7. A surface or superficies has D

length and breadth, but is considered

as having no thickness; as ABCD.

C

B

8. A superficies may be contained within one curved line, but cannot be contained within fewer than three straight lines.

9. The area of a figure is its superficial content, or the measure

11. Angles are of three kinds, viz., right, acute, and obtuse.

16. The supplement of an angle is what it wants of two right angles; as the angle ACB is the supplement of the angle BCD.

17. A triangle is a figure or superficies bounded by three right lines, and admits of three varieties; viz., equilateral, isosceles, and scalene.

21. Triangles are also right-angled, acute-angled, and obtuseangled.

22. A right-angled triangle has one right angle, the side opposite to which is called the hypotenuse, the other two being termed legs, or one the perpendicular, and the other the base; thus AC is the hypotenuse, BC the perpendicular, and AB the base.

C

B

A

25. The longest side AB of the triangle ABC is called the base; and the line ca falling upon it at right angles from the opposite angle c, is called a perpendicular.

26. A figure of four sides and angles is denominated a quadrangle or quadrilateral figure.

27. A parallelogram is a quadrilateral figure, having its opposite sides parallel and equal, and admits of four varieties; viz., the square, the rectangle, the rhombus, and the rhomboid.

35. Plane figures having more than four sides are generally called polygons, and receive their particular denominations from the number of their sides or angles.

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

37. A regular polygon has all its sides and angles equal. When

they are unequal the polygon is irregular.

38. A circle is a plane figure bounded by a curved line called the circumference, which is everywhere equidistant from a certain point within it, called the centre.

39. The circumference of every circle is supposed to be divided into 360 equal parts, called degrees; each degree into 60 equal parts, called minutes; and each minute into 60 equal parts, called seconds.