Mason & Dixon: Their Line And Its Legend


Mason took a keen interest in all his New World surroundings, both scientific and nonscientific. Once when winter weather suspended surveying, he and his colleague went to New York for a couple of weeks to enjoy the activities of that colonial community. En route they stopped at “Prince Town in the Jersies” and Mason admired “the most elegant built Colledge I’ve seen in America.” On another occasion, when the survey was getting under way, Mason took a short side trip to see a large cave, describing the churchlike atmosphere in a lugubrious vein:

On the side walls were drawn by the Pencil of time, with the tears of ye Rocks: the imitation of organ, pillar, collumns and monuments of a Temple; which, with the glinting, faint light, makes the whole an awful, solemn appearance, Striking its Visitors with a strong and melancholy reflection: That, such is the abodes of the Dead; Thy inevitable doom, O Stranger, soon to be numbered as one of Them.

The business at hand, however, occupied most of the scientists’ attention. On the Harlan farm, where they set up a more or less permanent headquarters, they erected a crude monument, known by the unindoctrinated for many years after as the “stargazers” stone,” and spent much of the snowbound winter making observations.

The Penns and Lord Baltimore had been unduly optimistic as to the period required for the survey, and the deadline had to be extended several times. Weather, transportation, the cumbersome procedure of meeting periodically with the commissioners to make progress reports, and, in the late stages of the project, the threat of Indian interference—all interposed delays. On the whole, however, the parties on both sides were quite satisfied with the work of Mason and Dixon. True, in April of 1764 Governor Horatio Sharpe of Maryland received a letter from Cecilius Calvert, secretary of the colony, alleging that the Penns had offered the Englishmen a contract for the surveying of Pennsylvania’s northern border as a douceur if the southern border survey treated the Quaker colony right. But on finding that Mason and Dixon had located their original point, fifteen miles south of Philadelphia, a quarter of a mile farther north than previous surveys had indicated, the Calverts’ suspicions were quieted.

Before actually surveying the line for the northern boundary of Maryland (39 degrees 43 minutes 17.6 seconds), Mason and Dixon proceeded to establish the Maryland-Delaware boundary. On June 25, 1764, they arrived at the southwest corner of Delaware as established by the colonial surveyors—a point midway on the transpeninsular line running west from where Cape Henlopen was indicated on the early maps. From this point Mason and Dixon ran the tangent line to the New Castle “twelve mile arc.” On September 25 they reported that this line, as finally run, lacked only two feet, two inches of tangency to the twelve-mile circle; and as the length of the line was over eighty-two miles, it was accepted by the commissioners.∗ This survey, when completed, included a 1.466-mile arc of the twelve-mile circle itself, plus a 3.574-mile “North Line” connecting the arc with the northeast corner of Maryland. The scientists stored their instruments for the winter, and the party disbanded to await more favorable weather. On April 4, 1765, Mason and Dixon returned to the Bryan plantation, fifteen miles south of the latitude of Philadelphia, where they set up the zero milestone for the survey of the main segment of their line, the Maryland-Pennsylvania boundary. There they placed a reference marker which in their notes they frequently described as the “Post mark’d West.”

In the running of the border Mason and Dixon, using astronomical observation and the laws of spherical geometry, checked their geographical positions every eleven and a half miles—more precisely, every ten minutes of great circle. Deviations were then corrected at each mile point. These points, temporarily marked by posts, were on the true parallel of latitude and represented the boundary between Maryland and Pennsylvania.

To facilitate sighting and marking, the surveyors employed axemen to clear a rough corridor (or “Visto,” as they called it) “8 or 9 yards wide” along the points of their periodic observations and measurements. Horizontal measurements were taken with a Gunter’s chain of sixty-six links on level ground, and with a triangular-shaped surveyor’s “level” on the slopes. By this procedure of making horizontal measurements controlled by astronomical observations they continued westward, until ultimately, on October 9, 1767, they reached a point about 233 miles from the “Post mark’d West,” beyond which the Indian tribes refused to permit further work.

The survey was consistently meticulous. “To prove that the Chain Carriers had made no error,” Mason wrote in his journal at one point, “I took a Man with me, a few days after, and measured it myself; and made it within a link of the same.” After the first twenty-five miles the party (about a dozen persons) retraced their steps and checked their work. By June, 1765, they were again moving westward, crossing the Susquehanna at; Peach Bottom, Pennsylvania, and continuing to the summit of the Blue Ridge mountains, which they reached in late October. Here Mason and Dixon suspended the survey for the season, left their instruments “not in the least damaged to our knowledge” with Captain Evan Shelby, a well-known frontiersman, and spent the next few weeks checking their distance measurements as they returned eastward.