River silt, the earth's shifting centre, and a vision test
Floods raise the sea and migrate the world's centre; a blowpipe-versus-open-air test of sight
In densely written mirror-script Leonardo argues that the surface of the sphere of water keeps rising because turbid rivers in flood dump their sediment along the coasts, narrowing the sea's banks and raising its bed. He reasons that as this soil is stripped from mountainsides and carried to the sea, the redistributed weight makes the centre of the world migrate toward our hemisphere. A short note headed "three transverse lines" then proposes proving a point about sight by looking at a white line on a black field with two eyes, one sighting through a blowpipe and the other through the open luminous air.
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Sediment building up the body of the earth
Soil stripped continuously from the shores and sides of the mountains is carried down to the seas, so the water's surface is forced ever higher above the centre of the world. Leonardo treats the earth's surface as something remade over time by this transport of material.
Turbid rivers in flood raise the sea bed
The floods of turbid rivers discharge the soil of their turbulence onto the sea shores, narrow the banks of that sea and, beyond this, raise its bottom. Of necessity, he concludes, the surface of the water rises.
The centre of the world migrates as weight shifts
The centre of the world always changes place in the body of the earth, fleeing toward our hemisphere. The more soil is removed from one side the lighter it becomes, and the more it weighs down the side where the waves deposit it, so the centre must shift.
A white line seen through a blowpipe
Under the heading "three transverse lines" Leonardo notes a demonstration: a white line on a black field is viewed by two eyes at once, one looking through a blowpipe (cerbottana) and the other through the open luminous air. The comparison sets up the pupil experiment continued on the facing leaf.