502 JAMES CLERK MAXWELL. 



planet itself. In 1665 William Bell observed a dark line 

 running round the northern surface of the ring. In 16*75 

 Cassini noticed the same on the other surface, and con- 

 cluded that the ring consists of two concentric rings, of 

 which the inner is the brighter. Hadley observed the 

 shadow of the planet thrown upon the surface of the ring, 

 and the shadow of the ring on the planet, and showed 

 that Saturn rotates in the same plane with the rings. In 

 1714 Moradi observed a want of symmetry in the ring on 

 opposite sides of the planet, and found that when it 'had 

 disappeared on the eastern side it was still visible on the 

 western, but he did not discover the cause of the phenomena. 

 The observations of Sir William Herschel, published in 1790, 

 corroborated the opinion of Cassini respecting the division. 

 By observing certain spots on the surface of the planet, 

 Herschel found that it rotated on its axis in 10 hours 

 29 minutes 16*8 seconds in the same direction as the rota- 

 tion of the earth. In 1789, as the earth passed through 

 the plane of Saturn's ring, Herschel noticed some bright 

 spots on the edge of the ring which were carried almost to 

 the end of the diameter, and appeared to rotate about 

 Saturn in about 10 hours 32^- minutes. 



The observations of the present century show that the 

 outer of the two bright rings is permanently divided into 

 two concentric rings by a very narrow gap, and when their 

 plane is inclined to the line of sight at a considerable angle, 

 so that the rings are widely open, a series of dark elliptic 

 curves near the extremities of the major axis of the elliptic 

 projection indicates that each ring is still further broken up 

 into a number of thin concentric rings, the gaps between 

 them being apparently filled by the edges of the inner 

 rings except near the extremity of the major axis of the 

 projections. 



In 1850 a dark ring encircling Saturn within the other 

 two rings was discovered independently by three astronomers 

 in England and America. The distance between the outer 

 edge of the dark ring and the inner edge of the adjoining 

 bright ring appeared to vary between narrow limits, the 



