The Rotation of the Wing and the Ellipse Traced by Its Tip
Does the wingtip circle like an oar-blade or like a swimmer's hand?
Leonardo studies the looping path of the wingtip, sketching a small bird and, at upper right, an ellipse with a cross-axis and lettered points that maps the tip's orbit. He compares the wingtip moving through air to an oar-blade or a swimmer's hand moving through water. He then poses a doubt about the direction of the loop: as the bird travels along line f a, does the tip drive forward along a b f and return by the upper curve f c a, or does it push backward along the upper line a c f and return by f b a, as a swimmer's hand does?
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Wingtip in air likened to an oar or a swimmer's hand
The tip of the bird's wing guides itself through the air as the tip of an oar does through water, or as the swimmer's arm or hand under water. A doubt then arises about which way round the tip travels its loop between points a, b, c and f.
Ellipse lettered a, c, f, b for the tip's orbit
At the upper right a drawn ellipse with a cross-axis and marked points serves as the geometric path of the wingtip. Its labelled ends are used to phrase the two rival routes, a b f versus a c f, discussed in the text.