Tail resistance and the circles of a bird's flight
The tail retards the motion; the less oblique it is held, the smaller the circles of flight
The leaf carries a passage in mirror-writing that continues an argument begun on the facing folio. Leonardo reasons that the tail meets somewhat more resistance than the wings, so it retards the motion, and that it moves less than the wings do. Necessity, he concludes, drives the bird into circular motion: the less oblique the tail, the smaller the diameters of those circles, and conversely.
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How the tail governs the curvature of flight
The tail receives somewhat more resistance than the wings and so retards the motion, moving less than they do. Necessity makes the bird travel in a circular path, and the less oblique the tail is held the smaller the diameter of its circles, and the reverse when the tail is more oblique.