Gustave Eiffel may be best known for the Parisian tower that bears his name, but the French engineer played a key role in the construction of another structure with worldwide fame: the Statue of Liberty. A recently discovered set of Eiffel’s original schematic drawings for Frédéric Auguste Bartholdi’s statue—which weren’t previously known to have survived—show the development of his innovative design. For more than a century, Eiffel’s engineering ideas helped the statue withstand the high winds and salty air of New York Harbor. More curiously, the drawings also hint at a last minute change to Lady Liberty’s upraised, torch-bearing arm.

In late 2018, California map dealer Barry Lawrence Ruderman purchased a folder of material from Eiffel’s workshop at an auction in Paris. The auction catalog said the lot included blueprints and other papers related to the Statue of Liberty, says Alex Clausen, director of Ruderman’s gallery. That alone would have made it a rare find: only two other copies of Eiffel’s blueprints were known to have survived: one at the Library of Congress and another in a private collection in France.

Clausen and Ruderman only realized the extent of what they’d purchased when it arrived in California several weeks later. At the bottom of the folder they discovered a tightly folded stack of papers. “It was too fragile to open,” Clausen says.

So they sent the stack to a conservator, who placed the papers in a humidified chamber to soften them. Once separable, the documents turned out to be 22 original engineering drawings of the Statue, many with handwritten annotations and calculations in the margins. ”To find the drawings from which all the blueprints were made, that’s just as good as it could get,” Clausen says.

The Statue was a gift from France to the United States, conceived in the mid-1860s and intended as a tribute to American democracy. Bartholdi, the French sculptor who got the commission for the Statue took inspiration from the Roman goddess of liberty. His elegant statue is 151 feet tall, but what tourists see from the outside is just a shell of hammered copper—thinner than two stacked pennies. “Without really good structural support, the copper would never stand up on its own,” says Edward Berenson, a historian at New York University and author of a 2012 book about the statue .

To design that support system, Bartholdi turned to Eiffel, who was at the time best known for his work designing railroad bridges. (The first engineer on the project, Eugène Viollet-le-Duc, died in 1879 before completing his plans). Eiffel’s ingenious use of wrought iron trusses made his bridges light enough to span long gaps and flexible enough to absorb the shock of moving trains, Berenson says. “It’s the same principle he used for the Statue of Liberty, and then for the Eiffel Tower.”

With the statue, it wasn’t the shock of moving trains but rather the howling winds of New York Harbor that Eiffel had to contend with. (According to the National Park Service, the statue sways up to 3 inches in a 50 mile an hour wind, and the torch sways up to 6 inches). The iron skeleton he devised acts like a network of springs that supports Bartholdi’s copper statue and absorbs the force as it gets buffeted by the wind. The design anticipates the support systems used in the early skyscrapers of Chicago and New York in the late 19th and early 20th century, Berenson says.

Eiffel also had to reckon with another problem created by the statue’s location: the potential for corrosion at the points of contact between the iron skeleton and copper exterior in the presence of saltwater spray from the harbor. Eiffel’s solution, sandwiching asbestos insulation between the two metals, helped stall this electrochemical process for decades, but it created headaches when the statue was restored in the 1980s, by which time asbestos was known to be carcinogenic.

The newly found drawings primarily show, from multiple angles, Eiffel’s designs for the iron trusswork that would support the statue. They also include closeups of key features, like the hardware that would be needed to attach the statue to its massive concrete base. Tables of numbers list the weights of various components and the loads they would need to bear. In many places, handwritten calculations in the margins appear to have been used to make adjustments or corrections.

Berenson thinks the drawings may nail down something that historians have long suspected but not been able to prove: that Bartholdi disregarded Eiffel's engineering plans when it came to the statue's upraised arm, electing to make it thinner and tilted outward for dramatic and aesthetic appeal. Several drawings appear to depict a bulkier shoulder and more vertical arm—a more structurally sound arrangement. But one of these sketches (below) was marked up by an unidentified hand with red ink that tilts the arm outward, as Bartholdi wanted. “This could be evidence for a change in the angle that we ended up with in the real Statue of Liberty,” Berenson says. “It looks like somebody is trying to figure out how to change the angle of the arm without wrecking the support.”

The date on that sketch, July 28, 1882, as well as dates on several pages of handwritten calculations and diagrams pertaining to the arm, suggest that this change was made after much of the statue had already been built. “It’s really late in the game,” Berenson says. (Construction of the statue stretched from 1876 to 1884; after being packed in crates and shipped to New York, it was finally dedicated on October 28, 1886.)

Historians have found very little record of the collaboration between Bartholdi and Eiffel, Berenson says, so it’s hard to know what transpired. “Bartholdi played down Eiffel’s contributions because he was kind of an egotistical guy,” Berenson says. One possibility, he suggests, is that Eiffel had largely moved on to other projects by that late stage, and assigned his assistants to wrap things up with Bartholdi. “That may be one reason why Bartholdi decided he could make modifications, because he knew that Eiffel was not totally hands-on,” Berenson says.

Whatever the case, Lady Liberty's more precarious arm has had consequences over the years, from exacerbating the damage to the statue caused in 1916 when a German saboteur blew up a nearby munitions depot , to a debate about how to strengthen the statue during the extensive renovations in the 1980s. At that time, engineers wanted to fortify the arm in accordance with what they suspected Eiffel had originally intended, but they were eventually overruled by preservationists who insisted on not making any visible changes to the statue to remain true to Bartholdi’s artistic vision.

Clausen says he and Ruderman initially hoped to partner with a museum or other institution to put the drawings on public display, but those plans are up in the air now as museums are closed due to the COVID-19 pandemic. In the meantime, the gallery has made digital versions available on their website.