1865.] Saturn and its System. 723 



actual phenomena observed on the rings are so utterly irreconcilable 

 with the idea of their continuous fluid or cloud-like structure as to 

 render necessary its adoption in place of the one more generally re- 

 ceived. The grounds on which he bases his opinion are partly a priori, 

 partly certain phenomena which the hypothesis is supposed best to 

 interpret, and partly exact mathematical inquiry. They may be thus 

 stated : — First, rings of disconnected satellites are not uncommon in the 

 solar system ; the ring of asteroids between the orbits of the Earth and 

 Mars ; the belt of small bodies through which the Earth is supposed to 

 pass twice in the year ; the Zodiacal light, &c, being examples. We 

 are not therefore to be unprepared for meeting with them in other 

 situations, as for instance, around Saturn. Secondly, the following 

 phenomena have been observed and require to be accounted for : all 

 the rings often exhibit traces of divisions which form portions of 

 different concentric circles, those on the bright ones being mottled, 

 dusky, and very evanescent. The dark ring is transparent and allows 

 the planet's disc to be seen through it undistorted. In 1856 a darken- 

 ing of the inner bright ring was observed at the points where it lay 

 nearest to the extremities of the apparent longer axis of the dark one. 

 On the disappearance of the rings, a faint hazy bordering on either 

 side of each ansa, and extending outwards to a variable distance from 

 the ball nearest to which it was thickest, has more than once been 

 seen. Lastly, as the result of independent mathematical inquiry, 

 it was proved by La Place that for a solid flat ring to remain in 

 equilibrium about a globe like Saturn's when subjected to the dis- 

 turbances which Saturn's rings actually experience, it must rotate so 

 as to give to each particle the same velocity that it would have if it 

 were a free satellite, and, unless it were imagined to be in unstable 

 equilibrium, that it must be biassed. The degree of bias, or the 

 distance at which the centre of gravity would require to be from the 

 centre of figure, he did not determine. Mr. Clerk Maxwell, taking up 

 the question at this point, has proved that it would require to be nine 

 times as far from its lightest as from its heaviest side, an arrangement 

 which cannot exist, because if it did it would make the appearance of 

 the system very different from what it actually is. Yet if the nebular 

 hypothesis be true the rings must be solid, for on it their formation 

 must have taken place in a period between that 'of the satellites 

 and that of the globe, and as these extremes are solid, the interme- 

 diate bodies would probably be so likewise. 



These, briefly, are the grounds on which Mr. Proctor constructs 

 his hypothesis. Space forbids our doing much more, so far as the 

 a priori part of the evidence is concerned, than to point out bow little 

 real analogy can be established among the cases of ring systems brought 

 forward. Take that of the asteroids. The entire mass of these must 

 fall very far short, on the most liberal calculation, of one-millionth 

 part of the bulk of the Sun ; while Saturn's rings, from their observed 

 effect in disturbing Titan, are considered to be not less than T j^ of his 

 mass. The Zodiacal light again has by no means been so certainly 

 proved to result from flights of small cosmical bodies as to make it 

 prudent to adduce this interpretation as a positive fact in Science, from 



