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.