23. The comet of Encke describes a spiral, gradually converging to the sun; but at what rate converging? In how many revolutions will it reach the sun? Of how many folds will its spire consist, before it attains the end of its course? The answer is:--Of very many. The retardation of Encke's Comet is very small: so small, that it has tasked the highest powers of modern calculation to detect it. Still, however, it is there: detected, and generally acknowledged, and confirmed by every revolution of the comet, which brings it under our notice; that is, commonly, about every three years. And having this fact, we must make what we can of it, in reasoning on the condition of the universe. No accuracy of calculation is necessary for our purpose: it is enough, if we bring into view the kind of scale of numbers to which calculation would lead us. 24. Encke's comet revolves round the sun in 1,211 days. The period diminishes at present, by about one-ninth of a day every revolution. This amount of diminution will change, as the orbit narrows; but for our purpose, it will be enough to consider it unchangeable. The orbit therefore will cease to exist in a number of periods expressed by 9 times 1,211; that is, in something more that 10,000 revolutions; and of course sooner than this, in consequence of its coming in contact with the body of the sun. In 30,000 years then, it may be, this comet will complete its spiral, and be absorbed by the central mass. This long time, this long series of ten thousand revolutions, are long, because the resistance is so small, compared with the inertia of the moving mass. However thin, and rare, and unsubstantial the comet may be, the medium which resists it is much more so. 25. But this spiral, converging to its pole so slowly that it reaches it only after 10,000 circuits, is very different indeed from the spirals which we see in the nebulæ of which we have spoken. In the most conspicuous of those, there are only at most three or four circular or oval sweeps, in each spiral, or even the spiral reaches the centre before it has completed a single revolution round it. Now, what are we to infer from this? How is it, that the comet has a spiral of so many revolutions, and the nebulæ of so few? What difference of the mechanical conditions is indicated by this striking difference of form? Why, while the Comet thus lingers longer in the outer space, and approaches the sun by almost imperceptible degrees, does the Nebular Element rush, as it were, headlong to its centre, and show itself unable to circulate even for a few revolutions?