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A plumb-bob or a plummet is a weight, usually with a pointed tip on the bottom, that is suspended from a string and used as a vertical reference line, or plumb-line. It is essentially the y-axis equivalent of a "water level".
The instrument has been used since at least the time of ancient Egypt to ensure that constructions are "plumb", or vertical. It is also used in surveying to establish the nadir with respect to gravity of a point in space. They are used with a variety of instruments (including levels, theodolites, and steel tapes) to set the instrument exactly over a fixed survey marker, or to transcribe positions onto the ground for placing a marker.
The "plumb" in "plumb-bob" comes from the fact that such tools were originally made of lead (Latin plumbum, French plomb). The adjective "plumb" developed by extension, as did the noun "aplomb", from the notion of "standing upright".
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.
Although a plumb-bob and line alone can only determine a vertical, if mounted on a suitable scale the instrument may also be used as an inclinometer to measure angles to the vertical.
The early skyscrapers used heavy plumb-bobs hung on wire in their elevator shafts. The weight would hang in a container of oil to dampen any swinging movement, functioning as a shock absorber.
Students of figure drawing will also make use of a plumb line to find the vertical axis through the center of gravity of their subject and lay it down on paper as a point of reference. The device used may be purpose-made plumb lines, or simply makeshift devices made from a piece of string and a weighted object, such as a metal washer. This plumb line is important for lining up anatomical geometries and visualizing the subject's center of balance.
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