VELOCITIES OF PLANETARY ROTATION. 271 



course of its aggregation : and conversely with a small 

 mass. Still more marked will be the difference where the 

 form of the genetic ring conspires to increase the rate of 

 rotation. Other things eqnal, 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 angle 

 to its plane • and if the ring is absolutely as well as rela- 

 tively broad, the rotation will be very rapid. These con- 

 ditions were, as we saw, fulfilled in the case of Jupiter ; 

 and Jupiter goes round his axis in less than ten hours. 

 Saturn, in whose case, as above explained, the conditions 

 were less favourable to rapid rotation, takes ten hours and 

 a half. While Mars, Earth, Venus, and Mercury, whose 

 rings must have been slender, take more than double the 

 time : the smallest taking the longest. 



From the planets, let us now pass to the satellites. 

 Here, beyond the conspicuous facts commonly adverted to, 

 that they go round their primaries in the same directions 

 that these turn on their axes, in planes diverging but 

 little from their equators, and in orbits nearly circular, 

 there are several significant traits which must not be passed 

 over. 



One of them is, that each set of satellites repeats in 

 miniature the relations of the planets to the sun, both in the 

 respects just named, and in the order of the sizes. On pro- 

 gressing from the outside of the Solar System to its centre, 

 we see that there are four large external planets, and four 

 internal ones which are comparatively small. A like con- 

 trast holds between the outer and inner satellites in every 

 case. Among the four satellites of Jupiter, the parallel is 

 maintained as well as the comparative smallness of the num- 

 ber allows : the two outer ones are the largest, and the 

 two inner ones the smallest. According to the most recent 

 observations made by Mr. Lassell, the like is true of the 

 four satellites of Uianus. In the case of Saturn, who haa 



