15. A chord is any line drawn from one part of the circumference of a circle to another part, not passing through the centre, as C D.

m

16. A diameter divides a circle into two equal parts, called semicircles, as C A B r, and C A B D.

17. A chord line divides a circle into two unequal parts, called segments, as C r B D, and C D m.

18. A radius, drawn at right angles to the diameter, cuts off a part of the circle, called a quadrant, as A BO.

19. All plane figures, bounded by three right lines, are called triangles.

20. An equilateral triangle has its sides all equal, as A B C; and all its angles are acute.

B

21. An isosceles triangle has two of its sides equal, as A B C.

A.

22. A scalene triangle has all its sides unequal, as

BC D.

B C is called

the hypothenuse, CD the base, and

BD the

perpendicular.

B

In navigation B D is called the difference of latitude, DC the departure from the meridian, B C represents the distance sailed, the rhumb line, or the point of the compass that the ship sails on; and C B D the angle that her course makes with the meridian, B D.

Triangles are likewise described by their angles. 23. A right-angled triangle, has in it one right angle.

24. An obtuse-angled triangle, has in it one obtuse angle.

25. An acute-angled triangle, has all its angles

acute.

26. All plane figures, bounded by four right lines, are called quadrilaterals.

27. A square is a quadrilateral, having its sides equal, and its angles right angles, as A B D C.

28. A rectangle, oblong square, or parallelogram, has its angles right angles, and its opposite sides are equal and parallel, as B C E D. A line passing from an angle to its opposite angle, in four-sided figures, is called a diagonal, as B E.

B

D

E

29. A rhombus and rhomboid are four-sided figures, whose opposite sides are parallel, but their angles are not right angles.

30. All other four-sided figures, beside those specified, are called trapeziums.

31. Figures having more than four sides are called polygons, and are distinguished by the number of their sides; polygons, of five sides, are called pentagons; of six, hexagons; of seven, heptagons; of eight, octagons, &c.-If their sides and angles are all equal, they are called regular polygons.

32. Parallel right lines are such, which, if infinitely produced, would never meet, as the lines A B and CD would never meet; but it is evident from the

nature of curves, that if A or B, or both, were produced, they would meet and form a circle; as in nature there is no such thing as a straight line.

33. A solid has length, breadth, and thickness, and its bounds are superficies, as ABCDEF is a solid cube, bounded by six square sides, which are equal.

34. A sphere is a solid, described by the revolution of a semicircle about its diameter, which is fixed, and represents the artificial globe: the true shape of the earth or natural globe, being an oblate spheroid, that is, flattened towards the poles.