How a bird sets its wings to descend and steer

Holding the wings tight to descend against the wind

The bird that descends above or below the wind holds its wings tight, so as not to be sustained or impeded by the air, keeping them firm above its body so the gust cannot turn it upside down.

Spreading the shoulders to retard the descent

When the bird opens its wing-shoulders wider than the tips, it wishes to retard its motion with greater power; by holding the shoulders tight and the tips wide it instead thickens the air ahead to slow itself without straying from its line.

Beating the wings backward to speed the descent

When the bird rows, that is beats, its wings backward during its descent, it is a manifest sign that it increases the speed of that descent. From these dispositions one reads the effects that together reveal the bird's will.

Gathering one wing to turn in a revolving descent

Above the wind, at the end of the reflex motion, the bird never holds its wings equally open, or the wind would overturn it; it gathers to itself the wing around which it will make its revolving motion, descending toward that wing and turning behind it to rise or descend.

Reply to the adversary: the heavier part must guide the fall

The adversary concedes that the bird cannot fall backward edgewise or head-down, but doubts whether it might descend along the line of its wingspan when that lies perpendicular to the earth. Leonardo answers that then the heavier part of the body would not guide the motion, which he has proved impossible.

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