SCIENCE 



[N. S. Vol. XXXIII. No. S38 



smaller and the variations of the eccen- 

 tricity due to this libration are also much 

 smaller. There is, in consequence, less 

 tendency toward instability due to this 

 cause and in fact the numbers do not indi- 

 cate such a phenomenon. The eccentricity 

 does not rise very greatly in value. 



The divisions of the ring system admit 

 of an explanation if we suppose them to 

 consist of clouds of small bodies subject 

 mainly to the attraction of Saturn and of 



on the average be much greater than the 

 number of small eccentricities. On both 

 accounts the number of bodies visible at 

 any one time will be least on the critical 

 radius, rise to a maximum on each side of 

 this radias, and then diminish to the aver- 

 age distribution. Thus we get a ring 

 darker than the average between two rings 

 brighter than the average — a result which 

 tends by contrast to increase the apparent 

 difference in illumination. 



M 



■sssrsmmm-mL- 



Mt 



m'z. 



wfeWpPw 



M 



its satellites. A body on or near a critical 

 radius will experience considerable varia- 

 tions of eccentricity which are of long 

 period. A narrow ring of these bodies will 

 therefore be moving in ellipses of different 

 eccentricities which vary for the different 

 bodies much more than for bodies not near 

 this ring. Hence the range of distance of 

 the bodies composing the narrow ring wiU 

 be spread out much more than for other 

 rings of the same width. At any given 

 time the number of large eccentricities will 



Another result of this variation of ec- 

 centricity may be mentioned. Observation 

 shows that the apparent form of the rings 

 is very nearly circular. It would seem that 

 the orbits of the separate bodies must have 

 small eccentricities in order to avoid col- 

 lisions in which the velocities differ con- 

 siderably. Near a libration region the ec- 

 centricities can not remain very small. In 

 the course of long periods of time it would 

 result from this that encounters have taken 

 place which changed the velocities of the 



