80.
The sphere of the earth has gravity which increases in proportion to the lightness of the element which contains it.
Fire is light in its sphere and its lightness increases in proportion to the weight of the element which contains it.
No primary element has gravity or lightness in its own sphere.
81.
The motion made by bodies which possess gravity to the common centre is not produced by the tendency of the body to find this centre, nor is it caused by attraction made by the centre, as by a magnet, drawing the weight towards it.
82.
Why does not the weight remain in its place?
It does not remain because it has no resistance.