dividing the circle into two equal parts.
XVIII. A Semicircle is a figure which is contained under the diameter, and under that part of the circumference which is cut off by the diameter.
In the circle EABCD, E is the center, AC the diameter, ABC the ſemicircle.
XIX. Right-lined figures are ſuch as are contained under right lines.
XX. Three-ſided or Trilateral figures are ſuch as are contained under three right lines.
XXI. Four-ſided or Quadrilateral figures are ſuch as are contained under four right lines.
XXII. Many-ſided figures are ſuch as are contained under more right lines than four.
XXIII. Of Trilateral figures, that is an Equilateral Triangle, which hath three equal ſides; as the Triangle A.
XXIV. Iſoſceles is a Triangle which hath only two ſides equal; as the Triangle B.
XXV. Scalenum is a Triangle whoſe three sides are all unequal; as C.
