 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.

164. Gnome.—The space included between the boundary lines of two similar parallelograms, the one within the other, with an angle in common.

165. Hyperbola.—A curve, A, formed by the section of a cone. If the cone is cut off vertically on the dotted line, A, the curve is a hyperbola. See Parabola. 167. Hypothenuse.—The side, A, of a right-angled triangle which is opposite to the right angle B, C. A, regular triangle; C, irregular triangle.

168. Incidence.—The angle, A, which is the same angle as, for instance, a ray of light, B, which falls on a mirror, C. The line D is the perpendicular.

169. Isosceles Triangle.—Having two sides or legs, A, A, that are equal.

170. Parabola.—One of the conic sections formed by cutting of a cone so that the cut line, A, is not vertical. See Hyperbola where the cut line is vertical.

171. Parallelogram.—A right-lined quadrilateral figure, whose opposite sides, A, A, or B, B, are parallel and consequently equal.

172. Pelecoid.—A figure, somewhat hatchet-shaped, bounded by a semicircle, A, and two inverted quadrants, and equal to a square, C.

173. Polygons.—Many-sided and many with angles.

174. Pyramid.—A solid structure generally with a square base and having its sides meeting in an apex or peak. The peak is the vertex.

175. Quadrant.—The quarter of a circle or of the circumference of a circle. A horizontal line, A, and a vertical line, B, make the four quadrants, like C.

176. Quadrilateral.—A plane figure having four sides, and consequently four angles. Any figure formed by four lines.

177. Rhomb.—An equilateral parallelogram or a quadrilateral figure whose sides are equal and the opposite sides, B, B, parallel.

178. Sector.—A part, A, of a circle formed by two radial lines, B, B, and bounded at the end by a curve.

179. Segment.—A part, A, cut from a circle by a straight line, B. The straight line, B, is the chord or the segmental line.

180. Sinusoid.—A wave-like form. It may be regular or irregular.

181. Tangent.—A line, A, running out from the curve at right angles from a radial line.

182. Tetrahedron.—A solid figure enclosed or bounded by four triangles, like A or B. A plain pyramid is bounded by five triangles.

183. Vertex.—The meeting point, A, of two or more lines.

184. Volute.—A spiral scroll, used largely in architecture, which forms one of the chief features of the Ionic capital.

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