Why the Sphere of Water Is Perfectly Round
Enclosed water finds no descent and stays still; weights roll from a plane's edge to its middle
Leonardo argues that water only moves by descending, and uses this to prove that the sphere of water on the earth is perfectly round. A margin proof labels the water sphere a b n and shows that an enclosed quantity of water c cannot move, since it finds no descent and is as far from the center of the world as points a and b. Further notes and diagrams treat a plane of water whose edges run to its middle, and spherical weights that stay put on the frozen water sphere but roll from the edge of a flat plane to its center.
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Water moves only by descending
Water does not move of itself unless it descends, and if it moves of itself it follows that it is descending. From this Leonardo asserts that the sphere of water is perfectly round.
Proof that enclosed water c remains immobile (a b n)
Let a b n be the sphere of water and c a quantity of water enclosed within it. Because a and b are as remote from the center of the world as c, the enclosed water finds no descent, and by the definition of the circle it cannot be surpassed by the surrounding water of equal height. It follows that c stays immobile.
Extremes of a water plane run to its middle (a b c)
Given a plane of water resting on the surface of the sphere of water, its outer extremes a b c will move toward the middle of that plane.
A spherical weight is stable on the frozen water sphere but not on a flat plane
A spherical weight set on the sphere of water where it is frozen will not shift from its place, because every point of that surface is equally distant from the center. But a spherical weight set at the edge of a perfect flat plane will not stay still; it moves at once to the middle of that plane.