A plumb line is a reference line guided by a string or cord weighted at the end with a large weight known as a plumb bob. It is used to create a vertical reference line, where a spirit level may be impractical or difficult to use. A plumb-bob is a weight with a pointed tip on the bottom that is suspended from a string, and is used as a reference line that is perpendicular to the ground. ... A spirit level is an instrument designed to indicate whether a surface is level. ...
The line has in every point the same direction as that of the force of gravity of the Earth; thus, an object dropped on the surface of the Earth tends to follow this line. Gravity is a force of attraction that acts between bodies that have mass. ...
Strictly speaking the plumb line is not straight: it is curved mainly in the North-South direction (approximately 0.2" per km of elevation). This is mathematically connected with the fact that gravity diminishes when going from pole to equator. The curvature is additionally affected by rough topography. The equator is an imaginary circle drawn around a planet (or other astronomical object) at a distance halfway between the poles. ...
An object is in a vertical position when it is aligned in an up-down direction, perpendicular to the horizon. ... The zenith, in astronomy, is the point in the sky which appears directly above the observer. ... The vertical deflections (deflections of the plumb line, astro-geodetic deflections) are important parameters of the local gravity field. ...
The verb is first recorded c.1380, with sense "to immerse;" meaning "take soundings with a plumb" is first recorded 1568; fig.
"on the plumbline," from L. plumbum "(the metal) lead" (see plumb), of which the weight at the end of the line was made.
perpendiculer, from L. perpendicularis "vertical, as a plumbline," from perpendiculum "plumbline," from perpendere "balance carefully," from per- "thoroughly" + pendere "to weigh, to hang" (see pendant).
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