Of the flyers: bearing weight and the wing that lifts an eighth
How a flyer's weight rests on feet or seat, and the levers of the flight instrument
Under the heading 'Of the flyers,' folio 83v reasons about where a flying creature's weight is borne — on its feet or on the place where it sits — and shows that the force left in the legs equals whatever load is taken off them. Turning from bird to machine, Leonardo resolves to make the flight instrument's mover exert force at the first degree of its power, through manageable levers and counter-levers that can be moved and withdrawn quickly at will. A small sketch of a bird with outspread wings (lettered a, b) accompanies a 'Demonstration' noting that the wing sustains an eighth of the bird's weight while it balances on the wind passing beneath it.
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Where the flyer's weight rests, and the force left in the legs
If the flyer's weight loads onto its feet, the feet exert force equal to that weight; if instead the weight rests where the creature sits, the legs retain in potential the same force they had sustained. Restoring the weight subtracts from the legs exactly the force added, so the two cases come to the same thing.
The flight instrument's levers and counter-levers
Attention turns to the instrument, whose mover is to employ its force at the first degree of its power. The levers and counter-levers are to be manageable and fit for every need, prepared and moved and withdrawn quickly according to the mover's will.
The wing bears an eighth of the bird's weight
The bird governs itself upon its wings, whose 'hand' sustains an eighth of its weight, doing so only while balancing on the course of the wind passing beneath it. When the wind fails and it will not descend, it must move the whole wing along its full length.
Demonstration sketch of a bird with outspread wings (a, b)
A small pen sketch of a bird with wings spread wide, its span lettered a and b, sits mid-text beside the word 'Demonstration.' It illustrates the balancing of the creature on the wind described just below.