A recent column in the London Daily Mail on Sunday began, “The first time I ever crossed the Mason Dixon Line, that imaginary boundary meant to separate the liberal North from the slave-owning South before the American Civil War …”

A recent column in the London Daily Mail on Sunday began, “The first time I ever crossed the Mason Dixon Line, that imaginary boundary meant to separate the liberal North from the slave-owning South before the American Civil War …”

First of all, the Mason Dixon Line was never meant to separate anything other than the Colonial provinces of Maryland and Pennsylvania; and second, it may be legendary and it may be iconic, but it is certainly not imaginary-it is quite real indeed. It represents the world’s first geodetic survey and the greatest scientific and engineering achievement of its age, and therein lies quite a tale.

The story begins with Sir George Calvert, a Catholic, who was the secretary of state to the Protestant King James I of England. Sir George had the unenviable task of shepherding the King’s anti-Catholic measures through parliament, and for his loyal service to the Crown, he was created Lord Baltimore, a baron in the Irish peerage (Baltimore is a little village in County Cork). He was also granted land in the New World. He was first given a colony named Avalon in Newfoundland, but the weather was too harsh. He was an investor in the Virginia Company and next tried Jamestown, but he was not accepted there due to his religion. It is reported that “the people looked upon him with an evil eye,”  so he petitioned Charles I, who had succeeded his father to the throne, for a province between Virginia and New England, which he named Maryland in honor of Henrietta Maria, consort to the king.

On June 20, 1632, the Royal Charter granted to Lord Baltimore all the territory from the Atlantic Ocean “unto the true meridian of the first fountain of the River Potowmack” and from the south bank of the Potomac River to include all land “which lieth under the Fortieth Degree of North Latitude.”

The grant precluded lands “hactenus inculta” or previously cultivated by which clause Lord Baltimore was to lose what is now Delaware based on the somewhat dubious claim of prior Dutch and Swedish settlement.

The other player in the story is Sir William Penn who had been a distinguished admiral in the Royal Navy during the Dutch wars and who had loaned the profligate 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. Penn was given all of the land for five degrees of longitude west of the Delaware River between the 43rd and 40th parallels excluding a “twelve mile circle” around New Castle in what is now Delaware.

Unfortunately, the maps at the time were based upon the map of the Chesapeake region made by Captain John Smith in 1608, which showed the 40th parallel too far south.  In fact, the 40th parallel of north latitude does not intersect a 12 mile circle around New Castle but lies much further north, and it was this discrepancy that set off the mother of all boundary disputes which raged for more than 80 years.

The dispute was so bitter because the stakes were so high-about 2.5 million acres of territory were in question. Philadelphia, then as now the leading city of Pennsylvania, settled at the limits of navigability of the Delaware River and lay about five miles south of the 40th parallel. Depending upon the location of its border, Pennsylvania could have been denied both Philadelphia and the all-important access to the sea and re-supply. It was also unclear to what proprietor taxes were due, and violence also broke out sporadically along the border.

In one famous case, Col. Thomas Cresap, a Maryland partisan, operated a ferry on the Susquehanna near the 40th parallel in what is now Pennsylvania, but after confiscating land for Maryland and refusing to pay taxes to Lancaster County in what became known as “Cresap’s War,” he was arrested and hauled through the streets of Philadelphia, where he is reported to have exclaimed, “Damn, this is one of the prettiest towns in Maryland.”

There were numerous, often-contentious attempts to settle the issue, but it finally came to a head when the King in Council ordered the proprietors to suppress the violence and to stop issuing patents in the disputed areas.

There were still lots of contention. The Calverts suggested simply sailing up the Delaware until the 40th parallel could be determined with a sextant, but  the  Penns, on the other hand, argued that their southern border should be no further than 12 miles north of New Castle.
Finally, on May 10, 1732, the parties entered into a settlement agreement. They agreed that the boundary should run west from Cape Henlopen on Fenwick Island to the midpoint of the peninsula, then north to intersect a tangent with the 12 mile arc around New Castle, then north and finally along an east-west line 15 miles south of Philadelphia. The red line showing the boundary on the exhibit accompanying the agreement is reported to have been drawn by Lord Baltimore himself. It was not a good deal for the Calverts placing the boundary about 19 miles south of the 40th parallel, and controversy over the interpretation of the agreement continued.

Numerous rather creative arguments were advanced: The Calverts argued that the 12 mile circle around New Castle was actually a periphery (a circumference) and not a radius and, therefore, the border was only 1.9 miles from New Castle. They also 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. Rather ingeniously, they also argued that because a man’s 40th year began at his 39th birthday, that the 40th degree of north latitude should begin at the 39th parallel. Finally, in 1735, the Penns filed a complaint for specific performance in the English courts (Penn v. Lord Baltimore 3 Ves. Sen 194), 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, Earl Hardwicke, 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 from the center of New Castle as closely as it could be determined, and the east-west line would run at a constant parallel of latitude 15 miles south of the city of Philadelphia.

