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9. An Acute Angle is less than a Right Angle; as EDB. Fig. 3. Note. When three letters are used to express an
Angle, the middle letter denotes the angular Point. 10. A Circle is a round Figure, bounded by a Line cqually distant from some Point, which is called the Centre. Fig. 4.
11. The Circumference or Periphery of a Circle is the bounding Line; as ADEB. Fig. 4.
12. The Radius of a Circle is a Line drawn from the Centre to the Circumference ; as CB. Fig. 4. Therefore all Radii of the same Circle are equal.
13. The Diameter of a Circle is a Right Line drawn from one side of the Circumference to the other, passing through the Centre ; and it divides the Circle into two equal parts, called Semicircles; as AB or DE. Fig. 5.
14. The Circumference of every Circle is supposed to be divided into 360 equal parts, called Degrees ; and each Degree into 60 equal parts, called Minutes ; and each Minute into 60 equal parts, called Seconds ; and these into Thirds, &c. Note. Since all Circles are divided into the same
number of Degrees, a Degree is not to be accounted a quantity of any determinate length, as so many Inches or Feet, &c. but is always to be reckoned as being the 360th part of the Circumference of any Circle, without regarding the big
ness of the Circle. 15. An Arch or Arc of a Circle is any part of the Circumference; as BF or FD, Fig. 5; and is said to be an Arch of so many Degrees as
as it contains parts of 360 into which the whole Circle is divided.
16. A Chord is a Right Line drawn from one end of an Arch to the other, and is the measure of the Arch; as HG is the Chord of the Arch HIG. Fig. 6. Note. The Chord of an Arch of 60 degrees is equal
in length to the Radius of the Circle of which the
17. The Segment of a Circle is a part of a Circle, cut off by a Chord ; thus the space comprehended between the Arch HIG and the Chord HG is called a Segment. Fig. 6.
18. A Quadrant is one quarter of a Circle ; as ACB. Fig. 6.
19. A Sector of a Circle is a space contained between two Radii and an Arch less than a Semicircle; as BCD or ACD. Fig. 6.
20. The Sine of an Arch is a Line drawn from one end of the Arch, perpendicular to the Radius or Diameter drawn through the other end : Or, it is half the Chord of double the Arch; thus HL is the Sine of the Arch HB. Fig. 7.
21. The Sines on the same Diameter increase in length till they come to the Centre, and so become the Radius. Hence it is plain that the Radius CD Fig. 7. is the greatest possible Sine, or Sine of 90 Degrees.
22. The Versed Sine of an Arch is that part of the Diameter or Radius which is between the Sine and the Circumference; thus LB is the Versed Sine of the Arch HB. Fig. 7.
23. The Tangent of an Arch is a Right Line touching the Circumference, and drawn perpendicular to the Diameter; and is terminated by a Line drawn from the Centre through the other end of the Arch; thus BK is the Tangent of the Arch BH. Fig. 7. Note. The Tangent of an Arch of 45 Degrees is
equal in length to the Radius of the Circle of which the Arch is a part.
24. The Secant of an Arch is a Line drawn from the Centre through one end of the Arch till it meets the Tangent; thus CK is the Secant of the Arch BH. Fig. 7.
25. The Complement of an Arch is what the Arch wants of 90 Degrees, or a Quadrant; thus HD is the Complement of the Arch BH. Fig. 7.
26. The Supplement of an Arch is what the Arch wants of 180 Degrees, or a Semicircle ; thus ADH is 27. The Sine, Tangent or Secant of the Complement of any Areh is called the Co-Sine, Co-Tangent or Co-Secant of the Arch; thus FH is the Sine, DI the Tangent and CI the Secant of the Arch DH; or they are the Co-Sine, Co-Tangent and Co-Secant of the Arch BH. Fig. 7.
