The tail as balance; the wing does not press all its area on air
A lever proof from the 'Mechanical Elements' and the porous surface of feathers
Leonardo treats the bird as a balance turning about a centre of rotation (circunvolubile). A bird facing upwind avoids being overturned by lowering the tail to catch a great sum of wind beyond its centre; he proves it 'by the first of the Mechanical Elements', labelling the body d e f, the centre e, and the wind striking a b d e and b c e f, so that the wind on the tail e f exceeds that on d e (text 1). A companion case with the bird lengthwise downwind (d n f, centre n) shows the wind on d n exceeding that on n f, driving that end down and raising the bird to the site of equality (text 2). A final note argues that the whole area of the wing is not employed in pressing the air, since the master feathers have perforations wider than the feathers themselves, so the speculator of birds must not count the wing's full size (text 3). Sketches of birds and a wing accompany the text at right.
On this page
A balance proof from the Mechanical Elements
A bird facing the wind can be overturned unless it lowers the tail to receive a great sum of wind. By the first of the Mechanical Elements, a body in balance struck beyond its centre of gravity sends down the opposite part: with the body d e f and centre e, the wind on the tail e f (beyond the centre) exceeds that on d e, so the bird cannot overturn, especially holding the wings edgewise to the wind.
The lengthwise-downwind case d n f
If the bird lies lengthwise downwind it risks being thrown over unless it raises the tail. With length d n f and centre n, the wind strikes d n more than n f, so d n yields to the wind's course and goes down, raising the bird back to the site of equality.
The wing does not press its whole area on the air
The whole magnitude of the wing is not employed in pressing the air: the perforations between the master feathers are of much wider spaces than the feathers' own width. Therefore the 'speculator of flying creatures' must not reckon the wing's full size, and should note the diverse qualities of wings among all birds.