Figure-eight soaring and hovering motionless in the wind
Right-angle turns of a figure-of-eight glide; two ways wind holds a bird still against cliffs and slopes
Leonardo traces a figure-of-eight glide made with wings and tail drawn in: turning south under an east wind, the bird makes a rectilinear incident motion under the wind, then a reflected motion above the wind toward the east, before turning its front south again, the incident and reflected motions always joining at about a right angle. He then gives two ways the wind holds a bird motionless without wingbeats: when the wind is deflected up the sides of steep mountains or sea-cliffs so the bird's weight exactly balances the wind's resistance, and when the bird sets itself so obliquely above the wind that its power to descend equals the wind's power to resist. A marginal note explains how a rising, circling bird, driven by both wind and its own weight, is lifted like a weight on a wedge when it turns its breast into the wind, so its reflected motion climbs higher than the start of the incident.
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The figure-of-eight glide with right-angle turns
Turning south with an east wind, the bird makes a rectilinear incident motion with wings drawn in under the wind, then a reflected motion with wings and tail open above the wind toward the east, at whose end it turns its front south and regenerates the incident motion. The junction of incident and reflected motion is always about a right angle.
Hovering motionless: weight balanced against wind
The first way the wind holds a bird still is when it is deflected up the sides of steep mountains or sea-cliffs: the bird tilts so that its weight against the wind's front equals the wind's resisting power, and since equal powers do not overcome one another, the bird stays motionless save for an imperceptible vibration. The second is when the bird sets itself so obliquely above the wind that its power to descend equals the wind's power to resist its descent.
Wind and weight lift the circling bird like a wedge
When the bird rises and circles without wingbeats, showing its tail to the wind's origin, it is driven by two powers: the wind striking the concavity beneath the wings, and its own weight descending by a composite obliquity. When it turns its breast into the wind, the wind acts beneath it like a wedge lifting a weight, so its reflected motion rises much higher than the start of the incident.