CONTRIBUTIONS TO SCIENCE. 503 



rings sometimes appearing to be in contact. This innermost 

 ring is transparent, so that the edge of the planet can be 

 seen through it, and as it is seen in all positions without 

 distortion it seems that the ring cannot consist of a trans- 

 parent gas or liquid. The rings appear to be much thicker 

 near the planet than at their outer edges. A comparison of 

 measurements of the diameters of the rings at different 

 times seems to indicate a rapid change in their breadths, the 

 circumference of the outer ring extending outwards, while the 

 inner bright ring appears to be approaching the planet, so 

 that the whole breadth of the ring system is increasing. At 

 least this is the conclusion to which Struve arrived from a 

 comparison of his measurements with those of Huyghens 

 and Herschel. 1 According to Hind the exterior diameter of 

 the outer ring is about 170,000 miles, its interior diameter 

 150,000 ; the exterior diameter of the inner bright ring is 

 14*7,000 miles, its interior diameter 114,000 miles; and 

 the equatorial diameter of the planet 75,500 miles; but, as 

 above stated, the breadth of the rings appears to be increas- 

 ing. Sir John Herschel was of opinion that the thickness 

 of the rings did not exceed 100 miles, and Bessel calculated 

 that their mass, as determined by the disturbance they pro- 

 duced in one of Saturn's satellites, did not exceed XTTB"^ f 

 the mass of the planet. 



If Saturn's rings were solid and at rest, but of course 

 subject to the attraction of the planet, the stress upon them 

 would be such that we can conceive of no material capable 

 of sustaining it. Maxwell remarked that iron would not 

 only be plastic but semi-fluid under such stresses. By 

 allowing the rings to rotate, the attraction of the planet 

 might expend itself in producing the necessary acceleration 

 towards the centre, or, as is generally stated, might be 

 balanced by the centrifugal force, and the stress might be 

 thus relieved to a great extent ; but if the rings rotated with 

 the velocity proper to the interior of the inner ring, the rest 



1 The evidence of this supposed change in the configuration of the 

 rings is far from conclusive. 



