1911 Encyclopædia Britannica/Mason and Dixon Line

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MASON AND DIXON LINE, in America, the boundary line (lat. 39° 43′ 26.3″ N.) between Maryland and Pennsylvania, U.S.A.; popularly the line separating “free” states and “slave” states before the Civil War. The line derives its name from Charles Mason (1730–1787) and Jeremiah Dixon, two English astronomers, whose survey of it to a point about 244 m. west of the Delaware between 1763 and 1767[1] marked the close of the protracted boundary dispute (arising upon the grant of Pennsylvania to William Penn in 1681) between the Baltimores and Penns, proprietors respectively of Maryland and Pennsylvania. The dispute arose from the designation, in the grant to Penn, of the southern boundary of Pennsylvania mainly as the parallel marking the “beginning of the fortieth degree of Northerne Latitude,” after the northern boundary of Maryland had been defined as a line “which lieth under the fortieth degree of north latitude from the equinoctial.” The eastern part of the line as far as Sideling Hill in the western part of the present Washington county, was originally marked with milestones brought from England, every fifth of which bore on one side the arms of Baltimore and on the opposite side those of Penn; but the difficulties in transporting them to the westward were so great that many of them were not set up. Owing to the removal of the stone marking the north-east corner of Maryland, this point was again determined and marked in 1849–1850 by Lieut.-Colonel J. D. Graham of the U.S. topographical engineers; and as the western part of the boundary was not marked by stones, and local disputes arose, the line was again surveyed between 1901 and 1903 under the direction of a commission appointed by Pennsylvania and Maryland.

The use of the term “Mason and Dixon Line” to designate the boundary between the free and the slave states (and in general between the North and the South) dates from the debates in Congress over the Missouri Compromise in 1819–1820. As so used it may be defined as not only the Mason and Dixon Line proper, but also the line formed by the Ohio River from its intersection with the Pennsylvania boundary to its mouth, thence the eastern, northern and western boundaries of Missouri, and thence westward the parallel 36° 30′—the line established by the Missouri Compromise to separate free and slave territory in the “Louisiana Purchase,” except as regards Missouri. It is to be noted, however, that the Missouri Compromise did not affect the territory later acquired from Mexico.

  1. These surveyors also surveyed and marked the boundary between Maryland and Delaware.