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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



building. How small would be the chance that 

 any two particles from that tiny heap would 

 come into collision during months of their aerial 

 wanderings ! Very much smaller would be the 

 risk of a single collision between asteroids dur- 

 ing millions of years as they travel (all the same 

 way round, be it noticed) on their wide orbits, 

 even though their number were a hundred-fold 

 greater than it is, and their volumes increased a 

 million-fold. 



Either, then, we must imagine innumerable 

 millions of years to have elapsed since the ring 

 of asteroids first existed, and that very gradually 

 the synchronous asteroids have been eliminated 

 by collisions, or else we are forced to the con- 

 clusion that the formation of this ring of worlds, 

 or rather this series of rings, belongs to an ear- 

 lier era of our solar system's history, when the 

 matter whence the rings were one day to be 

 formed was in the nebulous condition. It ap- 

 pears to us that the latter conclusion is alto- 

 gether the more probable. We escape none of 

 the difficulties of the problem by adopting the 

 former conclusion, while many other difficulties 

 are introduced. By the latter, we simply have 

 the same difficulties to encounter which apper- 

 tain to all forms of the nebular hypothesis re- 

 specting the origin of the solar system. These 

 difficulties are great, because the distance over 

 which we endeavor to look back is great ; but 

 they are not insuperable. The positive evidence 

 for the general theory becomes stronger and 

 stronger as astronomical research advances ; and 

 the mere circumstance that it is surrounded by 

 difficulties can in no sense lead us to abandon 

 it, although compelling us to admit that as yet 

 we have not thoroughly mastered its details. 

 The asteroids themselves supply an argument in 

 favor of the nebular theory, rendering its proba- 

 bility so strong as practically to amount to cer- 

 tainty ; for the antecedent probability against the 

 observed uniformity of direction of the 175 aste- 

 roids by chance, or in any conceivable way ex- 

 cept as the result of some process of evolution, 

 is equal to that of tossing either "head" or 

 "tail" 175 times running, or about 

 23,945,290,000,000,000,000,000,000,000,000,000,- 

 000,000,000,000,000,000 to 1. 



Adopting the nebular theory, we must of 

 course adopt with it the conclusion respecting 

 the origin of the asteroids, to which, as we have 

 seen above, we are led by the examination of the 

 relations presented by this system — viz., that 

 while still existing as a great ring of nebulous 

 masses, they were to such degree perturbed by 



Jupiter's mighty attraction upon them, as on the 

 one hand to be prevented from forming into a 

 single planet, and on the other to be sorted out, 

 if one may so speak, into several rings with well- 

 marked gaps between them, these gaps corre- 

 sponding exactly with the distances at which 

 planets would be most effectively disturbed by 

 Jupiter. The close accordance between the re- 

 sults to which we are lpd by a posteriori and a 

 priori considerations affords strong evidence in 

 favor of both lines of reasoning. But it is very 

 noteworthy, also, that when seeing the proba- 

 bility of the conclusions toward which we have 

 been led, we inquire whether any similar case 

 exists within our solar system, and, if so, whether 

 | the evidence in that case corresponds with that 

 which we have obtained in the case we have 

 been considering, we find the most striking evi- 

 dence of all. The ring system of Saturn has 

 long been regarded as consisting of multitudes of 

 minute satellites. Thus it resembles the zone of 

 asteroids, only it is relatively much more crowded. 

 Now, in the ring system of Saturn there are caps 

 or relatively vacant divisions separating rings of 

 closely-clustering satellites. Distinguished among 

 all these gaps by superior breadth and darkness 

 is the great division separating what were for- 

 merly called the two rings from each other. 

 Here, for a breadth of nearly 2,000 miles, so few 

 satellites travel, that to ordinary observation the 

 great division looks black, though, closely scruti- 

 nized, it is found to be simply very dark. Now 

 when we inquire whether satellites moving round 

 this open space would have periods synchronizing 

 with that of the innermost (and therefore most 

 effectively disturbing) of his moons, we discover 

 these remarkable facts — that a satellite would 

 travel in the very middle of the dark division or 

 open space if its period were one-half that of the 

 innermost of Saturn's moons, and almost on the 

 same track, if its period were one-fourth that of 

 the innermost moon but two, while it would be 

 well within the open space, but nearer its inner 

 edge, if its period were one-third that of the in- 

 nermost moon but one, or one-sixth that of the 

 innermost moon but three. It follows unmistak- 

 ably from these relations, first noted by Prof. 

 Kirkwood, that the great division in Saturn's 

 rings has been swept and garnished by the action 

 of the four innermost of Saturn's moons, but 

 especially by the innermost of all. This fact 

 corresponds so well with the nebular hypothesis, 

 and is so utterly inexplicable on any other, as 

 strongly to corroborate an opinion, expressed by 

 the present writer twelve years ago, that the 



