17 Angle of a section is that which is comprehended of the bounds of a section.
As here eai: And eia.
18 A section is either a semicircle: or that which is unequall to a semicircle.
A section is two fold, a semicircle, to wit, when it is cut by the diameter: or unequall to a semicircle, when it is cut by a line lesser than the diameter.
19 A semicircle is the half section of a circle.
Or it is that which is made the diameter.
Therefore
20 A semicircle is comprehended of a periphery and the diameter 18 d j.