As part of the Baltimore Festival of Maps, the “Borders and Boundaries” exhibition at the Maryland Historical Society in Baltimore displays the six-foot-long map of the “boundary between the provinces of Maryland and Pennsylvania,” the map of the Mason-Dixon survey printed by Robert Kennedy in Philadelphia in 1768 and signed and sealed by members of the Boundary Commission. The Map, however, is only one of many original documents related to the boundary dispute between Maryland and Pennsylvania, the survey of the Mason-Dixon Line is also on display as part of the exhibition.

For his loyal service to the Crown as secretary of state, Sir George Calvert was made Lord Baltimore, a Baron in the Irish peerage, and was given a colony in the New World, which was named Maryland in honor of Henrietta Maria, consort to King Charles I. Maryland’s charter granted to Lord Baltimore all of the lands from the Atlantic to the headwaters of the Potomac River and those lands from the south bank of the Potomac “lying under the 40th parallel of north latitude.”

Sir William Penn had been a distinguished admiral in the Royal Navy during the Dutch wars, he had loaned King Charles II the stupendous sum of 16,000 pounds sterling. In exchange for discharging the debt, his son, also William, was granted the province of Pennsylvania, the southern boundary of which was to have been co-terminus with Maryland’s border extending along the 40^th parallel until it intersected a circle 12 miles distant from the town of New Castle in what is now Delaware.

Unfortunately, the 40th parallel of north latitude does not intersect a 12 mile circle around New Castle, and this discrepancy set off the grand-daddy of all boundary disputes, which raged for over 80 years. It was only resolved by the world’s first geodetic survey and the greatest scientific achievement of its day – the survey of the Mason-Dixon Line.

The dispute was bitter because the stakes were high. Philadelphia, then, as now, was the leading city of Pennsylvania, and the limits of navigability of the Delaware River lay south of the 40^th parallel. Depending upon the location of its border, Pennsylvania could have been denied both Philadelphia and the all-important access to the sea. There were about 3,000 square miles of territory in dispute, and it was unclear to which proprietor taxes were due. Violence also broke out sporadically along the border.

There were numerous, often contentious, attempts to settle the issue. While the Calverts simply suggested sailing up the Delaware until the 40^th parallel could be determined with a sextant, the Penns, on the other hand, argued that their southern border should be no further than 12 miles north of New Castle. Numerous rather creative arguments were advanced: the Calverts argued that the 12-mile circle around New Castle was actually a periphery and not a radius and, therefore, the border was only 1.9 miles from New Castle. On the other hand, the Penns argued that the measurements should follow the contours of the ground and not a horizontal and so the border around New Castle would be less than 12 miles and irregularly shaped. They also argued that because a man’s 40^th year began at his 39^th birthday, the 40^th degree of north latitude should begin at the 39^th parallel.

Finally, in 1735, the Penns filed a complaint in the English courts, which became known as the Great Chancery suit. The case was litigated over a 15-year period at enormous cost to both sides until in 1750 the Lord Chancellor, Lord Harwick, rendered a decision: The southern boundary of the lower three counties of Pennsylvania (now Delaware) would be at the latitude of Cape Henlopen. The peninsula would be divided equally. The center of the 12-mile circle was to be measured horizontally as a radius, and the east-west line would run at a constant parallel of latitude 15 miles south of the City of Philadelphia.

The proprietors engaged colonial surveyors to begin marking the boundary. They were able to find the midpoint of the Maryland-Delaware Peninsula, but running the tangent line from the midpoint to intersect the tangent of a circle around New Castle-a distance of approximately 80 miles-eluded them. After three unsuccessful attempts were made, the Lord's proprietor consulted the Astronomer Royal for suggestions of surveyors who could complete the task, and Charles Mason and Jeremiah Dixon were recommended. Dixon was an experienced surveyor from County Durham, and Mason had been an assistant at the Royal Observatory in Greenwich.

On August 4, 1763, they entered into a contract with Lord Baltimore and the Penns. They would survey an exact tangent line from the midpoint of the trans-peninsula line to a tangent on the 12-mile boundary around New Castle and the east-west boundary beginning 15 miles south of the city of Philadelphia and running five degrees of longitude west from the Delaware River. Theirs would be one of the greatest scientific accomplishments of the age.

They had brought with them state-of-the-art equipment to perform their task. This included a “zenith sector” built by the famous London instrument maker John Bird. This was a six-foot telescope mounted on a six-foot radius protractor scale used to determine latitude by measuring the angles of reference stars from the zenith in the sky. The delicate zenith sector was transported on a feather mattress laid in a wagon. They also had a Hadley quadrant; a “transit and equal altitude instrument,” an instrument used to determine true north by tracking stars where they crossed the Meridian; Gunter’s chains; and wood and brass measuring rods used to measure level horizontal distance across sloping ground. They also had various reference books, including ephemerides and logarithm and trigonometric tables.

