Mason & Dixon: Their Line And Its Legend


The winter months of January and February, 1766, were spent sightseeing in adjacent colonies, including a trip to Williamsburg, “the Metropolis of Virginia,” and a call on Governor Sharpe at Annapolis. By late March, they were back at Captain Shelby’s and again moving westward until, on June 16, Mason noted that they had reached “the most westernmost Waters [the headwaters of the Potomac] that runs to the Eastward in these parts.” It seems, in fact, that the surveyors believed they were getting close to “the Boundary between the Natives and strangers, in these parts of His Brittanic Majesties Collonies.”

The party now set itself to the task of establishing the actual markers for the border they had fixed. Mason noted in his journal on June 18, 1766:

Set a Post (18 inches square 3 feet in the Ground and 5 out) … mark’d M on ye South Side, P on ye North Side, and W on the West: and began to cut a Visto in the true Parallel, or Line between Maryland and Pennsylvania … By drawing it thro’ Points, laid off from the Line we had run … toward the Post mark’d West in Mr. Bryan’s field.

On the tangent line of the boundary between Maryland and the Delaware counties, the party was provided with limestone markers which had been cut in England for the purpose and shipped directly to various points on Chesapeake Bay. These were set at every mile point, with every fifth marker distinguished by a “crown stone” on which were carved the arms of the proprietors in place of the M or P. On one occasion while setting the stones along the “West Line” the scientists had an opportunity to view it from the summit of a hill and observe its curvature. Mason wrote, “I saw the Line, still form’d the arch of a … circle, very beautiful, and agreeable to the Laws of a Sphere.” He also indulged his keen interest in landmarks along the way, and in recording the moral which might be drawn from them:

Went to see Fort Cumberland [he wrote on June 27, 1766] … Going to the Fort, I fell in to General Braddock’s Road, which he cut thro’ the Mountains to lead the Army under his command to the westward in the year 1755 but, fate: how hard: made thro” the desert a path, himself to pass; and never, never to return.

Most of the summer and autumn of 1766 was spent in prolonging the Maryland-Pennsylvania border to the approximate limit indicated by the Pennsylvania charter, five degrees in longitude westward from their starting point. The scientists also extended the line from the “Post marked West in Mr. Bryan’s Field” eastward to the Delaware River. This project they completed on December i, 1766. The commissioners, however, desired to have the main line go farther westward, and advised Mason and Dixon that Sir William Johnson, the royal agent for Indian affairs, was negotiating with the natives for its further extension into their territory. Additional work would await the outcome.

This was agreeable with Mason and Dixon. They had observed that the smooth terrain on what is now the Delmarva Peninsula was well adapted to a geodetic determination of the exact linear measure of one degree of latitude this far from the equator. They appealed to Maskelyne, then the Astronomer Royal at Greenwich, who arranged for the Royal Society to sponsor and finance the undertaking. The Society also granted their requested fee of £200 for the work, and assured them that if the proprietors should not be willing to give them their passage money after the delay which would be occasioned by this extra project, the Society would make it good.

But both Frederick, Lord Baltimore, and the Penns acquiesced in this research, and the Penns even sent the scientists some instruments which they had recently acquired. Not only would the work occupy the geodesists during the winter while negotiations between Sir William and the Indians were being carried on, but, as it developed later, the commissioners were interested in having the results of the investigation in order to fix the precise length of a degree of longitude along the east-west border. (This could be calculated from the value of a degree of latitude.)

Maskelyne sent his colleagues a long letter of instruction for the most accurate astronomical measurements, and through the instrument maker John Bird he arranged for the fashioning of a five-foot brass rod as a standard measure. With the aid of these and other improved instruments sent from abroad, Mason and Dixon determined, on the Delmarva Peninsula, the first precise value of dimensions of the earth ever made in North America.

In June, 1767, Sir William Johnson advised that the Six Nations had reluctantly consented to a limited extension of the border between Pennsylvania and Maryland. The situation could be rather delicate, the commissioners warned Mason and Dixon: “As the public Peace and your own Security may greatly depend on the Good Usage and Kind Treatment of these [Indian] Deputies” who were to accompany the party, “Spiritous Liquors” were to be given to them only in small quantities and not more than three times a day.

The Englishmen were eager to gather information about the country beyond the Atlantic seaboard for scientific and perhaps for political reasons. Much of Mason’s journal for this part of the survey was devoted to “a description of the Ohio and Mississippi, as describ’d to me by Mr. Hugh Crawford, our Interpreter, who has traversed these parts for 28 years.” From this description he noted: