For nearly 150 years, a story has circulated about a hidden Civil War message engraved inside one of Abraham Lincoln's pocket watches. Now we know what it says.

On Tuesday at the National Museum of American History, a watchmaker used tiny tools to open the pocket watch and reveal the message left during repairs in 1861.

The first line says: "April 13, 1861. Fort Sumter was attacked by the rebels on the above date. J. Dillon." A second line reads: "April 13, 1861. Thank God we have a government. Jonathan Dillon."

Dillon's story circulated among his family and friends, eventually reaching a New York Times reporter. In a 1906 article in the paper, Dillon said he was moved to engrave a message after the first shots of the Civil War were fired in South Carolina.