Of birds.

The flight of birds is always slower in the raising of their wings than in the lowering. And this is caused by the necessary rest that is required after the recent labour of the wearied limbs. And besides this, speed is not necessary in raising these wings, as it is in lowering them, since the impetus that carries the bird is generated over a long space of motion in such a bird, and it suffices it only to have raised the wings, whence they first descended, when the said impetus begins to decline, which is known by the declination of the bird. But when the bird wishes to make itself fast, it takes up the impetus again nearer to its beginning, and beats its wings more often and with as long and swift a motion as is possible to it.

Of the circling motion made by the kite in rising.

The circling motion made by birds in their elevation above the wind arises because with one of its wings it enters upwind and with the other it tempers itself in the straightness of the wind, and besides this it lowers one of the horns of the tail toward the centre of its circular motion; and for this the wind that strikes within it slows the motion of the side that is lower and more toward the centre of such a circle; and this is the cause of the circular motion, and the wing held upwind makes the bird rise to the topmost height of the wind.