Why the earth's center of gravity lies at the world's center
The sun lightens the earth, rain weighs it down; water issuing from an angled wall
Leonardo asks why the earth's natural center of gravity must coincide with the center of the world, given that parts of the earth continually grow lighter and rise while opposite parts sink, even as the sphere of water keeps its surface equidistant from the world's center. He explains that where the sun stands directly overhead the earth lightens as its waters and snow diminish, while rain and snow elsewhere weigh it down and drive it toward the center, so the water sphere preserves equal distance but not equal gravity. A lower note and drawing describe water pouring through a concave angle into the air as a 'membrane' that clings to the contact side and springs back on the other like an open sheath.
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Why the earth's center of gravity lies at the center of the world
The earth in some part is always growing lighter, and the lightened part thrusts upward while the opposite part submerges, so that the center of its gravity joins the center of the world. Meanwhile the sphere of the water keeps its surface equidistant from the world's center.
The sun lightens the earth; rain and snow weigh it down
Where the sun is directly overhead, the earth covered by the air grows lighter as its waters and snow diminish. On the opposite side rains and snow weigh the earth down and drive it toward the center of the world, thrusting the lightened parts away from that center, yet the water sphere preserves equality of distance but not of gravity.
Membraned water from a concave angle springs back like a sheath
Water pouring through a concave angle into the air becomes membraned. The membrane continues farther on the side of the angle where the water has contact, while on the opposite side it springs back and rejoins the first in the manner of an open sheath.