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_.

[Illustration: _Fig. 167.-Fig. 184._]

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