Erosion and the shifting centre of the earth's gravity
How rivers stripping the mountains lighten the land and move it from the earth's centre of gravity
A pen note argues that the surface of any heavy body grows more remote from its centre of gravity where it becomes lighter. Applying this to the earth, Leonardo reasons that where rivers strip the mountain slopes and carry the material to the sea, the lightened place rises away from the centre of the earth's gravity, which he takes to be always concentric with the centre of the universe. A faint red-chalk profile of a hill with a fortification runs across the top of the sheet.
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Rivers stripping the mountains to the sea
Where rivers erode the slopes of the mountains and carry the material to the sea, the place from which such weight is removed becomes lighter. This lightening is treated as the mechanism by which the body of the earth is reshaped over time.
Surface lightness and distance from the centre of gravity
The part of the surface of any heavy body becomes more remote from the centre of that gravity where it becomes of greater lightness. The eroded, lightened land therefore rises further from the earth's centre of gravity.
Earth's centre concentric with the centre of the universe
Leonardo identifies the centre of the earth's gravity with the centre of the universe, stating the two are always concentric. The lightened land is thus said to move away from the centre of the cosmos itself.
Red-chalk hill and fortification
A faint red-chalk (sanguigna) line across the top of the sheet gives the profile of a hill crowned by a fortification. It continues the series of hilltop stronghold profiles running through these folios.