<P> Until the modern age, plumb - bobs were used on most tall structures to provide vertical datum lines for the building measurements . A section of the scaffolding would hold a plumb line, which was centered over a datum mark on the floor . As the building proceeded upward, the plumb line would also be taken higher, still centered on the datum . Many cathedral spires, domes and towers still have brass datum marks inlaid into their floors, which signify the center of the structure above . </P> <P> Although a plumb - bob and line alone can determine only a vertical if they are mounted on a suitable scale, the instrument may also be used as an inclinometer to measure angles to the vertical . </P> <P> The early skyscrapers used heavy plumb - bobs, hung on wire in their elevator shafts . </P> <P> A plumb bob may be in a container of water (when conditions are above freezing temperatures), molasses, very viscous oils or other liquids to dampen any swinging movement, functioning as a shock absorber . </P>

Show me a picture of a plumb line