The Surveyor in Foure Bookes

half of which Diameter is called the Semidiameter of the same Circle.

Euclid, Book 1, Def. 17.

As the line A-B-C in this circle is the diameter thereof, for that it passeth by the centre B through the whole circle, as from A to C, and also divideth the circle into two equal parts, the one half towards D, the other towards E. Either half of which Diameter, as A-B or B-C, is called the Semidiameter of the same Circle.
PROB. 34.

A circle with a horizontal diameter line labeled A-B-C, with D above the center and E below the center.

DEFIN. XII.

A Semicircle is a Figure contained under the Diameter of a Circle, and the semicircumference of the same Circle.

Euclid, Book 1, Def. 18.

As supposing the circle A-B-C, the Diagram of the last DEFINITION, to be divided into two equal parts by the diameter thereof A-C, whereby two Figures are projected, namely, A-D-C and A-E-C. Now by this DEFINITION, each of these Figures are Semicircles, for that one of them is contained under the diameter of the same circle A-C and the semicircumference A-D-C, and the other under the same diameter, and the semicircumference A-E-C.

DEFIN. XIII.

A Segment, Section, or Portion of a Circle, is a Figure contained under a right line, and part of the circumference, either greater or less than the Semicircle.

Euclid, Book 1, 3. Def. 5.

As the diameter of a circle passing by the centre thereof, divideth the same into two equal parts; so any right line, drawn from any one part of the circumference to any other part thereof (and not passing by the centre) divideth the circle into two unequal parts, which are called Segments, Sections, or Portions of a circle: As in this circle, the figure A-C-B, because it is contained under the right line A-B and the lesser part of the circumference A-C-B, is called a lesser Segment, Section, or Portion of a circle; and the figure A-D-B, because it is contained under the right line A-B and the greater part of the circumference A-D-B, is called a greater Segment, Section, or Portion of a circle. Here note also, that these parts and such like of the circumference so divided, are commonly called Arches, or arch lines. And all lines (less than the diameter) drawn

A circle with a horizontal chord line labeled A-B, with C on the upper arc and D on the lower arc.