 Macroscopic objects, of course, do not move at a uniform and very great speed; the atoms within them do, but their motions are restricted and deflected by neighboring atoms, and so they vibrate. In the case of compound objects that are completely at rest, the resultant of internal atomic motions is zero, relative, at least, to the earth, which may have an average motion of its own. If so, and if for some reason the earth’s motion is slower in a downward direction than that of objects on or near its surface — because, say, the earth is disk-shaped, as Epicurus held, and hence sinks more slowly in the surrounding atomic medium, like a falling leaf — then Epicurus could explain as well why things like stones tend to fall to the earth’s surface when let go.