Earth as a moon, gravity of the elements, and membraned water
Why the earth loses light, whether fire can be condensed, and a fountain of filmed water
Leonardo argues that the earth, performing the office of a moon, has lost much of its ancient light in our hemisphere as its waters have receded, citing a fourth book 'On the world and the waters.' He then reasons about the heaviness of earth and lightness of fire within their spheres, holding that no simple element has weight or lightness in its own sphere and that condensed air (or even condensed fire) gains weight, as a bladder of compressed air shows on the balance. A lower note and drawing treat water that issues in a thin sheet or 'membrane' from a broken concave angle, forming a fountain of varied figures.
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No simple element has gravity or levity in its own sphere
The earth is heavy in its sphere, and the more so the lighter the element surrounding it; fire is light in its sphere, and the more so the heavier the element around it. A bladder filled with air weighs more than when empty because that air is condensed, and even fire, if condensed, could grow heavier than air or water and become equal to earth.
The earth, acting as a moon, has lost its ancient light
Leonardo states that the earth, performing the office of a moon, has lost much of its ancient light in our hemisphere through the falling away of the waters. He refers this proof to a fourth book titled 'On the world and the waters.'
A fountain of 'membraned' water
The value of such thin water-films lies in making a fountain that throws water in various membrane-like figures. Water issuing through the break of a concave angle, touching the angle on one side only, forms the figure shown in the drawing.