<P> Mason and Dixon could only do the work as accurately as they did due to the work of Nevil Maskelyne, some of whose instruments they used . There was keen interest in their work and much communication between the surveyors, Maskelyne and other members of the British Scientific establishment in the Royal Society in Britain, notably Henry Cavendish . During such survey work, it is normal to survey from point to point along the line and then survey back to the starting point, where if there were no errors the origin and re-surveyed position would coincide . Normally the return errors would be random--i.e. the return survey errors compared to the intermediate points back to the start point would be spatially randomly distributed around the start point . Mason and Dixon found that there were larger than expected systematic errors, i.e. non-random errors, that led the return survey consistently being in one direction away from the starting point . When this information got back to the Royal Society members, Henry Cavendish realised that this may have been due to the gravitational pull of the Allegheny Mountains deflecting the theodolite plumb - bobs and spirit levels . Maskelyne then proposed measuring the gravitational force causing this deflection induced by the pull of a nearby mountain upon a plumb - bob in 1772 and sent Mason (who had returned to England) on a site survey through central England and Scotland to find a suitable location during the summer of 1773 . Mason selected Schiehallion at which to conduct what became known as the Schiehallion experiment, which was carried out primarily by Maskelyne and determined the density of the Scottish mountain . Several years later Cavendish used a very sensitive torsion balance to carry out the Cavendish experiment and determine the average density of Earth . </P> <P> Mason and Dixon likely never heard the phrase, "Mason - Dixon line". The official report on the survey, issued in 1768, did not even mention their names . While the term was used occasionally in the decades following the survey, it came into popular use when the Missouri Compromise named "Mason and Dixon's line" as part of the boundary between slave territory and free territory . </P> <P> In popular usage, the Mason--Dixon line symbolizes a cultural boundary between the North and the South (Dixie). Originally "Mason and Dixon's Line" referred to the border between Pennsylvania and Maryland . After Pennsylvania abolished slavery, it served as a demarcation line for the legality of slavery . That demarcation did not extend beyond Pennsylvania because Delaware, then a slave state, extended north and east of the boundary . Also lying north and east of the boundary was New Jersey, where slavery was formally abolished in 1846, but former slaves continued to be "apprenticed" to their masters until the passage of the Thirteenth Amendment to the United States Constitution in 1865 . </P> <P> The Missouri Compromise line had a much clearer geographic connection to slavery in the United States leading up to the Civil War . </P>

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