# 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.

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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154 *apsides* Or *apsis*
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