29. An obtuse angled triangle has one obtuse angle, as A.

30. A figure of four sides and angles, is called a quadrangle, or quadrilateral, as B, C, D, E, F, and G.

31. A parallelogram, is a quadrilateral, which has both pairs of its opposite sides parellel, as B, C, D, and E; and takes the following particular names.

32. A rectangle, is a parallelogram, having all its angles right ones, as B and C.

33. A square, is an equilateral rectangle, having all its sides equal, and all its angles right ones, as C.

34. A rhombus, is an equilateral parallelogram, whose angles are oblique, as D.

35. A rhomboid, is an oblique angled parallelogram, as E. 36. A trapezium, is a quadrilateral, which has neither pair of its sides parallel, as F.

37. A trapezoid hath only one pair of its opposite sides parallel, as G.

38. Plane figures having more than four sides, are in ge. neral called polygons, and receive other particular names, according to the number of their sides or angles.

39. A pentagon is a polygon of five sides, a hexagon hath six sides, a heptagon seven, an octagon eight, a nonagon nine, a decagon ten, an undecagon eleven, and a dodecagon twelve sides.

40. A regular polygon hath all its sides and angles equal; and if they are not equal, the polygon is irregular.

41. An equilateral triangle is also a regular figure of three sides, and a square is one of four; the former being called a trigon, and the latter a tetragon.

42. A circle is a plane figure, bounded by a curve line, called the circumference, which is every-where equidistant from a certain point within called its centre.

43. The radius of a circle is a right line drawn from the centre to the circumference, as a b, at A.

44. A diameter of a circle is a right line drawn through the centre, terminating on both sides of the circumference, as, cd, at B.

45. An are of a circle is any part of the circumference.

46. A chord is a right line joining the extremities of an arc, as ab, at C.

47. A segment is any part of a circle, bounded by an arc and its chord, as D..

48. A semicircle is half the circle, or a segment cut off by the diameter, as E.

49. A sector is any part of a circle bounded by an arc and two radii, drawn to its extremities, as F.

50. A quadrant, or quarter of a circle, is a sector, having a quarter of the circumference for its arc, and the two radii are perpendicular to each other, as G.

51. The height or altitude of any figure, is a perpendicular let fall from an angle, or its vertex, to the opposite side, called the base, as a b, at H.

52. When an angle is denoted by three letters, the middle one is the place of the angle, and the other two denote the sides containing that angle; thus, let a b c be the angle as I, then b will be the angular point, and a b, and b c, will be the two sides containing that angle.

53. The measure of any right lined angle, is an arc of any circle contained between the two lines which form the angle, the angular point being in the centre, as K. Thus if the arc b c d be double of the arc b c then the angle b a d, will be double that of ba c.