§ 94. That spiral arrangement so general among the more diffused nebulæ—an arrangement which must be assumed by matter moving towards a centre of gravity through a resisting medium—shows us the progressive establishment of revolution, and therefore of rhythm; in those remote spaces which the nebulæ occupy. Double stars, moving round common centres of gravity in periods some of which are now ascertained, exhibit settled rhythmical actions in distant parts of our sidereal system. And another fact which, though of a different order, has a like general significance, is furnished by variable stars—stars which alternately brighten and fade. The periodicities of the planets, satellites, and comets, are so familiar that it would be inexcusable to name them, were it not needful here to point out that they are so many grand illustrations of this general law of movement. But besides the revolutions of these bodies in their orbits (all more or less excentric) and their rotations on their axes, the Solar System presents us with various rhythms of a less manifest and more complex kind. In each planet and satellite there is the revolution of the nodes—a slow change in the position of the orbit-plane, which after completing itself commences afresh. There is the gradual alteration in the length of the axis major of the orbit; and also of its excentricity: both of which are rhythmical alike in the sense that they alternate between maxima and minima, and in the sense that the progress from one extreme to the other is not uniform, but is made with fluctuating velocity. Then, too, there is the revolution of the line of apsides, which in course of time moves round the heavens—not regularly, but through complex oscillations. And further we have variations in the directions of the planetary axes—that known as nutation, and that larger gyration which, in the case of the Earth, causes the precession of the equinoxes. These rhythms, already more or less compound, are compounded with each other. Such an instance as the secular acceleration and retardation of the moon, consequent on the varying excentricity of the Earth’s orbit, is one of the simplest. Another, having more important consequences, results from the changing direction of the axes of rotation in planets whose orbits are decidedly excentric. Every planet, during a certain long period, presents more of its northern than of its southern hemisphere to the sun at the time of its nearest approach to him; and then again, during a like period, presents more of its southern hemisphere than of its northern—a recurring coincidence which, though causing in some planets no sensible alterations of climate, involves in the case of the Earth an epoch of 21,000 years, during which each hemisphere goes through a cycle of temperate seasons, and seasons that are extreme in their heat and cold. Nor is this all. There is even a variation of this variation. For the