On the Tremor of the Earth; a Bird Gliding Under the Wind
Underground streams deepen their beds, rivers build land, and new islands are born
Under the heading on the vibration of the earth, Leonardo observes that subterranean water courses, like surface ones, continually wear away and deepen their beds, while earth lifted by rivers is discharged in the lowest reaches of their course. He explains that fresh water welling up at the sea's surface is a sign that an island is forming, sooner or later according to how much water surges. A short note beside a small sketch of a bird in flight, at the lower right, describes how a gliding bird bends its course toward the wing it draws in.
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Underground streams deepen their own beds
The subterranean courses of water, like those made between the air and the earth, continually consume and deepen the beds along which they run. Leonardo treats erosion below ground as the same process he observes in open rivers.
How a new island is born in the sea
Where fresh waters well up at the surface of the sea, Leonardo reads a manifest sign that an island is being created, appearing sooner or later according to the quantity of water that surges. The island is generated from earth and from the stones worn away by the underground water course.
Rivers carry earth down to their lowest reaches
Earth lifted by the rivers from their high courses is discharged in the last, lowest parts of their motion. The moving water is thus both an agent of erosion upstream and of deposition downstream.
A gliding bird turns toward its drawn-in wing
The note beside the flight sketch states that a bird descending obliquely without beating its wings, beneath the coming of the wind, bends its straight course toward the side on which one wing draws itself in. The small pen sketch at the lower right shows just such a gliding bird.