754 Origin of Cometary Bodies and Saturn's Rings. 



from the fact that there are no distinctive marks on their 

 surfaces from which their rotations can be determined. 



Laplace and also Herschel were content to consider the 

 rings as one body, and both assigned the period of its 

 rotation to be 10 hours 32 minutes, as being the time of a 

 satellite revolving at the same distance as the middle of its 

 breadth. 



Later investigators have, however, found it necessary 

 to recognize, from the discrete constitution of the rings, 

 the different times of revolution of their outer and inner 

 circumferences, but have still treated them as one body, 

 and assigned a period of 12 hours 5 minutes for the outer 

 circumference, and 5 hours 50 minutes for the inner edge of 

 the dusky ring C. 



From the fact that the ring A is separated from the inner 

 ring B by a clear space of 2585 miles, the time of its revolution 

 may be determined independently of the times of B and 0. 



As the ring A is postulated to be the first annular ejection 

 from the planet, its outer edge would be the extreme limit 

 of the ejective force, and it would consequently revolve in 

 the same time as a satellite at the same distance, in accord- 

 ance with Kepler's third law. Now the period of Mimas, 

 the first satellite of Saturn, is 22 hours 37 minutes, hence 

 we have for the outer edge of the ring a periodic time of 

 12 hours 48 minutes ; and 11 hours 45 minutes as the time 

 of rotation at the middle of its breadth. 



Dealing with the second ring B in the same mannner, we 

 have for the outer edge a period of 10 hours 9 minutes, and 

 for the middle breadth, 8 hours 24 minutes as the period of 

 revolution. 



The determination of the time of revolution of the dusky 

 crape ring C presents some difficulty on account of the wide 

 separation of the discrete particles of which it is composed, 

 and its apparently close contact with the interior of the ring 

 B, but as by Kepler's law the time of revolution of the 

 interior of B would be 6 hours 44 minutes, the exterior parts 

 of G may be assumed to revolve at the same rate, and the 

 inner edge of C in 5 hours 15 minutes. 



From the principle of the transformation of energy it 

 may be rightly inferred that some of the molar motion of 

 the vast assemblage of discrete particles constituting the 

 rings would be converted into heat, with a consequent slow 

 contraction of their orbits. The observations collected by 

 0. Struve in favour of such contraction have been discussed 

 by astronomers, but without so far arriving at any definite 

 conclusion. 



