Mason & Dixon: Their Line And Its Legend


Territorial assignments in seventeenth-century charters were vague, and the technical capacity for accurate fixing of boundaries in the unsettled wilderness was extremely limited. Local surveyors had no particular difficulty in laying out sites for towns and individual plantations. There was a political as well as a surveying problem involved, however, in determining the point described in the Maryland charter as its northern boundary—“that Part of the Bay of Delaware on the North, which lieth under the Fortieth Degree of North Latitude, where New England is terminated.”

The fortieth parallel would have put Maryland’s northern border somewhere within the present city of Philadelphia. When Penn’s grant was made in 1681, Charles Calvert, third Lord Baltimore, insisted that his own prior charter should be interpreted literally. Not so, said Penn’s party; the original intent of the Maryland charter was to put the border “under” the fortieth parallel—how far “under” being the point at issue. The vicinity of the fortieth parallel was not unacceptable for the main boundary line, but at its eastern end a vital problem arose. A few miles up or down the Delaware estuary could insure or deprive Pennsylvania of a harbor of enormous commercial potential. In addition, there was the question of which colony was entitled to collect taxes from the settlers within the disputed zone. Bitterly but steadily over the years, the Calverts retreated or were pushed southward from their charter position, particularly in the region of Delaware Bay.

Three times—in 1685, 1732, and 1750—the boundary controversy was adjudicated in England by agencies of the Crown. The upshot was that, by 1760, the Penns’ proprietary was held to include roughly half of the northern part of the Delmarva Peninsula, separating Chesapeake Bay from Delaware Bay. It was to include, that is, the area now forming the state of Delaware, but at that time known as the “lower counties” of the Penn domain, or simply as “the counties of Delaware.”

Determination of their exact boundaries, however, was a prickly matter. The British Court of Chancery had ruled that the Delaware southern border should be a transpeninsular line extending westward from Cape Henlopen—as that point was indicated on contemporary maps. It turned out that the maps showed the Cape too far south by as much as twenty-two miles; but the Court was stubborn, and the transpeninsular line separating the Penns from the Calverts was drawn accordingly, to the detriment of the Culverts. The northern boundary of the Delaware counties—which had acquired a form of home rule while remaining within the Penn proprietary—was to be determined by the arc of a circle twelve miles in radius, with its center at New Castle Court House (now New Castle, Delaware).

But the final step—determining the Delaware western border—required the connecting of the midpoint on the transpeninsular line with a tangent point on the arc. The nice astronomical and mathematical steps needed to connect these points proved too much for the colonial commissioners charged with the survey. f n 1761, after several months of clearing a line through the wilderness, Thomas Penn’s commissioners were compelled to advise him that there had been some basic astronomical miscalculations and “the business set back almost as far as ever.” The trouble started, said the commissioners, with a survey telescope that had gone awry; after that everything went wrong.

The telescope “being extended and fixed on a strip of wood … after being exposed to a Shower of Rain they perceived the Strip had warped & the Glass did not represent Objects precisely in the places they possessed.” Apparently the Penns, accustomed to the considerable scientific sophistication of mid-eighteenth-century England, could not at first believe that the colonials were incapable of handling the problem. A letter from Dr. John Robertson, master of the Royal Naval Academy at Portsmouth, however, assured them of its difficulty and of the need for competent instruments competently used. Added to this was the prospect that erroneous surveys could run into money: as one Pennsylvania commissioner wrote Thomas Penn in 1763, if local scientists surveyed the border, “and if afterwards on Examination of the Work by Mathematicians in England it should be pronounc’d wrong, can Lord Baltimore take advantage of this, set it aside and procure an order to do it over again?”

The obvious solution was “to send over from England some able Mathematicians with a proper set of Mathematical instruments.” These persons, in addition to their scientific competence, were to be “of Great Integrity and totally unbiassed and unprejudiced on either side of the question.” Eighty years of argument had crystallized the Penns’ and Calverts’ mutual distrust of each other. Apparently the nomination of the qualified “mathematicians” was solicited from the Astronomer Royal, Charles Bradley, director of the Greenwich Observatory, and his successor, Nathaniel Bliss.

The Astronomer Royal, it happily developed, had the men and the instruments for the job. Charles Mason had been assistant observer at Greenwich from 1756 to 1760, during which time he had worked closely with Bradley on a monumental catalogue of positions of the moon, and on the designing of improved instruments for astronomical observations. And Jeremiah Dixon had early shown enough mathematical precocity to bring him to the attention of John Bird, the creator of man) astronomical instruments at Greenwich and a member of the Royal Society. Dixon was described to Penn as a competent surveyor from Durham County, England.