THE ELEMENTS OF EUCLID. BOOK 1. A P DEFINITIONS. I. POINT is that which hath no parts, or which hath no magnitude. II. A line is length without breadth. III. The extremities of a line are points. IV. A straight line is that which lies evenly between its extreme points. ས. A superficies is that which hath only length and breadth. VI. The extremities of a superficies are lines. VII. A plane superficies is that in which any two points being taken, the straight line between them lies wholly in that superficies. VIII. "A plane angle is the inclination of two lines to B one another in a plane, which meet together "but are not in the same direction." IX. 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. N. B. When several angles are at one point 'B, any one of them is expressed by three letters, of which the letter that is at the vertex "of the angle, that is, at the point in which the 'straight lines that contain the angle meet one another, is put between the other two letters, ' and one of these two is somewhere upon one of those straight lines, and the other upon the other line: Thus the angle which is contained 'by the straight lines, AB, CB, is named the 'angle ABC, or CBA; that which is contained 'by AB, DB, is named the angle ABD, or DBA ; and that which is contained by DB, CB, is called the angle DBC, or CBD; but, if 'there be only one angle at a point, it may be 'expressed by a letter placed at that point; as 'the angle at E.' X. 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. XI. An obtuse angle is that which is greater than a right angle. XII. An acute angle is that which is less than a right angle. XIII. "A term or boundary is the extremity of any thing." XIV. A figure is that which is inclosed by one or more boundaries. XV. 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. XVI. And this point is called the centre of the circle. XVII. A diameter of a circle is a straight line drawn through the centre, and terminated both ways by the circumference. XVIII. A semicircle is the figure contained by a diameter and the part of the circumference cut off by the diameter. XIX. "A segment of a circle is the figure contained by a straight line, and the circumference it "cuts off." XX. Rectilineal figures are those which are contained by straight lines. XXI. Trilateral figures, or triangles, by three straight lines. XXII. Quadrilateral, by four straight lines. XXIII. Multilateral figures, or polygons, by more than four straight lines. XXIV. Of three-sided figures, an equilateral triangle is that which has three equal sides XXV. An isosceles triangle is that which has only two sides equal. XXVI. A scalene triangle is that which has three unequal sides. XXVII. A right-angled triangle is that which has a right angle. XXVIII. An obtuse-angled triangle is that which has an obtuse angle. XXIX. An acute-angled triangle is that which has three acute angles. A A |