On Gravity, Weight and Levity Among the Elements
Gravity as daughter of motion, tested by a balance weighed under water
These closely written notes define gravity (gravita) as an accidental power, daughter and mother of motion and sister of impetus and percussion, and argue that every heavy body desires its centre to be common to all the elements. No element has weight within its own sphere; weight arises only when one element is forced into a lighter one and struggles to return home, so that what moves toward the centre is called weight and what flees it levity. A balance experiment shows a skin of air submerged in deep water balancing a like bag held in the air, with the water surface a b acting as the pole of the two weights. Small lettered and numbered diagrams of the balances appear in the right-hand column.
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Gravity, force and percussion as powers of motion
Every heavy body desires that its centre be common to all the elements. Gravity and force are called daughters of motion and sisters of impetus and percussion, generators of motion as much as generated by it; of these three accidental powers, two share one nature in birth, desire and end.
No element weighs in its own sphere; weight versus levity
No element has weight in its own sphere; an element that by accident passes into a lighter one at once causes gravity and, not being sustained, falls back to its element, where that gravity dies. That which moves toward the centre is weight, that which flees it levity, each of equal power and motion.
Balance of air and water skins; the surface as pole
A balance under water loaded with weights of air behaves as a balance in air loaded with weights of water. A skin full of air weighs in deep water as much as a like bag in the air; tying a water-skin and an air-skin together shows the water surface a b to be the pole of the two weights, and turning them over leaves both weights dead.