What kind of a line is _s t_? Why? What kind of a line is 9 10? Why? What is a spiral line? What kind of a line is _w x_? Why? LESSON SECOND. REVIEW. Read all the straight lines. (DIAGRAM 2.) Why is _m n_ a straight line? Define a straight line. Read all the crooked lines. Why is 7 8 a crooked line? Define a crooked line. Read all the curved lines. Why is 5 6 a curved line? What is a curved line? Read all the wave lines. Read all the spiral lines. Why is 3 4 a spiral line? Why is _u v_ a spiral line? What is a spiral line? [Illustration: Diagram 2.] [Illustration: Diagram 3.] LESSON THIRD. POSITIONS OF LINES. Let the pupils hold their books so that they will be straight up and down like the wall. VERTICAL LINES. The straight line _a b_ points to the center of the earth. (DIAGRAM 3.) It is called a vertical line. Name all the vertical lines. _A vertical line is a straight line that points to the center of the earth._ HORIZONTAL LINES. The straight line _o p_ points to the horizon. It is called a horizontal line. Read all the horizontal lines. _A horizontal line is a straight line that points to the horizon._ OBLIQUE LINES. The line _s t_ points neither to the center of the earth nor to the horizon. It is called an oblique line. Read all the oblique lines. _An oblique line is a straight line that points neither to the horizon nor to the center of the earth._ NOTE.—After going through with the lessons on angles, the pupils may be told that oblique lines are so called because they form oblique angles with the horizon. LESSON FOURTH. REVIEW. Read all the vertical lines. (DIAGRAM 4.) Why is _q r_ a vertical line? What is a vertical line? Read all the horizontal lines. Why is 5 6 a horizontal line? Define a horizontal line. Read all the oblique lines. Why is _s t_ an oblique line. What is an oblique line? NOTE.—Lines that point in the same direction do not approach the same point. [Illustration: Diagram 4.] [Illustration: Diagram 5.] LESSON FIFTH. ANGLES. Do the lines _a b_ and _c d_ (DIAGRAM 5.) point in the same direction? (See note, page 15.) Then they form an _angle_ with each other. What other line forms an angle with _a b_? Which of the two lines _c d_, _e f_, has the greater difference of direction from the line _a b_? Then which one forms the greater angle with _a b_? What line forms a still greater angle with the line _a b_? _An angle is the difference of direction of two straight lines._