b. Plumb Bob. A plumb bob is made of metal, usually brass. It
usually has a screwtype cap with a hole in the center. A string or
plumb line is inserted through the hole and fastened inside. The
bottom end has a point in direct line with the hole in the cap
(Figure 241). The string is absolutely perpendicular to the
horizontal when the plumb bob is suspended on it. It can be used for
Figure 2-41. Plumb bobs
same purpose as the plumb glass on a level; however, the plumb bob is
not accurate when used in the wind.
c. Chalk Line. This is a strong, lightweight cord used to make a
straight line between two widely separated points. To snap a
straight line, rub chalk on a cord held taut between two points.
Then pull the cord straight up from the center and release it, to
allow it to spring back into place. Chalk lines come in metal or
plastic cases. Figure 242 shows how to snap a chalk line.