Centre of gravity and centre of magnitude in a flying bird
Numbered propositions on descent, balance, and how the tail rights a fall
Leonardo works out the balance of a flying bird as a problem of two centres — the centre of gravity and the centre of magnitude (bulk). A small movement of the wing-tip puts it above or below the 'thread of the wind', and the bird's descent is always from the extreme nearest the centre of gravity, with the heaviest part leading; when it floats motionless in the 'site of equality' the two centres coincide (text 1). Numbered propositions then state that a head-down heavy part never rises above the lighter part, that throwing the tail backward rights a tail-down fall while throwing it forward overturns, and that shifting the wings' centre of resistance behind or ahead of the centre of gravity sends the bird head-down or tail-down respectively (texts 2-5). Several small sketches of birds in these attitudes run down the right margin.
On this page
The two centres and the site of equality
The descent of the bird is always from the extreme nearest the centre of its gravity, and its heaviest part stays in front of the centre of its magnitude. When it holds itself in the air without beating its wings in the 'site of equality', this shows the centre of gravity is concentric with the centre of magnitude.
Righting a fall by throwing the tail backward
If the bird falls tail-downward, by throwing the tail backward it straightens itself to the site of equality; if it threw the tail forward it would overturn. This gives the tail's role in recovering level flight.
Wings' centre of resistance behind or ahead of gravity
When the wings' centre of resistance is sent behind the centre of gravity, the bird descends head-downward (First); when it is more forward than the centre of gravity, the bird falls with the tail turned toward the earth (Second).