How the Tail Turns and Steadies a Bird in Flight
Lowering one side of the tail curves the flight; alternate beats hold balance
Leonardo analyses the flyer's tail: when one side of it drops rapidly, the air beneath condenses and resists more, so the bird is forced to incline the opposite way and its path becomes curved, circling around the lowered part of the tail. When instead the two 'horns' of the tail are lowered alternately, now right, now left, no circular motion results; this is simply a way of striking the air as the wings would. Finally, when the bird feels itself tilting to one side, the tail beats the air on the opposite side and so resists that inclination. The whole page is written in mirror script with no accompanying drawing.
On this page
Curving the flight by lowering one side of the tail
When one side of the tail lowers with rapid motion, the air it strikes condenses and resists more, so the bird necessarily inclines with its opposite part. Its motion then becomes curved, circling around the lowest part of the tail.
Alternating tail-beats to strike the air and hold balance
When the two horns of the tail are lowered by turns, right then left, no circular motion results; it is only a way of beating the air, as the wings do. And when the bird feels itself declining to one side, the tail beats the air on the opposite side and resists the tilt.