Sizing a man's wings from a bird's span and weight
The wing-span is the square root of the weight: a bird spans 5 braccia at 25 pounds
Leonardo works out how large a flying man's wings must be by comparing them with a water-bird. The bird spans 5 braccia and weighs 25 pounds, so the span is the square root of the weight; a man weighing 400 has root 20, so his wings must span 20 braccia. Taking the bird's wing width as 3/4 of a braccio (3/20 of its length), he scales the man's wing to about 3 braccia at its widest. Alongside are columns of arithmetic and drawings of wing mechanisms and bird wings.
On this page
Wing-span scaled by the square root of the weight
A water-bird spans 5 braccia and weighs 25 pounds, so the span is the root of the weight. A man is 400 and his root is 20, giving a 20-braccia wing-span. The bird's wing width is 3/4 of a braccio, i.e. 3/20 of its length, so the man's wing should be about 3 braccia wide at its widest.
Arithmetical computations
Below the figure stand columns of numbers, including 400, 55, 5, 198, 80, 31, 32 and 261, working through the proportions and roots used in sizing the wings. They are calculations rather than continuous prose.