The earth is in the very middle of the firmament as the centre for the descent of heavy bodies, i.e., a middle point in a round thing.
As I mentioned, the natural position of the water is around the earth, and if it got space without obstruction from the earth, since it is a heavy, liquid, movable body, it would not stop until it would reach the centre of the earth, and it would remain there, because that, as we mentioned, is the last point of the motion of heavy bodies. And the parts of the water are pressing against each other, seeking the centre of the universe as a natural position for themselves if the firmness of the earth permitted them. Since the earth is round and firm, contending with the water, preventing it going to the centre, the water must be spherical around the earth, thus the other two elements that move upwards from the centre to the surrounding circle have a round shape.
For fire, on account of its lightness, keeps drawing upwards until the firm indestructible sphere of the moon meets it, and since it cannot pass it, it keeps and covers itself under the round axle of that sphere, therefore it must itself be round as is the sphere of the moon that envelopes it (the fire) inside in itself. That sphere is the last course of the motion of light bodies. What makes the air spherical is that it has the surface of the spherical water forced up into its lower boundary and the upper part of the air itself is in the lower boundary of the fire, and since the fire and the water are spherical according to my proof, the air which is enclosed by them must be spherical in accordance with the shape of them.
Such is the position of those very close elements at each other's boundary, that nothing else can be between them, therefore there can be no vacuum in the whole of creation.