Plane Figures: Positions of a Straight Line

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  • A transparent pitcher and vase
  • Water
  • Red dye
  • A level, two plumb lines, with a red line (cord)
  • A red stick
  • A globe



  1. Dye the water in the pitcher red (because red is always used to highlight the subject of a presentation) and pour some into the vase, which is placed in the center of the table.
  2. Observe the surface of this water and describe it: it is still.
  3. This will be a point of reference.
  4. Agitate the pitcher and place it next to the vase.
  5. Let's wait without saying anything until the surface of this water becomes like the other.
  6. When it is exactly like the point of reference, the surface can be identified as horizontal.
  7. Place the red stick alongside the vase so that it aligns perfectly with the water.
  8. This stick represents a line which goes on in both directions.
  9. When it has the same position as the surface of the still water, it is a horizontal line.
  10. Drop the stick in the water and wait until it is still (like the point of reference).
  11. This stick represents one of the lines that make up the surface of the water.
  12. This is a horizontal line.
  13. Remove the stick.
  14. A straight line is horizontal when it follows the direction of still water (horizontal < horizon: Greek horizon < horas, boundary, limit; thus the horizon is the boundary of the visible earth in all directions, where it seems that the sky touches the water.
  15. Bring the children up the hill to see the horizon.
  16. Hold the plumb line until it is still.
  17. This will be the point of reference now.
  18. Get another plumb line and wait without touching it, without a word, until it is exactly like the point of reference.
  19. Place the red stick along the red cord, so that it coincides just as the stick on the surface on the water.
  20. This straight line which goes on in both directions infinitely is vertical, because it follows the direction of the plumb line.
  21. This is a vertical line (vertical: Latin verticalis < vertex, whirlpool, vortex, crown of the head, summit, highest point, <vertere, to turn; therefore vertex can be applied to anything which turns like a whirlpool, or to the highest point, like the crown of the head or the summit).
  22. A vertical line is one which points to the vertex, that is, the topmost point in the sky over our heads (zenith).
  23. It passes through the center of the earth and on to the nadir (opposite of zenith).
  24. Take the plumb line and hold it still again.
  25. Let's imagine that this is a straight line which goes on in both directions - up to the zenith and down through the center of the earth to the other side.
  26. Use the globe to show that a vertical line is relative to the position of the observer.
  27. Place the points of reference for the two opposite elements in front on the child.
  28. What is the median? Hold the red stick horizontally.
  29. When a straight line follows the direction of the surface of still water, what is it? A horizontal line.
  30. Hold the stick vertically.
  31. When a straight line follows the direction of the plumb line what is it? A vertical line.
  32. Hold the stick obliquely.
  33. Is this straight line like the surface of water? The plumb line?
  34. When a straight line is neither horizontal or vertical, it is oblique.
  35. Turn the stick 360o identifying its position as it turns - horizontal, oblique, oblique, oblique....vertical, oblique, oblique, oblique, oblique, horizontal, oblique ...etc. (oblique: Latin obliquus, slanting, sloping, not straight, not right, devious)
  36. So what is straight, right, and normal?
  37. The horizontal and vertical line.
  38. The oblique line runs contrary to the true, contrary to vertical or horizontal.

Control Of Error

Points Of Interest



Classified nomenclature and commands. Demonstrate use of the level for determining lines in the environment.