“Jonathan Dillon April 13- 1861 Fort Sumpter was attacked by the rebels on the above date. J Dillon,” the brass underside of the watch movement reads.

The inscription continues: “April 13- 1861 Washington thank God we have a government Jonth Dillon.”

The story of the engraving had been passed down through the years by descendants of Jonathan Dillon, the watchmaker, without ever being verified. Then recently one of his great-great-grandsons, Douglas Stiles, a lawyer from Waukegan, Ill., discovered an April 1906 article in The New York Times in which Mr. Dillon described making the engraving.

In the article Mr. Dillon, then 84, recounted that he was working at M. W. Galt & Company, a watch shop on Pennsylvania Avenue in Washington, in April 1861 when the shop’s owner, Mr. Galt, hurried upstairs to tell him, “War has begun; the first shot has been fired.”

“At that moment I had in my hand Abraham Lincoln’s watch, which I had been repairing,” Dillon told The Times, adding that he later learned it was the first watch that Lincoln ever owned.

An immigrant from Waterford, Ireland, he told The Times, “I was the only Union sympathizer working in the shop.”

The National Museum of American History acquired the watch in 1958 through a bequest by a great-grandson of Lincoln. Approached by Mr. Stiles last year about the Times article, the museum’s curators brought in expert watchmakers to open the timepiece.

“It’s a good opportunity to show how we do the research about the collection,” Brent D. Glass, the museum’s director, said on Tuesday. The opening of the watch was also timed to coincide with “Abraham Lincoln: An Extraordinary Life,” an exhibition that opened at the museum in January.

Harry R. Rubenstein, the curator of the exhibition, said the museum had considered opening the watch in private and then decided to include the public. “It’s a moment of discovery, and you can only discover things once,” he said. “We wanted to share it.”

Working under a strong light with magnifying glasses and minute tools, George Thomas, a master watchmaker from Towson, Md., had a bit of difficulty removing one of the pins but finally opened the back to reveal the underside of the watch movement.

“The moment of truth has come,” he said. “Is there or is there not an inscription?”

He called Mr. Stiles, who attended the event with his brother, Don, 57, of Bloomington, Minn., to deliver the verdict. “There is an inscription!” Mr. Stiles said with elation.

“My goodness, that’s Lincoln’s watch,” he said later in wonderment. “My ancestor put graffiti on it.”

The inscription is not entirely accurate. Aside from misspelling Sumter, Mr. Dillon was evidently unaware that the opening shot of the Civil War had been fired on April 12, not April 13.

It also seems that Mr. Dillon’s memory of the message was a little fuzzy as recounted in the 1906 article. He remembered his engraving as reading: “The first gun is fired. Slavery is dead. Thank God we have a President who at least will try.”

The back of the watch movement also bears two other inscriptions: “LE Grofs Sept 1864 Wash DC,” probably engraved by another repairer, and what appears to be “Jeff Davis.” Whether the latter was intended as a retort by another watchmaker to Dillon’s pro-Union sentiments is unknown. (Jefferson Davis was the president of the rebel Confederacy.)

The discovery of the Dillon message is likely to hearten enthusiasts of Lincoln lore. “It’s a different message, but it still has that hopeful sound that the union will hold together, the country will go on,” Mr. Rubenstein said. “That Lincoln carried this hopeful message in his pocket unbeknownst to him — it casts you back.”

Mr. Glass, the museum’s director, said it came as little surprise that the message did not mention slavery after all; Lincoln issued the Emancipation Proclamation in separate executive orders in October 1862 and January 1863.

“In 1861 the preservation of the union was the key issue, and the abolition of slavery came later,” he said.

Mr. Glass likened the watch’s engraving to a note Thomas Jefferson attached to the underside of the writing table on which he wrote the Declaration of Independence. In the note, Jefferson wrote of the importance of preserving the desk because of what it was used for.

“I think there is a human instinct to want to communicate to the future,” Mr. Glass said.

Mr. Thomas said the timepiece was in “mint condition” and had its original hands. He said that it was made in Liverpool in the 19th century, but that the gold case in which it resided for generations was made in the United States.

Although Mr. Rubenstein described the timepiece as Lincoln’s “everyday pocket watch,” acquired when he was practicing law in Springfield, Ill., Mr. Thomas said it looked almost untouched. “It seems the president did not wear it much,” he said.

Mr. Stiles said he had first heard the story of the watch engraving sometime in the 1970s from a great-uncle. Last year, he said, a cousin in Ireland mentioned the story to him, which prompted him to start searching the Internet.

At the end of Tuesday’s news conference, museum curators asked if the watch could be wound in the hope that those present could “hear the ticking, the sound that Abraham Lincoln heard,” as Mr. Rubenstein said. It was the sole letdown of the day.

“It’s frozen,” Mr. Thomas said. “It hasn’t been touched in a hundred years.”

The watch was reassembled and will be returned to view at the museum, along with a detailed photograph and a transcript of the engraving.

“Does it change our view of history? No,” Mr. Glass said. “But it adds to our understanding of how an ordinary person was affected by the events of the day.”