28. The measure of an Angle is the Arch of a Circle contained between the two Lines which form the Angle, the angular Point being the Centre; thus the Angle HCB. Fig. 7. is measured by the Arch BH; and is said to contain so many Degrees as the Arch does. Note. An Angle is esteemed greater or less accord
ing to the opening of the Lines which form it, or as the Arch intercepted by those Lines contains more or fewer Degrees. Hence it may be observ. ed, that the bigness of an Angle does not depend at all upon the length of the including Lines; for all Arches described on the same Point, and intercepted by the same Right Lines, contain exactly the same number of Degrees, whether the Radius
be longer or shorter. 29. The Sine, Tangent or Secant of an Arch is also the Sine, Tangent or Secant of the Angle whose measure the Arch is.
30. Parallel Lines are such as are equally distant from each other; as AB and CD. Fig. 8.
31. A Triangle is a Figure bounded by three Lines; as ABC. Fig. 9.
32. An Equilateral Triangle has its three sides equal in length to each other. Fig. 9.
33. An Isoceles Triangle has two of its sides equal, and the other longer or shorter. Fig. 10.
34. A Scalene Triangle has three unequal Sides, Fig. 11.
35. A Right Angled Triangle has one Right Angle. Fig. 12.
36. An Obtuse Angled Triangle has one Obtuse Angle. Fig. 13.
37. An Acute Angled Triangle has all its Angles
38. Acute and Obtuse Angled Triangles are called Oblique Angled Triangles, or simply Oblique Triangles; in all which the bottom Side is generally called the Base and the other two, Legs.
-39. In a Right Angled Triangle the longest Side is called the Hypothenuse, and the other two, Legs, or Base and Perpendicular. Note. The three Angles of every Triangle being
added together will amount to 180 Degrees; consequently the two Acute Angles of
a Right Angled Triangle amount to 90 Degrees, the Right Angle
being also 90. 40. The perpendicular height of a Triangle is a Line drawn from one of the Angles to its opposite Side; thus the dotted Line AD. Fig: 14. is the perpendicular height of the Triangle ABC. Note. This Perpendicular may be drawn from either
of the Angles; and whether it falls within the Triangle, or on one of the Lines continued be
yond the Triangle, is immaterial. 41. A Square is a Figure bounded by four equal Sides, and containing four Right Angles. Fig. 15.
42. A Parallelogram, or Oblong Square, is a Figure bounded by four Sides, the opposite ones being equal and the Angles Right. Fig. 16.
43. A Rhombus is a Figure bounded by four equal Sides, but has its Angles Oblique. Fig. 17.
44. A Rhomboides is a Figure bounded by four Sides, the opposite ones being equal, but the Angles Oblique. Fig. 18.
45. The perpendicular height of a Rhombus or Rhomboides is a Line drawn from one of the Angles to its opposite Side ; thus the dotted Lines A B. Fig. 17. and Fig. 18. represent the perpendicular height of the Rhombus and Rhomboides.
46. A Trapezoid is a Figure bounded by four Sides, two of which are parallel though of unequal lengths. Fig. 19. and Fig. 20. Note. l'ig: 19. is sometimes called a Right Angler
47. A Trapezium is a Figure bounded by four unequal Sides. Fig. 21.
48. A Diagonal is a Line drawn between two oppo. site Angles ; as the Line AB. Fig. 21.
49. Figures which consist of more than four Sides are called Polygons; if the Sides are equal to each other they are called regular Polygons, and are sometimes named from the number of their Sides, as Pentagon or Hexagon, a Figure of five or six Sides, &c; if the Sides are unequal they are called irregular Polygons:
PROBLEM I. To draw a Line parallel to another Line, at any given distance ; as at the Point D, to make a Line parallel to the Line AB. PLATE 1. Fig. 22.
With the Dividers take the nearest distance between the Point D and the given Line AB; with that distance set one foot of the Dividers any where on the Line AB, as at E, and draw the Arch C; through the Point D draw a Line so as just to touch the top of the Arch C.
A more convenient way to draw parallel Lines is with a parallel Rule.
PROBLEM II. To bisect a given Line; or to find the middle of it. Fig. 23.
Open the Dividers to any convenient distance, more than half the given Line AB, and with one foot in A describe an Arch above and below the Line, as at C and D; with the same distance, and one foot in B describe Arches to cross the former ; lay a Rule from C to D, and where the Rule crosses the Line, as at E, will