Of flight.

...the weight of the water, as has been said; and therefore the rudder was placed behind the ship, where the water, furrowed and opened by the course of the ship, falls back from the banks it has generated into the lowness of the created concavity, and in its descent strikes the rudder with more or less power, according as the falling water strikes it between angles of greater or lesser width. And besides this, the course of the water, closing up again that center of the aforesaid concavity, falls with impetus upon the percussion of the rudder, as has been said.

But at the tail of the kite it is the percussion of the air that with fury shuts up the vacuum which the motion of the bird leaves behind it; and this is done from every aspect of that left-behind vacuum.

p - m - n o r S - q; a - d c - b e

The vacuum that the bird, penetrating the air, successively leaves behind it is struck on its sides by that part of the bird which most exceeds the space that is outside its central line.

The percussion that the sides of the bird make on the sides of the air they have penetrated, with those parts that exceed by their distances around the central line of its motion: if they are forward of the middle on its right side, then the straight motion will curve toward the right side; and if they are behind the middle on the opposite side, then that straight motion will curve toward the left side; and it will likewise do the same when struck forward of the middle on the left side. As for the left side behind, and the side below or above, and from every aspect, it will be told in its place, etc.

The bird that, without motion of its wings, rises on high by favor of the wind descends half of what it rises when moving up-wind, when it turns its tail to that wind — and the more so the greater the circle.

The spread tail of the bird feels such a proportion of the whole weight of the bird as is the proportion that the open tail has to the remainder of the bird — with trunk, neck, head, and open wings. And the less so, the nearer the center of gravity of the whole bird is to the center d e of the trunk than to that of the tail.

The air runs behind, into the vacuum that the air-piercing bird leaves behind it, as fast as the bird flees before the air that is continually joined to it by its contact. Therefore it is not the closing-up of the air, which is behind the bird, that drives the bird forward before it; rather, the impetus that moves the bird forward opens and drives away the air, which makes itself a sheath for it, and behind it draws the air after it, etc.