Pantometria — A Geometrical Treatize of Measuring

Longimetra.

A semicircle or half-circle contains both the dimetient (diameter) and center of its circle, with the precise half of its circumference.

A right line drawn through the center unto the circumference on both sides is named its diameter or dimetient; the half of it is called its semidiameter.

A semi-circular diagram with labels "An", "half", "circle", and "Dimetient".

All straight lines besides the diameter in any circle pulled from one part of the circumference to the other are called chords.

The portion of the circumference comprehended from that chord is named an arc.

A touch line (tangent) is that which touches a circle at one point.

A circular diagram with horizontal lines labeled "A Chord", "Diameter", "A Chord", and "A touch line", with an arc labeled "an arc".

A triangle labeled "A triangle".

Among right-lined figures, such as have only three sides are triangles, whereof there be sundry sorts bearing several names, according to the diversity of their sides and angles.

If the triangle's three sides are every one of them of like length, it is called an equilateral triangle.