Squaring the circle: sectors, chords, and Archimedes' rule
Geometric quadrature of circles and their portions, with a 96-sided polygon giving the ratio 3 1/7
A crowded geometry sheet devoted to squaring the circle and its parts. Leonardo asserts that circles stand in proportion as the squares of their diameters, and sketches sectors, chords, inscribed squares and triangles. He outlines a method using a pyramid and 'the rule of Archimedes' to approximate the quadrature of a lesser circle, a marginal note gives a 96-sided polygon's side-to-diameter ratio as 3 and 1/7, and a lower passage explains squaring a portion by arc and chord. Dense mirror-script surrounds the many small figures.
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Squaring the circle by Archimedes' rule
Leonardo states that circle is to circle as square is to square built on their diameters, so a thousandth part of a circle of 'a million' equals a circle of 'a thousand.' He proposes forming a pyramid whose length is the semidiameter of the greater circle and whose base, multiplied by half its length, yields five hundred thousand, giving the quadrature of the lesser circle to within an imperceptible trifle. By the rule of Archimedes fewer than 999 such trifles remain, each reducible near to a mathematical point.
The 96-sided polygon and the ratio 3 1/7
A marginal note records that the sides of a figure having 96 equal sides stand to the diameter as 3 and 1/7 to one, 'or a little more.' This is Leonardo working with the classical inscribed-polygon approximation to the ratio of circumference to diameter.
Squaring a portion of a circle by arc and chord
To square any portion of a circle, Leonardo says one must first see what fraction its curved arc is of the whole circumference, complete the sector to that portion, and detach the portion from the sector. After removing the rectilinear triangle from the square f, the remainder of f equals the portion; and here, he adds, arithmetic is of no use.
Circles as the squares of their diameters
The sheet pairs squares and semicircles labelled a thousand and a million, and squares of 1000 and 1,000,000 braccia. Leonardo compares their areas as the ratio of the squares of their linear measures, noting proportions of 1 to 100 and 1 to 10000.