The proprietors appointed boundary commissioners who engaged Colonial surveyors to begin marking the boundary. They started with the transpeninsular line on April 26, 1751. Their field notes begin:
“(We) began at the Verge of the Ocean. Thence extending due west 139 perches to a stone fixed near four Mulberry trees on Fenwick’s Island marking on the south side with the Arms of Lord Baltimore and on the north side with the Arms of the proprietor of Pennsylvania. Then 1745 perches to Fenwick’s ditch 105 perches to a branch of the Sound running into Sinnepuxent Inlet.

(Signed) John Emory, Wm Parson, Tho. Jones, Wm Shankland, Wm Killen”         

Although swamps and dense vegetation made work on the transpeninsular line difficult and, as usual, there was much bickering, the Colonial surveyors were able to locate and mark the midpoint of the peninsula, which was the southwestern corner of the three lower counties.

The transpeninsular survey and midpoint weren’t approved in London until 1760, and on Dec. 13, 1760, Colonial surveyors began the task of running the tangent line-the line from the midpoint of the peninsula to the point of tangency with the 12 mile circle around New Castle. This was a lot harder than it looked on paper given that the line was more than 80 miles long, the terrain difficult and the equipment poor.

The geometry of the corner was also very complex. A line more than 80 miles long would have to be run to intersect the 12 mile circle at a tangent, which then ran due north to intersect another line 15 miles south of Philadelphia. Thirteen acres of land, the secant, inside the circle but outside of the North line would belong to Maryland, but 800 acres south of the West line would belong to Pennsylvania. Known as the Wedge and claimed by both Pennsylvania and Delaware its legal jurisdiction was uncertain and it became a no man’s land--a notorious hideout for moonshiners and others avoiding the law. In 1921, it was finally ceded to Delaware by act of Congress.

The Colonial surveyors first ran their line due north from the midpoint until it was within the 12 mile radius. Then they measured from the dome of the courthouse, which the commissioners had agreed would be considered the center of New Castle, and tried to adjust to the tangent point, but their first attempt was a half-mile too far east.

Two other attempts were too far west, and as the task was apparently beyond the capability of the local surveyors, the proprietors consulted the Astronomer Royal for suggestions of surveyors who could complete the task. 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. They had worked together on the famous Transit of Venus observation of 1761. (The Transit of Venus is an astronomical event and that occurs twice each century. Edmund Halley had predicted the event and had left instructions on how it could be measured. By taking measurements at different points on the earth, the solar parallax, or distance from the earth to the sun, can be calculated.)

Mason and Dixon entered into a contract with the proprietors and arrived in Philadelphia on Nov. 15, 1763. They had brought with them state-of-the-art equipment specially made for the Penns by the famous London instrument maker John Bird to perform their task. These included:
  • A zenith sector--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;
  • A transit and equal altitude instrument used to determine true north by tracking stars where they crossed the Meridian; and
  •  A Hadley quadrant.
They also brought an astronomical regulator--a long case pendulum clock, Gunter’s chains and wood and brass measuring rods used to measure level horizontal distance across sloping ground and various reference books, including ephemeredes and logarithmic and trigonometric tables.           

After a few days meeting with the commissioners and checking their instruments, they began their task at the north wall of a house on Cedar Street (now South Street), which had been determined to be the southern point of Philadelphia. They constructed a temporary observatory near the house and from detailed astronomical observations, determined its latitude.

As going 15 miles due south would have taken them through the Delaware River, they decided to proceed west 31 miles to the farm of John Harland in what is now 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.” 

Tradition has that their observatory was built over the Star Gazer’s Stone, but recent research by Todd Babcock, chair of the Mason Dixon Line Preservation Partnership, suggests that the observatory was somewhat to the south and that the Star Gazer’s Stone was actually used for daylight sightings.

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 evenings in the local taverns no doubt sharing political gossip as tensions with the mother country rose.

In the spring of 1764, they set off south where they placed an oak post, which they 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-39°56’18.9” North. This was to become the starting point of the famous West Line.

They then headed to the middle point of the transpeninsula 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 marks remained on the ground and Mason and Dixon took advantage of them. The actual technique used to run the tangent line has been somewhat of a mystery, but a letter from Mason and Dixon to Lord Baltimore dated Dec. 4, 1764, explains their procedure. Mason’s elegantly simple 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 been 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.

