ca. v.] PLANETARY EVOLUTION. 369 



further confirmation ; for " other things equal, a genetic ring 

 that is broadest in the direction of its plane will produce a 

 mass rotating faster than one that is broadest at right angles 

 to its plane " ; and accordingly Jupiter and Saturn, originating 

 from relatively quoit-shaped rings, rotate very swiftly ; while 

 all the inner planets, originating from relatively hoop-shaped 

 rings, rotate with much less rapidity. 



Here we may profitably consider the singular instance in 

 the history of the solar system in which a detached ring has 

 failed to become integrated into a single planetary mass. 

 Everyone remembers how, in accordance with the law of 

 Titius concerning planetary intervals, Kepler was led to pre- 

 dict the existence of a planet between Mars and Jupiter ; and 

 how, at the beginning of the present century, not one only, 

 but four such planets, were suddenly discovered. More than 

 a hundred of these little bodies have now been detected, and 

 each year adds new names to the list. The four earliest 

 observed — Vesta, Juno, Ceres, and Pallas — are of respectable 

 dimensions ; Pallas having a diameter of 600 miles, or more 

 than one fourth the diameter of our moon. Most of the 

 others are quite tiny, the smallest having a surface perhaps 

 not larger than the state of Rhode Island. Not only do they 

 occupy the position which would normally belong to a ingle 

 planet between Mars and Jupiter, but it is hardly question- 

 able that they have all originated from a single ring ; for 

 their orbits are interlaced in such a complicated way that, if 

 they were material rings instead of ideal lines in space, it 

 would be possible to lift them all up by lifting any one of 

 them. Why should just one of the solar rings have failed to 

 develope into a single planet, and why should such an arrest 

 of development have occurred in just this part of the solar 

 system ? 



common mode of genesis for both planets and satellites ; and are likely, when 

 completely generalized, to yield important testimony in behalf of the nebular 

 theory. 



VOL. I. B B 



