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Plumb bob
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 vertical equivalent of a "water level".
The instrument has been used since at least the time of ancient Egypt〔Denys A. Stocks. ''(Experiments in Egyptian archaeology: stoneworking technology in ancient Egypt )''. Routledge; 2003. ISBN 978-0-415-30664-5. p. 180.〕 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.
== Etymology ==
The "plumb" in "plumb-bob" comes from the fact that such tools were originally made of lead (Latin ''plumbum'', French ''plomb'').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".
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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