Mason & Dixon: Their Line And Its Legend


In 1761, England and several leading continental powers had joined in an international scientific project—securing data from more than a hundred points in Europe, Africa, and the Far East during the transit of Venus across the face of the sun, in order to determine more precisely the mean distance between earth and sun. Mason and Dixon drew an assignment to proceed to the island of Sumatra and make observations. A French man-of-war interfered with their sailing schedule, however, and they were able to get only as’ far as the Cape of Good Hope by the date of the transit. Even so, their observations from that point were praised more than a century later by a scientist at the United States Naval Observatory as being among the most accurate of the whole project.

When in 1762 the two men returned to England by way of the island of St. Helena, they brought information about two instruments that were to figure substantially in their coming assignment in the New World. One was an astronomical clock, made for the Royal Society to aid in determining the ellipticity of the earth. It was this same clock which was later to be shipped to Mason and Dixon to make the New World’s first accurate determination of longitude by means of the eclipses of the satellites of Jupiter.

Perhaps more important was the zenith sector which had been used at St. Helena by Mason’s mentor, the astronomer Nevil Maskelyne. This instrument, a graduated arc of a vertical circle used in conjunction with a telescope and plumb line, Maskelyne found to have a serious flaw, owing to the manner in which the plumb line was suspended. As a consequence of his findings, the Royal Society at once set to work to construct a corrected sector, and this new instrument was brought by Mason and Dixon when they came to America. It was thus that Thomas Penn could write with confidence to the Reverend Richard Peters, former secretary of the colony, that the right men and the right equipment had at length been found. Messrs. Mason and Dixon, he wrote, would bring with them “the line Sector, two Transit Instruments, and two reflecting Telescopes, fit to look at the Posts in the Line for ten or twelve miles.”

The Penns and Frederick, the sixth Lord Baltimore, having agreed on Mason and Dixon as the realm’s bestqualified surveyors, a contract was drawn up on July so, 1763, stipulating their responsibilities and compensation, the latter to be ten shillings, six pence a day until their arrival in America, and one pound one shilling daily for the period of the survey. The expenses of the project were to be shared equally by the proprietors, and the English scientists were to file identical reports of their findings with the commissioners from each colony. Mason and Dixon were to come to America as soon as possible.

In the course of the protracted dispute over their border, the proprietors had finally agreed that the boundary line between Maryland and Pennsylvania proper should be run east and west along a latitude circle fifteen miles south of the southernmost limit of the city of Philadelphia. The first task for Mason and Dixon on arriving in the colonies was therefore to accurately fix this starting point. The men reached Philadelphia November 15, 1763, unpacked their instruments, and began construction of a small building to serve as their first observatory. By the first of December the “gentlemen commissioners” from Maryland arrived, and in company with those from Pennsylvania, they inspected and confirmed the spot which marked the southernmost limit of the city. After some sixty observations of stars selected from Bradley’s Catalogue , made over a period of three weeks, Mason and Dixon determined that this spot was at latitude 39 degrees 56 minutes 29.1 seconds north—a finding which later observations showed to be in error only by 2.5 seconds.

Moving westward along this line of latitude to the farm of John Harlan on the Brandywine, “the Telescope &c of the Sector … carry’d on the Springs (with leather beds under it) of a Single Morse Chair,” the surveyors made further observations from a point where they could run a direct line fifteen miles south—to “a plantation belonging to Mr. Alexander Bryan,” the precise spot being in the middle of the front of Mr. Bryan’s house. This finding was duly accepted by the two commissions, and the way finally opened for an official determination of the boundary between Pennsylvania and Maryland.

It was long overdue; Mason noted in his journal that the former sheriff of Lancaster had described to him an incident in which a “Mr. Crisep,” living on the Susquehanna in territory he maintained was in Maryland, had been set upon by fifty men from Pennsylvania who burned his house and shot one of the besieged party as they ran out. “Mr. Crisep” appears to have been Colonel Thomas Cresap, whose house indeed was burned in 1736—in retaliation, it was alleged, for numerous acts of violence which Cresap’s “border ruffians” had precipitated. (The Colonel, incidentally, survived the attack reported to Mason and lived to earn laurels as a patriot in both the French and Indian War and the Revolution.) There were frequent occasions when the border claims of both proprietaries had flared into armed combat.