307 forced; because, of course, there is resistance exerted at the pivots of the axis. For since it happens that the sphere is outside its proper place, it happens also that it exerts weight and is in need of support; that is why the extremities of the axis of the sphere, by exerting weight on the pivots, hinder the motion. But the more the ends of the axis are polished and thin, the less they will suffer resistance: so that, if we imagine them to be indivisibles {1}, then no resistance will develop from them. It also happens that such a motion is retarded by the surface of the sphere, if the surface is rough and unevenly cut: for the air flowing round and retained in the cavities of the surface will hinder the motion, and will not help it, as someone has believed; this will be explained in its place {1}. But if the sphere is heterogeneous, in such a way as to have a center of heaviness outside the center of size [i.e. magnitude], but it rotates around the center of magnitude, in this case, independently of the other accidental causes adduced above, there will be in addition a cause per se why such a motion is not, as the other was, neither natural neither forced, but sometimes natural, sometimes forced. For since the center of heaviness in such a circular motion describes a circle about the center of magnitude, when it goes up from the lowest point towards the highest, it will be moved by force, since it recedes from the center of the world; but when it tends from the highest to the lowest, it will surely be carried by nature.
But since it cannot be raised by the impetus {1} it has received as much as it descends by nature (the cause of this will be explained in its own place) {2}, consequently the difficulty in ascent is greater than the propensity in descent: from this it follows, both because of this and because of the other accidental causes, that it smacks more of the nature of the forced than of the natural.
Aristotle, as in almost all he has written concerning local motion, has written the opposite of the truth on this question also: and this is surely not astonishing, for who concludes true things from the false? Aristotle could not defend the view that that the mover must be in contact with the mobile, unless by saying that projectiles are moved by the air.And he has given testimony of this opinon of his in many places, {1}