154. Apsides or Apsis.—One of two points, A, A, of an orbit, oval or ellipse farthest from the axis, or the two small dots.
155. Chord.—A right line, as A, uniting the extremities of the arc of a circle or a curve.
156. Convolute (see also Involute).—Usually employed to designate a wave or folds in opposite directions. A double involute.
157. Conic Section.—Having the form of or resembling a cone. Formed by cutting off a cone at any angle. See line A.
158. Conoid.—Anything that has a form resembling that of a cone.
159. Cycloid.—A curve, A, generated by a point, B, in the plane of a circle or wheel, C, when the wheel is rolled along a straight line.
160. Ellipsoid.—A solid, all plane sections of which are ellipses or circles.
161. Epicycloid.—A curve, A, traced by a point, B, in the circumference of a wheel, C, which rolls on the convex side of a fixed circle, D.
162. Evolute.—A curve, A, from which another curve, like B, on each of the inner ends of the lines C is made. D is a spool, and the lines C represent a thread at different positions. The thread has a marker, E, so that when the thread is wound on the spool the marker E makes the evolute line A.
163. Focus.—The center, A, of a circle; also one of the two centering points, B, of an ellipse or an oval.