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Mason-Dixon instrument, found and restored, displayed in Baltimore

The job was the 18th-century equivalent of putting a man on the moon: charting a straight east-west line more than 200 miles through forests, swamps and mountain terrain to establish a boundary between the colonies of Maryland and Pennsylvania.

When surveyors Charles Mason and Jeremiah Dixon pulled off the engineering feat, they did it using a world-class brass instrument that became lost to the world.

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Visitors can now see that device, newly rediscovered and fully refurbished, along with related original documents, in a pristine setting in Baltimore.

The apparatus — a tripod-mounted surveying tool known as a transit — will be on public display at the Maryland Historical Society for the next two weeks, thanks largely to the persistence of a civil engineer, surveyor and amateur sleuth named David S. Thaler.

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"That really was the 18th century's most amazing scientific achievement, at least in the realms of surveying and measuring, achieving constant latitude over such a distance, and the instrument itself is magnificent," said Edwin Danson, author of the book "Drawing the Line: How Mason and Dixon Surveyed the Most Famous Border in America."

It was Danson's friend, the Baltimore resident Thaler, who located, identified and spearheaded the repair of the instrument — known variously as the Bird Transit or the Bird Instrument after its 18th-century creator, John Bird — completing a journey that lasted five years and climaxed in an unveiling reception Thursday.

Thaler calls the process "an Indiana Jones story," the kind of unpredictable tale anyone who loves unearthing the facts of history dreams of. It started when one of the curators of the Maryland Historical Society — where Thaler was president-elect — brought him something from the museum's vast collection: a map on a 5-foot sheet of linen-backed paper.

Thaler took one look, he says, and "my eyes popped out of my head: It was the original map of the Mason-Dixon survey."

Part of a crate of documents known as the Calvert papers, which the museum had acquired in 1870, it had lain unnoticed among the 8.5 million artifacts stored in the basement, many of them still being combed through by the institution's small staff.

"What else you got?" Thaler remembers asking, only to be stunned to find the box included other priceless treasures: the original contract for the expedition, signed by Mason, Dixon and William Penn and Charles Calvert, the governors of Maryland and Pennsylvania at the time; the bill to the King of England for the group's five years of work (the tab was 3,516 pounds, roughly the equivalent of $300,000 in today's U.S. currency); and the original field journal that Mason kept.

"We've got to have an exhibition!" Thaler remembers crying out.

The findings only whetted Thaler's appetite, he says, for information on the Mason-Dixon expedition, which took place between 1763 and 1768, and the 250th anniversary of which was on the horizon.

It wasn't much later that a friend who had visited Independence Hall in Philadelphia sent Thaler a photograph he had taken of an old surveying instrument then displayed as a decoration in a rarely seen chamber on the second floor.

Independence Hall curators had long believed it was an instrument used in a mid-1800s ocean voyage known to mark the transit of Venus, a mission on which surveyors took measurements intended to help establish the distance between Earth and the sun.

Thaler, who collects historic surveying instruments, thought it looked more like the instrument Mason and Dixon might have used during their expedition, a device known to have been created by Bird, a British instrument maker with a worldwide reputation.

When he went to Philadelphia and investigated, he was proved right: the telescope portion of the transit instrument bore Bird's engraved signature.

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The instrument had, in fact, been used on a transit of Venus expedition, as it turned out, but it was also the one Mason and Dixon used in settling the long-standing Maryland-Pennsylvania border dispute.

"They were right, but I was more right," jokes Thaler, who believes it was brought to the Philadelphia landmark in the 1800s to help establish local time.

The surveying tool was in bad shape, with only four of its main parts intact, so Thaler led an effort by the Maryland Society of Professional Engineers and the Maryland Society of Surveyors to raise money for its repair.

They hired one of the nation's top restorers of Colonial instruments, Jeff Lock of Ohio, to conduct research on some of Bird's other surviving pieces and do the job.

Tens of thousands of dollars later, it is in pristine condition and, in Danson's words, a "magnificent work of history and art."

It will be on display at the Maryland Historical Society for two weeks, Thaler said, along with the original Mason-Dixon documents, before being moved to its permanent home in William Penn's former office on the second floor of Independence Hall.

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