Constructing an Angle Equal to a Given Angle
A Euclidean construction worked through intersecting circles and an inscribed triangle
The folio sets out a Euclidean construction headed "To make an angle equal to a given one," begun from a line left indefinite and an open angle whose two ends are joined into a triangle marked a and b. Below, a segment carrying the points h, g, f, d is redrawn with two overlapping circles that meet the line at d and h, and a triangle is inscribed in their common area with base vertices f and g. A closing note, "By the eighth it is proved," refers the demonstration to an earlier proposition. Further faint drawing and writing show through the sheet but were not transcribed.
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Problem: copy a given angle
The heading states the task, "To make an angle equal to a given one." A line is set out and left indefinite, and an open angle is drawn as the starting point of the construction.
The angle closed into a triangle a b
The two ends of the open angle are joined to form a triangle, the joining being called the "ligature a b." The vertices are lettered a and b, written with the notebook turned sideways.
Two circles and an inscribed triangle prove the equality
A segment marked h, g, f, d is redrawn with two superposed circles meeting the line at d and h. A triangle is inscribed in the area common to both circles, its base vertices lettered f and g, and the result noted as "proved by the eighth."