f e d

c b a

compound circular motion

The rising with reflex motion is always made with the wings and tail entirely open, and the descending of the incident motion is made with the wings and tail largely closed.

The imperceptible fanning of the wings, without any other beating of them, keeps the bird balanced and level within the moving air.
The reflex motion against the oncoming wind will always be greater than its incident motion; and the reflex motion, made together with the course of the wind, will be increased by that wind and will become equal to the incident. The modes of birds' rising without beating of wings, but with the favour of the wind, made circularly, are of two natures, that is simple and compound. Simple are those which with the incident motion proceed along the flight of the wind, at the end of which they turn against the oncoming wind, receiving beneath themselves its impact, and finish the reflex motion against the wind. The compound motion of birds' rising is likewise circular and has incident and reflex motion against the oncoming wind in the course made in a semicircle, and has incident and reflex motion that follows the course of the wind. The simple circular motion of rising without beating of wings is always made in the great motions of the winds; and that this is true, the bird is carried far in such rising together with the course of the wind. The compound motion will be made in weak winds, because experience shows that in such compound motions the bird rises within the air without being too much carried by the wind toward the places where it travels. The down and feathers under the wings: much use is made of the flexible, or bendable, edges at the extremities of the wings and of the tail, and hard in front, where the air is cleft.