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Fine Maps

Walk the Line

With Mason carrying out the tasks of the astronomer and Dixon the surveyor, the crew, on a good day, could measure out about eight chains or 528 feet an hour. About every twenty miles their progress would be checked through astronomical observation. Using the zenith sector, Mason might make as many as 134 observations on five different stars, a process that could take as many as three consecutive nights.

Winter came early in the high altitudes. Knee-deep snow and frigid temperatures hampered the survey crew’s trek back to Philadelphia from Dunkard Creek. Due to the lateness of the season, they were unable to set the last of the four-hundred-pound rectangular limestone boundary markers that had been shipped from England. For most of the border, however, the stones were ejected every mile, with every fifth stone, or “crown stone,” marked with the coat of arms of the two English patrons. Standing on a high summit earlier in the survey, Mason looked back on the twenty-five-foot-wide path that their crew had cut through the bush and called the line of white marker stones “very beautiful, and agreeable to the Laws of a Sphere.”

Once back in Philadelphia, Mason and Dixon immediately set to work preparing “fair copies” of their daily journals and field data for the benefit of the Penns and the Calverts. And thank goodness they did because soon after the copies were made, the original notes went missing. A century later, they turned up in a Halifax, Nova Scotia, trash heap and were eventually deposited in the National Archives in Washington, D.C. Mason and Dixon also prepared two hundred copies of an engraved copperplate map that showed the border area in great detail. And last but not least, they submitted financial statements for expenses amounting to £3,512/9s, the equivalent to nearly a million dollars today.

In their attempt to leave a dependable border that both sides could agree on, Mason and Dixon set the stage for an engineering feat that has gone down in the annals of cartography. Their estimated error of fifty-sixty feet for the 233-mile survey made it the longest, straightest line of its day. If they had been able to account for gravitational pull on their plumb bobs, no doubt the error factor would have been more in line with modern GPS surveys. Eighteenth-century crews, however, had no experimental proof of the effect of mountain mass and dense bedrock on their measuring devices and no way to account for it in their calculations.

Considering the tools at their disposal and the uncharted country that they had to traverse, surely the Mason-Dixon survey is an achievement to be celebrated. To the average American, however, developments in the nineteenth century overshadowed the scientific triumphs of the eighteenth. The Mason-Dixon line is now more of a household phrase not as a technological marvel, but as a symbolic division between the northern free states and southern slave states.

Jeffrey S. Murray is a former senior archivist with Library and Archives Canada.
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