The philosophers declare that there are three motions, i.e., the motion from the centre, the motion towards the centre, and the motion around the centre.
Motion from the centre is the motion that proceeds equally out from the earth to every portion of the surrounding circle. Motion to the centre is the motion that proceeds downwards from the surrounding circle to the earth. Motion around the centre is the motion that revolves with the circle, and from its prime motions is produced every motion that is in the world, and one of these is the motion of the four elements, for some of them move from the centre, and some to the centre, and nothing moves around the centre naturally but the firmament or some portion of it.
Of the two elements that move to the centre, earth and water, the motion to the centre is swifter in earth than in water. Of the two elements that move from the centre, air and fire, fire moves more swiftly from the centre than air. And thus the elements that move to the centre are heavy, and the elements that move from the centre are light. From these facts let us conclude that earth is the heaviest element and fire the lightest. And although the water and the air are temperate between them, compared with each other and with the other elements, heaviness and lightness are found in them, for, although water is heavy compared to air, it is light compared to earth, and thus though air is light compared to water, it is heavy compared to fire.
In order to demonstrate this subject more clearly I shall make a figure here below that will elucidate the meaning, of these words. First I shall make a figure of the earth and write A in the middle of it and I shall put the letter B at the top of it, and then I shall make the circle of the firmament around the earth and put C on the east side of it, and D on the top of it, and E on the west of it, and F on the lower part of it, and on the model of the figure there are two of the elemental bodies moving from the centre to the surrounding circle from A to B and these are fire and air.
There are also the two other bodies which move from the surrounding circle to the centre, i.e., from B to A, and these are earth and water. The third prime motion that exists, i.e., the motion around the surrounding circle, which is the motion of the firmament, moves thus, from C to D and from D to E and from E to F and from F to C, and that is sufficient for the experienced.