B INTERCALARY 49 If a straight line in a plane be prolonged, its prolongation lies wholly in that same plane. Prop. 13. Nom. If a given straight line in a plane be turned in the situation from which it set out, the plane When a circle is said to be described about the centre A with the radius AB, the meaning is, that it is described by the revolution of the given straight line AB about the extremity A. Prop.13. Cor.3. A circle may be described about any centre and with any radius. Prop.13. Cor. 4. All the radië of the same circle are equal. And circles that have equal radii, are equal. Prop.13. Cor.5. A straight line from the centre of a circle to a point outside, coincides with the circumference only in a point. Prop. 14. Any three points are in the same plane. [That is to say, one plane may be made to pass through them all.] Prop. 14. Cor.1. Any three points which are not in the same straight line being joined, the straight lines which are the sides of the three-sided figure that is formed lie all in one plane. "Prop. 14. Cor.2. Any two straight lines which proceed from the same point, lie wholly in one plane. Prop. 14. Cor.3. If three points in one plane (which are not in the same straight line) are made to coincide with three points in another plane; the planes shall coincide throughout, to any extent to which they may be prolonged. (The above Recapitulation contains the principal matters likely to be referred to. But should reference be made to any thing that is not found in it, recourse is to be had to the Intercalary Book.) FIRST BOOK continued from page 6. NOMENCLATURE. which are not in one and the same straight line, and surfaces called curved. XXXIV. Straight lines which proceed from the same point but do not afterwards coincide, are said to be divergent. XXXV. If through two divergent straight lines of unlimited * Interc,14. length a plane be* made or supposed to pass, and another straight Cor. 2. line of unlimited length be turned about the point from which the two divergent straight lines proceed, continuing ever in the same plane with them, and so travel from the place of one to the place of the other; such travelling straight line is called the radius vectus. XXXVI. The plane surface (of unlimited extent in some direc tions but limited in others) passed over by the radius vectus in travelling from one of the divergent straight lines to the other, See Note. is called the angle between them. A D B' -C E4 the radius vectus goes by the nearest road, direct. When several direct angles are at one point B, any one of them is ex pressed by three letters, of which the letter that is at the vertex of the angle, [that is, at the point from which the straight lines that make the angle, proceed], is put in the middle, and one of the remaining letters is somewhere upon one of those straight lines, and the other upon the other. Thus the angle between the straight lines BA and BC, is named the angle ABC, or CBA ; that between BA and BD, is na ed the angle ABD, or DBA; and that between BD and BC, is named the angle DBC, or CBD. But if there be only one such angle at a point, it may be named from a letter placed at that point; as the angle at E, or more briefly still, the angle E. If the angle intended is the circuitous one, it must be expressed by the use of the term, or something equivalent. But whenever the contrary is not expressed, it is always the direct angle that is meant. XXXVII. 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; and is also said to be at right angles to it. XXXVIII. An angle greater than a right angle, is called obtuse. XXXIX. An angle less than a right angle, is called acute. XL. Angles greater or less than right angles, are called by the common title of oblique. XLI. A straight line joining the ex tremities of any portion of the circum- A B angles A and B, the angles at the cusps. XLII. Figures which are bounded by straight lines, are called rectilinear. Linear figures of all kinds are understood to lie wholly in one plane, when the contrary is not expressed. XLIII. Of rectilinear figures, such as are contained by three straight lines, are called triangles. XLIV. Those contained by four straight lines, are called quad rilateral. XLV. Those contained by more than four, are called polygons. Figures in which a number of sides is specified or intimated, are always understood to be rectilinear, when the contrary is not expressed. D D XLVI. Of triangles, such as have two sides equal, are called isoskeles. XLVII. A triangle which has all its three sides equal, is called equilateral. Hence all equilateral triangles are at the same time isoskeles ; but isoskeles triangles are not all equilateral. XLVIII. A triangle which has a right angle, is called right-angled. The side opposite to the right angle is called the hypotenuse. The other two sides are sometimes called the base and perpendicular. XLIX. A triangle which has an obtuse angle, is called obtuse-angled. L. A triangle which has all its angles acute, is called acule-angled. LI. A triangle which has all its angles oblique, is called oblique-angled. LII. The nomenclature of the various kinds of quadrilateral figures cannot with propriety be given, till these figures have been shown to be capable of possessing certain properties from which their distinctions are derived. It is therefore to be found in the places where such properties are demonstrated. (See the Nomenclature at the end of Propositions XXVIII A, XXXIII, and XXXIV bis, of the First Book. LIII. In any quadrilateral figure, a straight line joining two of the opposite angular points is called a diagonal. For brevity, quadrilateral figures may be named by the letters at two of their opposite angles, when no obscurity arises therefrom. LIV. Of polygons, such as have five, six, seven, eight, nine, ten, eleven, twelve, and fifteen sides respectively, are called a pentagon, hexagon, heptagon, oktagon, enneagon, dekagon, hendekagon, dodekagon, pendekagon. For polygons with other numbers of sides, names might probably be found, or be framed from the Greek ; but they are not in common use. SCHOLIUM.—Henceforward all lines, angles, and figures linear or superficial, whether single or formed by the junction of many, will be understood to lie wholly in one plane, viz. the plane of the paper on which they are represented; when the contrary is not expressed. PROPOSITION I. See Note. PROBLEM.-To describe an equilateral triangle upon a given straight line. Let AB be the given straight line. It is required to describe an equilateral triangle upon it. About the centre A, with the radius AB, de- scribe* the circle BCD; and about the centre B, D A B E crossed by it. From a point in which the circles meet (as for + INTERC. 9. instance C) drawt the straight lines CA, CB, to the points A and Cor. B. ABC shall be an equilateral triangle. Because the point A is the centre of the circle BCD, and C INTERC.13. and B are points in the circumference, AC ist equal to AB. And Cor. 4. because the point B is the centre of the circle ACE, and C and A are points in the circumference, BC is equal to AB. But it has been shown that AC is equal to AB; therefore AC and BC are each of them equal to AB. And things which are equal to the *INTERC. 1. same, are* equal to one another; therefore AC is equal to BC. Wherefore AC, BC, AB are equal to one another, and the triangle ABC is equilateral ; and it is described upon the given straight line AB. Which was to be done. And by parity of reasoning, the like may be done in every other instance. SCHOLIUM. --It has not yet been proved that the place where the two circles cross one another is only a point. There might, therefore, for all that has yet been proved, be more equilateral triangles than one, describable on the same side of AB. Which if it were possible (though it will hereafter be shown that it is not), would in no way affect the accuracy of the assertion that it has been shown how to construct an equilateral triangle upon AB.Referred back to, in the Scholium at the end of Prop. VII of the First Book. * PROPOSITION II. See Note. PROBLEM.- From a point assigned, to draw a straight line equal to a given straight line. First Case. Let A be the point assigned, and BC the given |