They began their task at the north wall of a house on Cedar Street (now 30 South Street), which had been determined to be the southern point of Philadelphia. They set up a temporary observatory near the house and, from detailed astronomical observations, determined its latitude.

Going 15 miles due south would have taken them through the Delaware River, so they decided to proceed west 31 miles to the farm of John Harland in Embreeville, Pennsylvania at the “forks of the Brandywine.” There they set up another observatory and a reference stone, which came to be know as the “Star Gazer’s Stone.” By observing various astronomical events and comparing them with the published times, they were able to determine their longitude. They seemed to enjoy their winter in Pennsylvania and whiled away many an evening in the local taverns.

In the spring of 1764, they set off south where they placed a post called “Post Mark’d West” in John Bryan’s field at a point exactly 15 miles south of the latitude they had determined in Philadelphia.

They then headed to the middle point of the trans-peninsula line that had been previously marked by the colonial surveyors. Although the colonial surveyors had tried unsuccessfully to run the tangent line several times, their survey points remained on the ground and Mason and Dixon took advantage of them. The actual technique used to run the tangent line has been something of a mystery but was recently solved by Edwin Danson in his book "Drawing the Line: How Mason and Dixon Surveyed the Most Famous Border in America," (John Wiley & Sons, Inc. 2001).

Mason’s solution was to run a dead-straight line and then adjust it proportionally. Accordingly, Mason and Dixon selected a convenient star in the tail of Ursa Minor, proceeded northwards and measured offsets to the previously run American line until they reached the vicinity of the tangency. The distance to the tangent point, which had previously been established by the Colonial surveyors, was measured, and the distance was proportioned back along the line with offsets calculated to bring each marker to its proper location. They then checked the angle at the tangent with their Hadley quadrant, and it measured a perfect 90 degrees.

In April of 1765, they then began the monumental task of surveying the west line. Setting out from the Point Mark’d West in Mr. Bryan’s field, they laid out sections of a great circle, each ten minutes of longitude long. At the end of each segment, they observed their latitude and calculated offsets to the true latitude at one-mile intervals. This was a tedious and time consuming process. Their work party was huge, sometimes consisting of more than 100 men employed as cooks, tent keepers, axe men, shepherds and even a milk maid. They cleared a so-called “visto” 6 yards wide. They continued for two years proceeding westward hauling stones quarried in England and setting them every mile with an M on one side and a P on the other.

On October 9, 1767, 231 miles, 20 chains west from the Post Mark’d West, the party crossed a “war path.” This is shown on the map, which they later created. Known as the Great Warrior Trail, it was one of the most important Indian trails in North America, running from New York to South Carolina. The French and Indian War had just ended, and English relations with the French were still dicey. The English were allied with the Six Nations against the French, who had their own Indian allies. George III did not want to antagonize his Indian allies and prohibited settlement west of the Alleghenies. The Colonists, on the other hand, were eager to expand westward into the fertile valleys of Ohio. This was one of the underlying tensions that led to the Revolutionary War. At the Great Warrior Trail, the representatives of the Six Nations informed Mason and Dixon that they had come to the end of their commission and that they “would not precede one step further westward.” The line was extended to the top of the next ridge and at a distance of 233 miles, 17 chains, and 48 links from the Post Mark’d West in Mr. Bryan’s field, Mason and Dixon’s line came to an end.

At the conclusion of their work, Mason and Dixon were instructed to draw a map of their survey, and 200 copies were printed. On September 11, 1768, after having completed their labors, Mason wrote in his journal “at 11h 30m a.m. went on board the Halifax packet boat for Falmouth, Thus ends my restless progress in America.”

Dixon returned to his family and surveying practice in Durham, where he died in 1779. Mason returned to America in 1786 with his wife and children but died shortly thereafter and is buried in an unmarked grave in the Christ Church burial grounds in Philadelphia. Their line lived on, however.

In 1820, Congress adopted the Missouri Compromise and first used the term “Mason-Dixon Line” to describe the Maryland-Pennsylvania border. States north of the Mason-Dixon Line were to be free and those south were slave states. And so in addition to being the first geodetic survey and one of the greatest scientific achievements ever, the Mason-Dixon Line became an icon: the boundary between slavery and freedom.

In 1888, the Maryland Historical Society purchased the Calvert papers, a collection of approximately 1,300 documents. Included among the papers are many original documents related to the boundary dispute with the Penn family, a number of which are currently exhibited at the Maryland Historical Society. The exhibition includes: