terrestrial spheroid, from the 

 attraction of the sun and moon on the prominent matter at the 

 equator. Jupiter revolves in less than ten hours round an axis at 

 right angles to certain dark belts or bands, which always cross his 

 equator. (See Plate 1.) This rapid rotation occasions a very great 

 compression in his form. His equatorial axis exceeds his polar axis 

 by 6000 miles, whereas the difference in the axes of the earth is 

 only about twenty-six and a half. It is an evident consequence 

 of Kepler's law of the squares of the periodic times of the planets 

 being as the cubes of the major axes of their orbit's, that the 

 heavenly bodies move slower the farther they are from the sun. 

 In comparing the periods of the revolutions of Jupiter and 

 Saturn with the times of their rotation, it appears that a year of 

 Jupiter contains nearly ten thousand of his days, and that of 

 Saturn about thirty thousand Saturnian days. 



The appearance of Saturn is unparalleled in the system of the 

 world. He is a spheroid nearly lj)00 times larger than the earth, 

 surrounded by a ring even brighter than himself, which always 

 remains suspended in the plane of his equator : and, viewed with 

 a very good telescope, it is found to consist of two concentric 

 rings, divided by a dark band. The exterior ring, as seen 

 through Mr. Lassell's great equatorial at Malta, has a dark- 

 striped band through the centre, and is altogether less bright 

 than the interior ring, one half of which is extremely brilliant ; 

 while the interior half is shaded in rings like the seats in 

 an amphitheatre. Mr. Lassell made the remarkable discovery 

 of a dark transparent ring, whose edge coincides with the inner 

 edge of the interior ring, and which occupies about half the 

 space between it and Saturn. He compares it to a band of dark- 

 coloured crape drawn across a portion of the disc of the planet, 

 and the part projected upon the blue sky is also transparent. At 

 the time these observations were made at Malta, Captain Jacob 

 discovered the transparent ring at Madras. It is conjectured to be 



