[10] We assume here that the number of revolutions to the centre is greater in proportion as the relative density of the resisting medium is less; which is by no means mechanically true; but the calculation may serve, as we have said, to show the scale of the numbers involved.
[11] Humboldt, whom nothing relative to the history of science escapes, quotes from Seneca a passage in which mention is made of a Comet which divided into two parts; and from the Chinese Annals, a notice of three "coupled Comets," which in the year 896 appeared, and described their paths together. Cosmos, iii. p. 570, and the notes.
[12] Laplace has proved that the masses of comets are very small. He reckons their mean mass as very much less than 1-100000th of the Earth's mass. And hence, considering their great size, we see how rare they must be. See Expos. du Syst. du Monde.
[13] Humboldt repeatedly expresses his conviction that our Solar System contains a greater variety of forms than other systems. (Cosmos, iii. 373 and 587.)
CHAPTER VIII.
THE FIXED STARS.
1. We appear, in the last chapter, to have cleared away the supposed inhabitants of the outskirts of creation, so far as the Nebulæ are the outskirts of creation. We must now approach a little nearer, in appearance at least, to our own system. We must consider the Fixed Stars; and examine any evidence which we may be able to discover, as to the probability of their containing, in themselves or in accompanying bodies, as planets, inhabitants of any kind. Any special evidence which we can discern on this subject, either way, is indeed slight. On the one side we have the asserted analogy of the parts of the universe; of which point we have spoken, and may have more to say hereafter. Each Fixed Star is conceived to be of the nature of our Sun; and therefore, like him, the centre of a planetary system. On the other side, it is extremely difficult to find any special facts relative to the nature of the fixed stars, which may enable us in any degree to judge how far they really are of a like nature with the Sun, and how far this resemblance goes. We may, however, notice a few features in the starry heavens, with which, in the absence of any stronger grounds, we may be allowed to connect our speculations on such questions. The assiduous scrutiny of the stars which has been pursued by the most eminent astronomers, and the reflections which their researches have suggested to them, may have a new interest, when discussed under this point of view.
2. Next after the Nebulæ, the cases which may most naturally engage our attention, are Clusters of stars. The cases, indeed, in which these clusters are the closest, and the stars the smallest, and in which, therefore, it is only by the aid of a good telescope that they are resolved into stars, do not differ from the resolvable nebulæ, except in the degree of optical power which is required to resolve them. We may, therefore, it would seem, apply to such clusters, what we have said of resolvable nebulæ: that when they are thus, by the application of telescopic power, resolved into bright points, it seems to be a very bold assumption to assume, without further proof, that these bright points are suns, distant from each other as far as we are from the nearest stars. The boldness of such an assumption appears to be felt by our wisest astronomers.[1] That several of the clusters which are visible, some of them appearing as if the component stars were gathered together in a nearly spherical form, are systems bound together by some special force, or some common origin, we may regard, with those astronomers, as in the highest degree probable. With respect to the stability of the form of such a system, a curious remark has been made by Sir John Herschel,[2] that if we suppose a globular space filled with equal stars, uniformly dispersed through it, the particular stars might go on forever, describing ellipses about the centre of the globe, in all directions, and of all sizes; and all completing their revolutions in the same time. This follows, because, as Newton has shown, in such a case, the compound force which tends to the centre of the sphere would be everywhere proportional to the distance from the centre; and under the action of such a force, ellipses about the centre would be described, all the periods being of the same amount. This kind of symmetrical and simple systematic motion, presented by Newton as a mere exemplification of the results of his mechanical principles, is perhaps realized, approximately at least, in some of the globular clusters. The motions will be swift or slow, according to the total mass of the groups. If, for instance, our Sun were thus broken into fragments, so as to fill the sphere girdled by the earth's orbit, all the fragments would revolve round the centre in a year. Now, there is no symptom, in any cluster, of its parts moving nearly so fast as this; and therefore we have, it would seem, evidence that the groups are much less dense than would be the space so filled with fragments of the sun. The slowness of the motions, in this case, as in the nebulæ, is evidence of the weakness of the forces, and therefore, of the rarity of the mass; and till we have some gyratory motion discovered in these groups, we have nothing to limit our supposition of the extreme tenuity of their total substance.