GEOMETRY. EUCLID'S ELEMENTS, BOOK I. Definitions, 1. A point is that which has position, but not magnitude. 2. A line is length without breadth. 3. The extremities of a line are points. 4. A straight line is that which lies evenly between its extreme points. 5. A superficies (or surface) is that which has only length and breadth. 6. The extremities of a superficies are lines. 7. A plane superficies is that in which any two points being taken, the straight line between them lies wholly in that superficies. 8. A plane angle is the inclination of two lines to one another in a plane, which meet together, but are not in the same direction. 9. A plane rectilineal angle is the inclination of two straight lines to one another, which meet together, but are not in the same straight line. NOTE.—When several angles are at one point B, any one of them is expressed by three letters, of which the middle letter is B, and the first letter is on one of the straight lines which contain the angle, : and the last letter on the other line. Thus, the angle contained by the straight lines AB and BC is ex. pressed either by ABC or CBA, and the angle contained by AB and BD is expressed either by ABD or DBA. When there is only one angle at any given point, it may be expressed by the letter at that point, as the angle E. 10. When a straight line standing on another straight line makes the adjacent angles equal to one another, each of the angles is called a right angle; and the straight line which stands on the other is called a perpendicular to it. 11. An obtuse angle is that which is greater than a right angle. 12. An acute angle is that which is less than a right angle. 13. A term or boundary is the extremity of anything. 14. A figure is that which is enclosed by one or more boundaries. 15. A circle is a plane figure contained by one line, which is called the circumference, and is such, that all straight lines drawn from a certain point within the figure to the circumference are equal to one another. 16. And this point is called the centre of the circle, [and. any straight line drawn from the centre to the circumference is called a radius of the circle]. a 17. A diameter of a circle is a straight line drawn through the centre, and terminated both ways by the circumference. 18. A semicircle is the figure contained by a diameter and the part of the circumference cut off by the diameter. 19. A segment of a circle is the figure contained by a straight line and the part of the circumference which it cuts off. 20. Rectilineal figures are those which are contained by straight lines. 21. Trilateral figures, or triangles, by three straight lines. 22. Quadrilateral figures, by four straight lines. 23. Multilateral figures, or polygons, by more than four straight lines. 24. Of three-sided figures an equilateral triangle is that which has three equal sides. 25. An isosceles triangle is that which has only two sides equal. 26. A scalene triangle is that which has three unequal sides. 27. A right-angled triangle is that which has a right angle. 28. An obtuse-angled triangle is that which has an obtuse angle. 29. An acute-angled triangle is that which has three acute angles. 30. Of four-sided figures, a square is that which has all its sides equal, and all its angles right angles. 31. An oblong is that which has all its angles right angles, but not all its sides equal. 32. A rhombus is that which has all its sides equal, but its angles are not right angles. 33. A rhomboid is that which has its opposite sides equal to one another, but all its sides are not equal, nor its angles right angles. 34. Parallel straight lines are such as are in the same plane, and which being produced ever so far both ways do not meet. 35. A parallelogram is a four-sided figure of which the opposite sides are parallel ; and the diagonal is the straight line joining two of its opposite angles. All other four-sided figures are called trapeziums. Postulates. 1. Let it be granted that a straight line may be drawn from any one point to any other point. 2. That a terminated straight line may be produced to any length in a straight line. 3. And that a circle may be described from any centre, at any distance from that centre. Axioms. 1. Things which are equal to the same thing are equal to one another. 2. If equals be added to equals the wholes are equal. |