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A plumb-bob or a '''plummet''' is a weight with a pointed tip on the bottom that is suspended from a string and used as a vertical reference line. This Instrument has been used since the time of the ancient Egyptians by bricklayers, Masons , and carpenters to ensure that their constructions are "plumb", or perfectly upright. It may also be used in surveying to sight a point on the ground that is not readily visible. Small plumb bobs are included in the kits of various instruments such as levels and Theodolites . They are used to set the instrument exactly over a fixed Datum marker, prior to taking fresh readings. ETYMOLOGY OF THE NAME The ''plumb'' in the name ''plumb-bob'' comes from the fact that such tools were archetypically made of ''). The adjective ''plumb'' developed by extension. USE Up until the modern age, on most tall structures, plumb-bobs were used to provide vertical datum lines for the building measurements. A section of the scaffolding would hold a plumb line that was centered over a datum mark on the floor. As the building proceeded upwards 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, that signify the center of the structure above. The early skyscrapers used heavy plumb-bobs hung on wire in their lift wells. The weight would hang in a container of oil to dampen any swinging movement. REFERENCES |
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