26. Regarding the question as a mechanical problem, the answer must be this:--It is so, because the nebula is so much more rare than the matter of the comet, or the resisting medium so much more dense; or combining the two suppositions, because in the case of the comet, the luminous matter has _much_ more inertia, more mechanical reality and substance, than the medium through which it moves; but in the nebula very _little_ more. 27. The numbers of revolutions of the spiral, in the two cases, may not exactly represent the difference of the proportions; but, as I have said, they may serve to show the scale of them; and thus we may say, that if Encke's comet, approaching the centre by 10,000 revolutions, is 100,000 times as dense as the surrounding medium, the elements of the nebula, which reach the centre in a single revolution, are only ten times as dense as the medium through which they have to move.[10] 28. Nor does this result (that the bright element of the nebulæ is so few times denser than the medium in which it moves) offer anything which need surprise us: for, in truth, in a diffused nebula, since we suppose that its parts have mechanical properties, the nebula itself is a resisting medium. The rarer parts, which may very naturally have cooled down in consequence of their rarity, and so, become non-luminous, will resist the motions of the more dense and still-luminous portions. If we recur to the supposition, which we lately made, that the Sun were expanded into a nebulous sphere, reaching the orbit of Neptune, the diffused matter would offer a far greater resistance to the motions of comets than they now experience. In that case, Encke's comet might be brought to the centre after a few revolutions; and if, while it were thus descending, it were to be drawn out into a string of luminous masses, as Biela's comet has begun to be, these comets, and any others, would form separate luminous spiral tracks in the solar system; and would convert it into a spiral nebula of many branches, like those which are now the most recent objects of astronomical wonder. 29. It seems allowable to regard it as one of those coincidences, in the epochs of related yet seeming unconnected discoveries, which have so often occurred in the history of science; that we should, nearly at the same time, have had brought to our notice, the prevalence of spiral nebulæ, and the circumstances, in Biela's and in Encke's comets, which seem to explain them: the one by showing the origin of luminous broken lines, one part drifting on faster than another, according to its different density, as is usual in incoherent masses;[11] and the other by showing the origin of the spiral form of those lines, arising from the motion being in a resisting medium.