Definition IV.
BEsides these names of the sections, there are several others made use of to denote various lines drawn and considered both within and without those sections, the chief whereof we shall here explain. And first of all, in general the line EF so let fall thro' the middle of the section from its top E (which is called the Vertex of the section) to the diameter of the base of the Cone AC (produced if occasion be) that it shall bisect the line GH and all others parallel to it, is called the Di∣ameter of that Section; and particularly it is called the Axis of the Section if it cuts them at right angles or perpendicularly; as also they name those lines GH, KN, &c. which are cut in∣differently by the diameters, but at right angles by the Axis, those, I say, they call Ordinates, or Ordinate Applicates, and their halves, FG, IK, &c. Semiordinates, (or some also call the latter Ordinates, and the former double Ordinates) and the parts of the Ax or Diameter EF, EI, &c. are called Abscis∣sa's (by some intercepted axes and diameters)