The flux and reflux of the sea continually moves the earth, with all its elements, away from the center of the elements. This is proved by the first proposition of this book, which said that the center of the world takes account of that which is greater than it, because no basin is smaller than it. The center of the world is immobile in itself, but the site where it is found is always in motion in various respects. The center of the world continually changes its site; of which changes some have a slower motion than others, inasmuch as some vary every 6 hours and some are accomplished over many thousands of years. But that of 6 hours is born of the flux and reflux of the sea; the other derives from the wearing-away of the mountains by the motions of the waters, born of the rains and of the continual course of the rivers. The site changes about the center of the world, and not the center about the site, because such center is immobile, and the site is continually moved by rectilinear motion, and never will such motion be curvilinear.