On March 11, 1765, they returned to the “Post Mark’d West” to begin the monumental task of running the West line. Poor weather delayed their start, but on April 5, they were able to finally begin. They ran as far as the Susquehanna River stopping along the way to take astronomical observations to determine their latitude. Before crossing the river, they returned to run the lines from the tangent point to the now established West line, and they set stones to mark the line in the presence of the boundary commissioners. They were then instructed by the commissioners to “continue the parallel of latitude as far as the Country is settled.”

They were accompanied by a sometimes huge work party, which cleared a “visto" six or seven yards wide, and they set stones which had been sent from England every mile and “crown stones” every five miles. By Oct. 7, 1765, they had proceeded 117 miles 12 chains and 97 links from the “Post Mark’d West” in Mr. Bryan’s field. They stored their instruments at the home of Captain Evan Shelby, a local magistrate, for the winter and returned east to the Harlan Farm. They resumed again in the spring and proceeded as far as Sidling Hill, where their wagons were unable to proceed any further.

On June 18, 1766, they reached the Allegheny Mountains, which had been established by King George as the western limit of English settlement. The Seven Year War (known in the Americas as 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 English 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. Mason wrote in his Journal:

At present, the Allegheny Mountains is the boundary between the Natives and the strangers; in these parts of his Britanic Majesties Collonies. From the solitary tops of  these mountains, the Eye gazes round with pleasure; filling the mind with adoration to that pervading spirit that made them.      

Unable to proceed, the instruments were again sent to Captain Shelby’s for safekeeping and the hands discharged.

While they waited for permission to proceed, Mason and Dixon applied to the Royal Society for a grant of £200 to measure a degree of latitude. Their proposal was accepted, and using the lines previously cleared, they measured the first degree of latitude ever measured in the New World.

Meanwhile, negotiations with the Six Nations continued and greased by a payment of 500 pounds sterling, permission was secured to proceed beyond the Alleghenies. In July 1767, three Onondagas, 11 Mohawks and an interpreter were dispatched by the Six Nations to guide the survey party, which had now grown to 115 men.

On Oct. 9, 1767, the party encountered “the Great Warrior Trail” (shown on their map as “War Path”). This was one of the most important Indian trails in the country running from New York to South Carolina, sort of an Indian Interstate 95. The chief of the Indians informed the surveyors that the War Path was the extent of (their commission from the chiefs of the Six Nations “and that they would not proceed one step further westward.”

The line was extended an additional 250.8 feet to the top of the next ridge, Brown’s Hill, and after setting up a tall post and a conical mound at 233 miles 17 chains and 48 links from the “Post Mark’d West” in Mr. Bryan’s field, the Mason Dixon survey came to an end.

At the conclusion of their work, Mason and Dixon returned to Philadelphia and were instructed to draw a map of their survey and 200 copies were printed, two copies of which are here at the MdHS. On Sept. 11, 1768, their work done, 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 County Durham, where he died in 1779. Mason returned to America in 1786 with his wife and eight children but died shortly thereafter and is buried in an unmarked grave in the Christ Church burial grounds in Philadelphia. There is no known portrait of either of them. 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 dividing line between slavery and freedom.
Although Mason and Dixon returned to England, the Bird instruments, which belonged to the Penns, remained behind. A small observatory operated by the American Philosophical Society was constructed on the grounds of the Pennsylvania State House, which we now know as Independence Hall, and the instruments were stored inside, and the second Transit of Venus was observed there in 1769. The observatory had steps, and being on the grounds of the State House, it provided a convenient stage from which public announcements were made during the unrest leading up to The Revolution.

It was from these steps that Col. John Nixon, leader of  Pennsylvania’s Committee of Safety,  first proclaimed the Declaration of Independence to a thronging crowd on July 8, 1776.

In 1912, during renovations to the bell tower of the State House, floorboards were removed under the old supports for the Liberty Bell and to great surprise the transit and equal altitude instrument was discovered. No one knows why it was hidden there and forgotten. It was theorized that when the Continentals lost the Battle of Brandywine in 1777, panic ensued in Philadelphia in anticipation of the British occupation and items that might be useful to the enemy were either hidden or hurriedly sent away and, perhaps, that is the reason the transit was hidden in the bell tower. However, the Bird transit was later used by David Rittenhouse and Andrew Ellicott in 1784 for the extension of the Mason Dixon line and in 1785 to run the western boundary of Pennsylvania up to Lake Erie. So the reason that the transit was hidden under the floor boards remains a mystery.

But whatever the reason, the little transit on display at the Maryland Historical Society may be the most historic American scientific instrument ever, having witnessed two of the seminal events in American history--the survey of the Mason Dixon line and the first reading of the Declaration of Independence.