« AnteriorContinuar »
13. An angle which is greater than a right angle, is called. an obtuse angle. Plate 1. fig. 5.
14. A figure is that which is inclosed by one or more boundaries.
15. A triangle is bounded by three straight lines. 16. Quadrilateral figures are bounded by four straight lines. 17. Polygons are bounded by more than four straight lines.
18. An equilateral triangle is that which has all its fides equal. Plate i. fig. 6.
19. An isosceles triangle is that which has two of its fides equal. Plate 1. fig. 7.
20. A scalene triangle is that whose sides are all unequal. Plate 1. fig. 8.
21. A right-angled triangle is that which has one right angle. Plate 1. fig. 9.
22. The longest side of a right-angled triangle is called the hypothenuse.
23. An acute angled triangle is that whose angles are all acute. Plate 1. fig. 10.
24. An obtuse angled triangle is that which has one obtuse angle. See plate 1. fig. u.
25. A Tquare is a figure whose fides are equal, and all its angles right angles. See plate 1. fig. 12.
26. An oblong is that whose parallel sides only are equal, and all its angles right angles. Plate 1. fig. 13.
27. A rhombus is that which has all its fides equal, but its angles not right angles. Plate 1. fig. 12.
28. A rhomboid is that whose opposite sides only are equal, but its angles not right angles. Plate 1. fig. 13.
29. A trapezium is a four-sided figure, which has none of its sides parallel. Plate 1. fig. 14.
30. A trapezoid is a quadrilateral figure, with two of its sides parallel. Plate 1. fig. 15:
31. A diagonal is a straight line, which joins any two oppofate angles of a quadrilateral figure. Plate 1. fig. 16.
32. A circle is a figure bounded by one curve line, which is called the circumference. Plate 1. fig. 17.
33. The centre of a circle is a point A, within the figure, equidistant from every point in the circumference.
34. The radius of a circle is the distance between the centre and circumference.
35. The diameter of a circle is a straight line drawn through the centre, and terminated both ways by the circumference.
A is the centre.
CD the diameter.
36. An arch is any part of the circumference.
37. The chord of an arch is a straight line, drawn between the extremities of an arch.
38. The segment of a circle is that space contained between the chord and arch of the fame circle.
39. A regular polygon is that whose fides are all equal.
40. An irregular polygon is a figure whose fides are not all equal.
41. Polygons receive names' according to the number of their sides and angles. Thus, A trigon has 3 Gides.
A tetragon 4
42. A mixed angle is that which is formed by one curved line meeting another straight line.
43. A curve-lined angle is that which is formed by the meeting of two curved lines.
1. To make an Equilateral Triangle upon a given line AB.
FROM the centre A, at the distance AB, describe an arch; and from the centre B, with the same radius, describe another arch, cutting the former in C; join CA and CB. Platei. fig.18.
To bisect any given line AB into two equal parts.
Upon B for a centre, with a radius more than the half of AB, describe an arch; and on A for a centre, with the same radius, describe another arch, cutting the former in the points CD: Join CD, and CD will bisect AB in the point E. Plate 2. fig. 19.
To erect a perpendicular from a given point A, in a given line AB.
UPON any point, C for a centre, with the radius CA, de. scribe a circle, cutting the given line also in D; draw the diameter DCE, and join EA; then shall EA be the perpendicular. Plate 2. fig. 20.
To erect a perpendicular from a given point A, in a given line AB,
a nother way.
FROM the given point A, with any radius AC, describe an arch, cutting the given line in C; from C, with the same radius, cut the former arch in D and E; and upon these points as centres, describe arches cutting in R; join RA, and it will be the perpendicular required. Plate 2. fig. 21,
From a given point C, to drop a perpendicular upon a given line AB.
On C, the given point, as centre, with any convenient distance, sweep an arch, cutting the given line in the points DE; and from these points, with any radius more than half their distance, describe arches cutting each other either above or below the line ; join the point of intersection and C, and it will be the perpendicular. Plate 2. fig. 22.
To bifect a given angle ABC.
From B the angular point as centre, describe an arch cutting the containing sides in D,F; on D,F for centres, describe arches of equal radii, cutting each other in E; join BE, which will bisect the angle ABC. Plate 2. fig. 23.
To trifect a right angle ABC.
FROM the angular point B, with any radius describe the arch AC; from C as centre, with the same radius, cut the arch AC in D; and from the centre A, with the same radius cut the arch AC in E; then join DB, EB, and they will trisect the angle. Plate 2. fig. 24.
To draw a line parallel to a given line AB.
FROM any two points, D and E, describe arches of equal radii; draw CF to touch these arches, and CF will be parallel to AB.
To divide a line AB into any number of equal parts.
LET it be required to divide AB into seven equal parts, from A draw AD at any angle; and from B draw BC parallel to AD. On each of these parallel lines lay off as many equal parts as AB is to be divided into : Join the opposite points of division by ftraight lines, passing through AB, and they will divide AB as required. Plate 2. fig. 26.
To find a fourth proportional to three given lines.
MAKE any angle ABC: Set off the first term from B to D, the second from D to A, the third from B to E; join DE, and