line DE, and the angles on each side of the perpendicular, are right angles. D A E 6. A Circle is any distance taken in the compasses, with one point fixed, and the other carried round, and the line described thereby, is called the circumference. 7. The Diameter of a circle is a line which divides it into two equal parts, as AB. 8. The Radius of a circle is the distance taken in the compasses to describe it, and is half the diameter, as AC. 9. A Semicircle is either segment of the circle, made by the diameter, as AGB, or AFB. 10. A Quadrant is half a semicircle, and is contained between half the diameter, and a line perpendicular thereto, drawn from the centre of the circle, as CG. 11. A Chord of a circle is a line which divides the circle into two unequal parts, as DE, and it is a chord to both segments of the circle, as DFE and DGE. 12. A Circle is actually, or supposed to be divided into 360 equal parts, called Degrees, and each degree into 60 equal parts, called Minutes; and these into seconds, &c. 13. An Angle is so many degrees, as lines drawn from the centre of a circle, include those parts of the circle, thus BAC is an angle of 30 degrees; because the lines, AB and AC, drawn from the centre of the circle, include it is less than 90 degrees, and BAD contains 90 of those parts, and is therefore an angle of 90°, and is called a right angle, because AD is perpendicular to AB, and DAE contains 110 of those parts, and is an angle of 110 degrees, and is called an obtuse angle, because it is greater than a right angle; consequently CAE must be 130o and an obtuse angle. 14. A Superfices is a plain, bounded by any number of lines; but the fewest number which can inclose a superfices, are three, and it is called a triangle, as ABC, and it is a right angled triangle, because it has one right angle, and the side AC opposite the right angle, is called the hypothenuse, and is always the longest; the other two are called the legs; the upright one BC the perpendicular, and the other, AB, the base.. B 15. All triangles which have not one right angle, are called oblique angled triangles, whether all the angles be acute, as ABC, or one angle obtuse, as DEF. C D F E 16. Any figure of four sides, is called a quadrilateral figure; and if the opposite sides be parallel, it is called a parallelogram; and if all the sides be equal, and angles right, it is called a square, as ABCD. D B 17. A parallelogram, whose opposite sides are equal, and angles right, is called an Oblong, as EFGH. |